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# About
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This document is an updated version of the original design documents
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by Spencer Kimball from early 2014.
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# Overview
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Cockroach is a distributed key:value datastore (SQL and structured
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data layers of cockroach have yet to be defined) which supports **ACID
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transactional semantics** and **versioned values** as first-class
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features. The primary design goal is **global consistency and
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survivability**, hence the name. Cockroach aims to tolerate disk,
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machine, rack, and even **datacenter failures** with minimal latency
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disruption and **no manual intervention**. Cockroach nodes are
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symmetric; a design goal is **homogenous deployment** (one binary) with
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minimal configuration.
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Cockroach implements a **single, monolithic sorted map** from key to
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value where both keys and values are byte strings (not unicode).
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Cockroach **scales linearly** (theoretically up to 4 exabytes (4E) of
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logical data). The map is composed of one or more ranges and each range
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is backed by data stored in [RocksDB](http://rocksdb.org/) (a
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variant of LevelDB), and is replicated to a total of three or more
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cockroach servers. Ranges are defined by start and end keys. Ranges are
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merged and split to maintain total byte size within a globally
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configurable min/max size interval. Range sizes default to target `64M` in
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order to facilitate quick splits and merges and to distribute load at
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hotspots within a key range. Range replicas are intended to be located
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in disparate datacenters for survivability (e.g. `{ US-East, US-West,
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Japan }`, `{ Ireland, US-East, US-West}`, `{ Ireland, US-East, US-West,
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Japan, Australia }`).
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Single mutations to ranges are mediated via an instance of a distributed
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consensus algorithm to ensure consistency. We’ve chosen to use the
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[Raft consensus
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algorithm](https://ramcloud.stanford.edu/wiki/download/attachments/11370504/raft.pdf).
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All consensus state is stored in RocksDB.
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A single logical mutation may affect multiple key/value pairs. Logical
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mutations have ACID transactional semantics. If all keys affected by a
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logical mutation fall within the same range, atomicity and consistency
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are guaranteed by Raft; this is the **fast commit path**. Otherwise, a
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**non-locking distributed commit** protocol is employed between affected
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ranges.
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Cockroach provides [snapshot isolation](http://en.wikipedia.org/wiki/Snapshot_isolation) (SI) and
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serializable snapshot isolation (SSI) semantics, allowing **externally
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consistent, lock-free reads and writes**--both from a historical
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snapshot timestamp and from the current wall clock time. SI provides
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lock-free reads and writes but still allows write skew. SSI eliminates
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write skew, but introduces a performance hit in the case of a
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contentious system. SSI is the default isolation; clients must
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consciously decide to trade correctness for performance. Cockroach
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implements [a limited form of linearizability](#linearizability),
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providing ordering for any observer or chain of observers.
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Similar to
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[Spanner](http://static.googleusercontent.com/media/research.google.com/en/us/archive/spanner-osdi2012.pdf)
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directories, Cockroach allows configuration of arbitrary zones of data.
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This allows replication factor, storage device type, and/or datacenter
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location to be chosen to optimize performance and/or availability.
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Unlike Spanner, zones are monolithic and don’t allow movement of fine
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grained data on the level of entity groups.
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A
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[Megastore](http://www.cidrdb.org/cidr2011/Papers/CIDR11_Paper32.pdf)-like
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message queue mechanism is also provided to 1) efficiently sideline
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updates which can tolerate asynchronous execution and 2) provide an
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integrated message queuing system for asynchronous communication between
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distributed system components.
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# Architecture
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Cockroach implements a layered architecture. The highest level of
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abstraction is the SQL layer (currently unspecified in this document).
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It depends directly on the [*structured data
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API*](#structured-data-api), which provides familiar relational concepts
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such as schemas, tables, columns, and indexes. The structured data API
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in turn depends on the [distributed key value store](#key-value-api),
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which handles the details of range addressing to provide the abstraction
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of a single, monolithic key value store. The distributed KV store
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communicates with any number of physical cockroach nodes. Each node
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contains one or more stores, one per physical device.
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Each store contains potentially many ranges, the lowest-level unit of
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key-value data. Ranges are replicated using the Raft consensus protocol.
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The diagram below is a blown up version of stores from four of the five
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nodes in the previous diagram. Each range is replicated three ways using
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raft. The color coding shows associated range replicas.
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Each physical node exports a RoachNode service. Each RoachNode exports
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one or more key ranges. RoachNodes are symmetric. Each has the same
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binary and assumes identical roles.
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Nodes and the ranges they provide access to can be arranged with various
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physical network topologies to make trade offs between reliability and
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performance. For example, a triplicated (3-way replica) range could have
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each replica located on different:
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- disks within a server to tolerate disk failures.
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- servers within a rack to tolerate server failures.
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- servers on different racks within a datacenter to tolerate rack power/network failures.
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- servers in different datacenters to tolerate large scale network or power outages.
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Up to `F` failures can be tolerated, where the total number of replicas `N = 2F + 1` (e.g. with 3x replication, one failure can be tolerated; with 5x replication, two failures, and so on).
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# Cockroach Client
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In order to support diverse client usage, Cockroach clients connect to
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any node via HTTPS using protocol buffers or JSON. The connected node
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proxies involved client work including key lookups and write buffering.
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# Keys
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Cockroach keys are arbitrary byte arrays. If textual data is used in
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keys, utf8 encoding is recommended (this helps for cleaner display of
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values in debugging tools). User-supplied keys are encoded using an
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ordered code. System keys are either prefixed with null characters (`\0`
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or `\0\0`) for system tables, or take the form of
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`<user-key><system-suffix>` to sort user-key-range specific system
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keys immediately after the user keys they refer to. Null characters are
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used in system key prefixes to guarantee that they sort first.
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# Versioned Values
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Cockroach maintains historical versions of values by storing them with
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associated commit timestamps. Reads and scans can specify a snapshot
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time to return the most recent writes prior to the snapshot timestamp.
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Older versions of values are garbage collected by the system during
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compaction according to a user-specified expiration interval. In order
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to support long-running scans (e.g. for MapReduce), all versions have a
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minimum expiration.
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Versioned values are supported via modifications to RocksDB to record
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commit timestamps and GC expirations per key.
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# Lock-Free Distributed Transactions
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Cockroach provides distributed transactions without locks. Cockroach
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transactions support two isolation levels:
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- snapshot isolation (SI) and
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- *serializable* snapshot isolation (SSI).
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*SI* is simple to implement, highly performant, and correct for all but a
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handful of anomalous conditions (e.g. write skew). *SSI* requires just a touch
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more complexity, is still highly performant (less so with contention), and has
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no anomalous conditions. Cockroach’s SSI implementation is based on ideas from
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the literature and some possibly novel insights.
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SSI is the default level, with SI provided for application developers
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who are certain enough of their need for performance and the absence of
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write skew conditions to consciously elect to use it. In a lightly
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contended system, our implementation of SSI is just as performant as SI,
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requiring no locking or additional writes. With contention, our
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implementation of SSI still requires no locking, but will end up
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aborting more transactions. Cockroach’s SI and SSI implementations
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prevent starvation scenarios even for arbitrarily long transactions.
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See the [Cahill paper](https://drive.google.com/file/d/0B9GCVTp_FHJIcEVyZVdDWEpYYXVVbFVDWElrYUV0NHFhU2Fv/edit?usp=sharing)
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for one possible implementation of SSI. This is another [great paper](http://cs.yale.edu/homes/thomson/publications/calvin-sigmod12.pdf).
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For a discussion of SSI implemented by preventing read-write conflicts
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(in contrast to detecting them, called write-snapshot isolation), see
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the [Yabandeh paper](https://drive.google.com/file/d/0B9GCVTp_FHJIMjJ2U2t6aGpHLTFUVHFnMTRUbnBwc2pLa1RN/edit?usp=sharing),
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which is the source of much inspiration for Cockroach’s SSI.
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Each Cockroach transaction is assigned a random priority and a
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"candidate timestamp" at start. The candidate timestamp is the
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provisional timestamp at which the transaction will commit, and is
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chosen as the current clock time of the node coordinating the
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transaction. This means that a transaction without conflicts will
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usually commit with a timestamp that, in absolute time, precedes the
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actual work done by that transaction.
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In the course of coordinating a transaction between one or more
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distributed nodes, the candidate timestamp may be increased, but will
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never be decreased. The core difference between the two isolation levels
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SI and SSI is that the former allows the transaction's candidate
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timestamp to increase and the latter does not.
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Timestamps are a combination of both a physical and a logical component
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to support monotonic increments without degenerate cases causing
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timestamps to diverge from wall clock time, following closely the
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[*Hybrid Logical Clock
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paper.*](http://www.cse.buffalo.edu/tech-reports/2014-04.pdf)
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1. Start the transaction by writing a new entry to the system
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transaction table (keys prefixed by *\0tx*) with state “PENDING”. In
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parallel write an "intent" value for each datum being written as part
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of the transaction. These are normal MVCC values, with the addition of
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a special flag (i.e. “intent”) indicating that the value may be
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the transaction id (unique and chosen at tx start time by client)
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is stored with intent values. The tx id is used to refer to the
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transaction table when there are conflicts and to make
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tie-breaking decisions on ordering between identical timestamps.
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selects the maximum from amongst all writes as the final commit
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timestamp.
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Each range maintains a small (i.e. latest 10s of read timestamps),
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*in-memory* cache from key to the latest timestamp at which the
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key(s) were read. This *latest-read-cache* is consulted on each
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write. If the write’s candidate timestamp is earlier than the low
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water mark on the cache itself (i.e. its last evicted timestamp)
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or if the key being written has a read timestamp later than the
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write’s candidate timestamp, this later timestamp value is
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returned with the write. The cache’s entries are evicted oldest
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timestamp first, updating low water mark as appropriate. If a new
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range replica leader is elected, it sets the low water mark for
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the cache to the current wall time + ε (ε = 99^th^ percentile
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clock skew).
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transaction table (keys prefixed by *\0tx*). The value of the
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commit entry contains the candidate timestamp (increased as
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necessary to accommodate any latest read timestamps). Note that
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the transaction is considered fully committed at this point and
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control may be returned to the client.
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In the case of an SI transaction, a commit timestamp which was
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increased to accommodate concurrent readers is perfectly
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acceptable and the commit may continue. For SSI transactions,
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however, a gap between candidate and commit timestamps
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necessitates transaction restart (note: restart is different than
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abort--see below).
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After the transaction is committed, all written intents are upgraded
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in parallel by removing the “intent” flag. The transaction is
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considered fully committed before this step and does not wait for
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it to return control to the transaction coordinator.
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In the absence of conflicts, this is the end. Nothing else is necessary
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to ensure the correctness of the system.
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**Conflict Resolution**
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Things get more interesting when a reader or writer encounters an intent
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record or newly-committed value in a location that it needs to read or
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write. This is a conflict, usually causing either of the transactions to
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abort or restart depending on the type of conflict.
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***Transaction restart:***
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This is the usual (and more efficient) type of behaviour and is used
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except when the transaction was aborted (for instance by another
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transaction).
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In effect, that reduces to two cases; the first being the one outlined
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above: An SSI transaction that finds upon attempting to commit that
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its commit timestamp has been pushed. The second case involves a transaction
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actively encountering a conflict, that is, one of its readers or writers
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encounter data that necessitate conflict resolution
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(see transaction interactions below).
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When a transaction restarts, it changes its priority and/or moves its
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begins anew reusing the same tx id. The prior run of the transaction might
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have written some write intents, which need to be deleted before the
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transaction commits, so as to not be included as part of the transaction.
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These stale write intent deletions are done during the reexecution of the
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transaction, either implicitly, through writing new intents to
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the same keys as part of the reexecution of the transaction, or explicitly,
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by cleaning up stale intents that are not part of the reexecution of the
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transaction. Since most transactions will end up writing to the same keys,
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the explicit cleanup run just before committing the transaction is usually
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a NOOP.
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***Transaction abort:***
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This is the case in which a transaction, upon reading its transaction
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table entry, finds that it has been aborted. In this case, the
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transaction can not reuse its intents; it returns control to the client
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before cleaning them up (other readers and writers would clean up
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dangling intents as they encounter them) but will make an effort to
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clean up after itself. The next attempt (if applicable) then runs as a
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There are several scenarios in which transactions interact:
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- **Reader encounters write intent or value with newer timestamp far
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enough in the future**: This is not a conflict. The reader is free
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to proceed; after all, it will be reading an older version of the
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value and so does not conflict. Recall that the write intent may
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be committed with a later timestamp than its candidate; it will
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never commit with an earlier one. **Side note**: if a SI transaction
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reader finds an intent with a newer timestamp which the reader’s own
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- **Reader encounters write intent or value with newer timestamp in the
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near future:** In this case, we have to be careful. The newer
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intent may, in absolute terms, have happened in our read's past if
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the clock of the writer is ahead of the node serving the values.
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In that case, we would need to take this value into account, but
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we just don't know. Hence the transaction restarts, using instead
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a future timestamp (but remembering a maximum timestamp used to
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limit the uncertainty window to the maximum clock skew). In fact,
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this is optimized further; see the details under "choosing a time
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stamp" below.
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- **Reader encounters write intent with older timestamp**: the reader
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must follow the intent’s transaction id to the transaction table.
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If the transaction has already been committed, then the reader can
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just read the value. If the write transaction has not yet been
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committed, then the reader has two options. If the write conflict
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is from an SI transaction, the reader can *push that transaction's
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commit timestamp into the future* (and consequently not have to
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read it). This is simple to do: the reader just updates the
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transaction’s commit timestamp to indicate that when/if the
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transaction does commit, it should use a timestamp *at least* as
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high. However, if the write conflict is from an SSI transaction,
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the reader must compare priorities. If the reader has the higher priority,
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it pushes the transaction’s commit timestamp (that
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transaction will then notice its timestamp has been pushed, and
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a new priority `max(new random priority, conflicting txn’s
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If the other write intent has been written by a transaction with a lower
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priority, the writer aborts the conflicting transaction. If the write
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intent has a higher or equal priority the transaction retries, using as a new
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priority *max(new random priority, conflicting txn’s priority - 1)*;
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the retry occurs after a short, randomized backoff interval.
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- **Writer encounters newer committed value**:
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The committed value could also be an unresolved write intent made by a
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transaction that has already committed. The transaction restarts. On restart,
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the same priority is reused, but the candidate timestamp is moved forward
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to the encountered value's timestamp.
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**Transaction management**
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Transactions are managed by the client proxy (or gateway in SQL Azure
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parlance). Unlike in Spanner, writes are not buffered but are sent
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directly to all implicated ranges. This allows the transaction to abort
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quickly if it encounters a write conflict. The client proxy keeps track
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of all written keys in order to cleanup write intents upon transaction
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completion.
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If a transaction is completed successfully, all intents are upgraded to
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committed. In the event a transaction is aborted, all written intents
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are deleted. The client proxy doesn’t guarantee it will cleanup intents;
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but dangling intents are upgraded or deleted when encountered by future
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readers and writers and the system does not depend on their timely
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cleanup for correctness.
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In the event the client proxy restarts before the pending transaction is
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completed, the dangling transaction would continue to live in the
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transaction table until aborted by another transaction. Transactions
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heartbeat the transaction table every five seconds by default.
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Transactions encountered by readers or writers with dangling intents
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which haven’t been heartbeat within the required interval are aborted.
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An exploration of retries with contention and abort times with abandoned
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transaction is
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[here](https://docs.google.com/document/d/1kBCu4sdGAnvLqpT-_2vaTbomNmX3_saayWEGYu1j7mQ/edit?usp=sharing).
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**Transaction Table**
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Please see [proto/data.proto](https://github.com/cockroachdb/cockroach/blob/master/proto/data.proto) for the up-to-date structures, the best entry point being `message Transaction`.
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**Pros**
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- No requirement for reliable code execution to prevent stalled 2PC
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protocol.
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- Readers never block with SI semantics; with SSI semantics, they may
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abort.
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- Lower latency than traditional 2PC commit protocol (w/o contention)
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because second phase requires only a single write to the
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transaction table instead of a synchronous round to all
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transaction participants.
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- Priorities avoid starvation for arbitrarily long transactions and
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always pick a winner from between contending transactions (no
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mutual aborts).
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- Writes not buffered at client; writes fail fast.
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- No read-locking overhead required for *serializable* SI (in contrast
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to other SSI implementations).
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- Well-chosen (i.e. less random) priorities can flexibly give
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probabilistic guarantees on latency for arbitrary transactions
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(for example: make OLTP transactions 10x less likely to abort than
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low priority transactions, such as asynchronously scheduled jobs).
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**Cons**
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- Reads from non-leader replicas still require a ping to the leader to
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update *latest-read-cache*.
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- Abandoned transactions may block contending writers for up to the
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heartbeat interval, though average wait is likely to be
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considerably shorter (see [graph in link](https://docs.google.com/document/d/1kBCu4sdGAnvLqpT-_2vaTbomNmX3_saayWEGYu1j7mQ/edit?usp=sharing)).
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This is likely considerably more performant than detecting and
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restarting 2PC in order to release read and write locks.
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- Behavior different than other SI implementations: no first writer
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wins, and shorter transactions do not always finish quickly.
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Element of surprise for OLTP systems may be a problematic factor.
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- Aborts can decrease throughput in a contended system compared with
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two phase locking. Aborts and retries increase read and write
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traffic, increase latency and decrease throughput.
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**Choosing a Timestamp**
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A key challenge of reading data in a distributed system with clock skew
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is choosing a timestamp guaranteed to be greater than the latest
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timestamp of any committed transaction (in absolute time). No system can
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claim consistency and fail to read already-committed data.
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Accomplishing this for transactions (or just single operations)
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accessing a single node is easy. The transaction supplies 0 for
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timestamp, indicating that the node should use its current time (time
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for a node is kept using a hybrid clock which combines wall time and a
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logical time). This guarantees data already committed to that node have
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earlier timestamps.
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For multiple nodes, the timestamp of the node coordinating the
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transaction `t` is used. In addition, a maximum timestamp `t+ε` is
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supplied to provide an upper bound on timestamps for already-committed
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data (`ε` is the maximum clock skew). As the transaction progresses, any
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data read which have timestamps greater than `t` but less than `t+ε`
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cause the transaction to abort and retry with the conflicting timestamp
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t<sub>c</sub>, where t<sub>c</sub> \> t. The maximum timestamp `t+ε` remains the same.
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Time spent retrying because of reading recently committed data has an
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upper bound of `ε`. In fact, this is further optimized: upon restarting,
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the transaction not only takes into account the timestamp of the future
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value, but the timestamp of the node at the time of the uncertain read.
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The larger of those two timestamps (typically, the latter) is used to
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bump up the read timestamp, and additionally the node is marked as
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“certain”. This means that for future reads to that node within the
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transaction, we can set `MaxTimestamp = Read Timestamp` (and hence avoid
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further uncertainty restarts). Correctness follows from the fact that we
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know that at the time of the read, there exists no version of any key on
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that node with a higher timestamp; if we ran into one during a future
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read, that node would have happened (in absolute time) after our
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transaction started.
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We expect retries will be rare, but this assumption may need to be
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revisited if retries become problematic. Note that this problem does not
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apply to historical reads. An alternate approach which does not require
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retries would be to make a round to all node participants in advance and
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choose the highest reported node wall time as the timestamp. However,
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knowing which nodes will be accessed in advance is difficult and
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potentially limiting. Cockroach could also potentially use a global
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clock (Google did this with Pinax TODO: link to paper), which would be
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feasible for smaller, geographically-proximate clusters.
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# Linearizability
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First a word about [***Spanner***](http://research.google.com/archive/spanner.html).
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By combining judicious use of wait intervals with accurate time signals,
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Spanner provides a global ordering between any two non-overlapping transactions
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(in absolute time) with \~14ms latencies. Put another way:
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Spanner guarantees that if a transaction T<sub>1</sub> commits (in absolute time)
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before another transaction T<sub>2</sub> starts, then T<sub>1</sub>'s assigned commit
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timestamp is smaller than T<sub>2</sub>'s. Using atomic clocks and GPS receivers,
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Spanner reduces their clock skew uncertainty to \< 10ms (`ε`). To make
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good on the promised guarantee, transactions must take at least double
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the clock skew uncertainty interval to commit (`2ε`). See [*this
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article*](http://www.cs.cornell.edu/~ie53/publications/DC-col51-Sep13.pdf)
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for a helpful overview of Spanner’s concurrency control.
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Cockroach could make the same guarantees without specialized hardware,
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at the expense of longer wait times. If servers in the cluster were
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configured to work only with NTP, transaction wait times would likely to
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be in excess of 150ms. For wide-area zones, this would be somewhat
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mitigated by overlap from cross datacenter link latencies. If clocks
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were made more accurate, the minimal limit for commit latencies would
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improve.
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However, let’s take a step back and evaluate whether Spanner’s external
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consistency guarantee is worth the automatic commit wait. First, if the
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commit wait is omitted completely, the system still yields a consistent
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view of the map at an arbitrary timestamp. However with clock skew, it
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would become possible for commit timestamps on non-overlapping but
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causally related transactions to suffer temporal reverse. In other
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words, the following scenario is possible for a client without global
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ordering:
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- Start transaction T<sub>1</sub> to modify value `x` with commit time *s<sub>1</sub>*
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- On commit of T<sub>1</sub>, start T<sub>2</sub> to modify value `y` with commit time
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\> s<sub>2</sub>
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- Read `x` and `y` and discover that s<sub>1</sub> \> s<sub>2</sub> (**!**)
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The external consistency which Spanner guarantees is referred to as
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**linearizability**. It goes beyond serializability by preserving
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information about the causality inherent in how external processes
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interacted with the database. The strength of Spanner’s guarantee can be
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formulated as follows: any two processes, with clock skew within
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expected bounds, may independently record their wall times for the
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completion of transaction T<sub>1</sub> (T<sub>1</sub><sup>end</sup>) and start of transaction
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T<sub>2</sub> (T<sub>2</sub><sup>start</sup>) respectively, and if later
499
compared such that T<sub>1</sub><sup>end</sup> \< T<sub>2</sub><sup>start</sup>,
500
then commit timestamps s<sub>1</sub> \< s<sub>2</sub>.
501
This guarantee is broad enough to completely cover all cases of explicit
502
causality, in addition to covering any and all imaginable scenarios of implicit
503
causality.
504
505
Our contention is that causality is chiefly important from the
506
perspective of a single client or a chain of successive clients (*if a
507
tree falls in the forest and nobody hears…*). As such, Cockroach
508
provides two mechanisms to provide linearizability for the vast majority
509
of use cases without a mandatory transaction commit wait or an elaborate
510
system to minimize clock skew.
511
512
1. Clients provide the highest transaction commit timestamp with
513
> successive transactions. This allows node clocks from previous
514
> transactions to effectively participate in the formulation of the
515
> commit timestamp for the current transaction. This guarantees
516
> linearizability for transactions committed by this client.
517
>
518
> Newly launched clients wait at least 2 \* ε from process start
519
> time before beginning their first transaction. This preserves the
520
> same property even on client restart, and the wait will be
521
> mitigated by process initialization.
522
>
523
> All causally-related events within Cockroach maintain
524
> linearizability. Message queues, for example, guarantee that the
525
> receipt timestamp is greater than send timestamp, and that
526
> delivered messages may not be reaped until after the commit wait.
527
528
2. Committed transactions respond with a commit wait parameter which
529
> represents the remaining time in the nominal commit wait. This
530
> will typically be less than the full commit wait as the consensus
531
> write at the coordinator accounts for a portion of it.
532
>
533
> Clients taking any action outside of another Cockroach transaction
534
> (e.g. writing to another distributed system component) can either
535
> choose to wait the remaining interval before proceeding, or
536
> alternatively, pass the wait and/or commit timestamp to the
537
> execution of the outside action for its consideration. This pushes
538
> the burden of linearizability to clients, but is a useful tool in
539
> mitigating commit latencies if the clock skew is potentially
540
> large. This functionality can be used for ordering in the face of
541
> backchannel dependencies as mentioned in the
542
> [AugmentedTime](http://www.cse.buffalo.edu/~demirbas/publications/augmentedTime.pdf)
543
> paper.
544
545
Using these mechanisms in place of commit wait, Cockroach’s guarantee can be
546
formulated as follows: any process which signals the start of transaction
547
T<sub>2</sub> (T<sub>2</sub><sup>start</sup>) after the completion of
548
transaction T<sub>1</sub> (T<sub>1</sub><sup>end</sup>), will have commit
549
timestamps such thats<sub>1</sub> \< s<sub>2</sub>.
550
551
# Logical Map Content
552
553
Logically, the map contains a series of reserved system key / value
554
pairs covering accounting, range metadata, node accounting and
555
permissions before the actual key / value pairs for non-system data
556
(e.g. the actual meat of the map).
557
558
- `\0\0meta1` Range metadata for location of `\0\0meta2`.
559
- `\0\0meta1<key1>` Range metadata for location of `\0\0meta2<key1>`.
560
- ...
561
- `\0\0meta1<keyN>`: Range metadata for location of `\0\0meta2<keyN>`.
562
- `\0\0meta2`: Range metadata for location of first non-range metadata key.
563
- `\0\0meta2<key1>`: Range metadata for location of `<key1>`.
564
- ...
565
- `\0\0meta2<keyN>`: Range metadata for location of `<keyN>`.
566
- `\0acct<key0>`: Accounting for key prefix key0.
567
- ...
568
- `\0acct<keyN>`: Accounting for key prefix keyN.
569
- `\0node<node-address0>`: Accounting data for node 0.
570
- ...
571
- `\0node<node-addressN>`: Accounting data for node N.
572
- `\0perm<key0><user0>`: Permissions for user0 for key prefix key0.
573
- ...
574
- `\0perm<keyN><userN>`: Permissions for userN for key prefix keyN.
575
- `\0tree_root`: Range key for root of range-spanning tree.
576
- `\0tx<tx-id0>`: Transaction record for transaction 0.
577
- ...
578
- `\0tx<tx-idN>`: Transaction record for transaction N.
579
- `\0zone<key0>`: Zone information for key prefix key0.
580
- ...
581
- `\0zone<keyN>`: Zone information for key prefix keyN.
582
- `<>acctd<metric0>`: Accounting data for Metric 0 for empty key prefix.
583
- ...
584
- `<>acctd<metricN>`: Accounting data for Metric N for empty key prefix.
585
- `<key0>`: `<value0>` The first user data key.**
586
- ...
587
- `<keyN>`: `<valueN>` The last user data key.**
588
589
There are some additional system entries sprinkled amongst the
590
non-system keys. See the Key-Prefix Accounting section in this document
591
for further details.
592
593
# Node Storage
594
595
Nodes maintain a separate instance of RocksDB for each disk. Each
596
RocksDB instance hosts any number of ranges. RPCs arriving at a
597
RoachNode are multiplexed based on the disk name to the appropriate
598
RocksDB instance. A single instance per disk is used to avoid
599
contention. If every range maintained its own RocksDB, global management
600
of available cache memory would be impossible and writers for each range
601
would compete for non-contiguous writes to multiple RocksDB logs.
602
603
In addition to the key/value pairs of the range itself, various range
604
metadata is maintained.
605
606
- range-spanning tree node links
607
608
- participating replicas
609
610
- consensus metadata
611
612
- split/merge activity
613
614
A really good reference on tuning Linux installations with RocksDB is
615
[here](http://docs.basho.com/riak/latest/ops/advanced/backends/leveldb/).
616
617
# Range Metadata
618
619
The default approximate size of a range is 64M (2\^26 B). In order to
620
support 1P (2\^50 B) of logical data, metadata is needed for roughly
621
2\^(50 - 26) = 2\^24 ranges. A reasonable upper bound on range metadata
623
locations and 220 bytes for the range key itself*). 2\^24 ranges \* 2\^8
624
B would require roughly 4G (2\^32 B) to store--too much to duplicate
625
between machines. Our conclusion is that range metadata must be
626
distributed for large installations.
627
628
To distribute the range metadata and keep key lookups relatively fast,
629
we use two levels of indirection. All of the range metadata sorts first
630
in our key-value map. We accomplish this by prefixing range metadata
631
with two null characters (*\0\0*). The *meta1* or *meta2* suffixes are
632
additionally appended to distinguish between the first level and second
634
we first locate the range information for the lower bound of
635
`\0\0meta1<key1>`, and then use that range to locate the lower bound
636
of `\0\0meta2<key1>`. The range specified there will indicate the
637
range location of `<key1>` (refer to examples below). Using two levels
638
of indirection, **our map can address approximately 2\^62 B of data, or
639
roughly 4E** (*each metadata range addresses 2\^(26-8) = 2\^18 ranges;
640
with two levels of indirection, we can address 2\^(18 + 18) = 2\^36
641
ranges; each range addresses 2\^26 B; total is 2\^(36+26) B = 2\^62 B =
642
4E*).
643
644
Note: we append the end key of each range to meta[12] records because
645
the RocksDB iterator only supports a Seek() interface which acts as a
646
Ceil(). Using the start key of the range would cause Seek() to find the
647
key *after* the meta indexing record we’re looking for, which would
648
result in having to back the iterator up, an option which is both less
649
efficient and not available in all cases.
650
651
The following example shows the directory structure for a map with
652
three ranges worth of data. Ellipses indicate additional key/value pairs to
653
fill an entire range of data. Except for the fact that splitting ranges
654
requires updates to the range metadata with knowledge of the metadata layout,
655
the range metadata itself requires no special treatment or bootstrapping.
656
657
**Range 0** (located on servers `dcrama1:8000`, `dcrama2:8000`,
658
`dcrama3:8000`)
659
660
- `\0\0meta1\xff`: `dcrama1:8000`, `dcrama2:8000`, `dcrama3:8000`
661
- `\0\0meta2<lastkey0>`: `dcrama1:8000`, `dcrama2:8000`, `dcrama3:8000`
662
- `\0\0meta2<lastkey1>`: `dcrama4:8000`, `dcrama5:8000`, `dcrama6:8000`
663
- `\0\0meta2\xff`: `dcrama7:8000`, `dcrama8:8000`, `dcrama9:8000`
664
- ...
665
- `<lastkey0>`: `<lastvalue0>`
666
667
**Range 1** (located on servers `dcrama4:8000`, `dcrama5:8000`,
668
`dcrama6:8000`)
669
670
- ...
671
- `<lastkey1>`: `<lastvalue1>`
672
673
**Range 2** (located on servers `dcrama7:8000`, `dcrama8:8000`,
674
`dcrama9:8000`)
675
676
- ...
677
- `<lastkey2>`: `<lastvalue2>`
678
679
Consider a simpler example of a map containing less than a single
680
range of data. In this case, all range metadata and all data are
681
located in the same range:
682
683
**Range 0** (located on servers `dcrama1:8000`, `dcrama2:8000`,
684
`dcrama3:8000`)*
685
686
- `\0\0meta1\xff`: `dcrama1:8000`, `dcrama2:8000`, `dcrama3:8000`
687
- `\0\0meta2\xff`: `dcrama1:8000`, `dcrama2:8000`, `dcrama3:8000`
688
- `<key0>`: `<value0>`
689
- `...`
690
691
Finally, a map large enough to need both levels of indirection would
692
look like (note that instead of showing range replicas, this
693
example is simplified to just show range indexes):
694
695
**Range 0**
696
697
- `\0\0meta1<lastkeyN-1>`: Range 0
698
- `\0\0meta1\xff`: Range 1
699
- `\0\0meta2<lastkey1>`: Range 1
700
- `\0\0meta2<lastkey2>`: Range 2
701
- `\0\0meta2<lastkey3>`: Range 3
702
- ...
703
- `\0\0meta2<lastkeyN-1>`: Range 262143
704
705
**Range 1**
706
707
- `\0\0meta2<lastkeyN>`: Range 262144
708
- `\0\0meta2<lastkeyN+1>`: Range 262145
709
- ...
710
- `\0\0meta2\xff`: Range 500,000
711
- ...
712
- `<lastkey1>`: `<lastvalue1>`
713
714
**Range 2**
715
716
- ...
717
- `<lastkey2>`: `<lastvalue2>`
718
719
**Range 3**
720
721
- ...
722
- `<lastkey3>`: `<lastvalue3>`
723
724
**Range 262144**
725
726
- ...
727
- `<lastkeyN>`: `<lastvalueN>`
728
729
**Range 262145**
730
731
- ...
732
- `<lastkeyN+1>`: `<lastvalueN+1>`
733
734
Note that the choice of range `262144` is just an approximation. The
735
actual number of ranges addressable via a single metadata range is
736
dependent on the size of the keys. If efforts are made to keep key sizes
737
small, the total number of addressable ranges would increase and vice
738
versa.
739
740
From the examples above it’s clear that key location lookups require at
741
most three reads to get the value for `<key>`:
742
743
1. lower bound of `\0\0meta1<key>`
744
2. lower bound of `\0\0meta2<key>`,
745
3. `<key>`.
746
747
For small maps, the entire lookup is satisfied in a single RPC to Range 0. Maps
748
containing less than 16T of data would require two lookups. Clients cache both
749
levels of range metadata, and we expect that data locality for individual
750
clients will be high. Clients may end up with stale cache entries. If on a
751
lookup, the range consulted does not match the client’s expectations, the
752
client evicts the stale entries and possibly does a new lookup.
753
754
# Range-Spanning Binary Tree
755
756
A crucial enhancement to the organization of range metadata is to
757
augment the bi-level range metadata lookup with a minimum spanning tree,
758
implemented as a left-leaning red-black tree over all ranges in the map.
759
This tree structure allows the system to start at any key prefix and
760
efficiently traverse an arbitrary key range with minimal RPC traffic,
761
minimal fan-in and fan-out, and with bounded time complexity equal to
762
`2*log N` steps, where `N` is the total number of ranges in the system.
763
764
Unlike the range metadata rows prefixed with `\0\0meta[1|2]`, the
765
metadata for the range-spanning tree (e.g. parent range and left / right
766
child ranges) is stored directly at the ranges as non-map metadata. The
767
metadata for each node of the tree (e.g. links to parent range, left
768
child range, and right child range) is stored with the range metadata.
769
In effect, the tree metadata is stored implicitly. In order to traverse
770
the tree, for example, you’d need to query each range in turn for its
771
metadata.
772
773
Any time a range is split or merged, both the bi-level range lookup
774
metadata and the per-range binary tree metadata are updated as part of
775
the same distributed transaction. The total number of nodes involved in
776
the update is bounded by 2 + log N (i.e. 2 updates for meta1 and
777
meta2, and up to log N updates to balance the range-spanning tree).
778
The range corresponding to the root node of the tree is stored in
780
781
As an example, consider the following set of nine ranges and their
782
associated range-spanning tree:
783
784
R0: `aa - cc`, R1: `*cc - lll`, R2: `*lll - llr`, R3: `*llr - nn`, R4: `*nn - rr`, R5: `*rr - ssss`, R6: `*ssss - sst`, R7: `*sst - vvv`, R8: `*vvv - zzzz`.
785
786
787
788
The range-spanning tree has many beneficial uses in Cockroach. It makes
789
the problem of efficiently aggregating accounting information of
790
potentially vast ranges of data tractable. Imagine a subrange of data
791
over which accounting is being kept. For example, the *photos* table in
792
a public photo sharing site. To efficiently keep track of data about the
793
table (e.g. total size, number of rows, etc.), messages can be passed
794
first up the tree and then down to the left until updates arrive at the
795
key prefix under which accounting is aggregated. This makes worst case
796
number of hops for an update to propagate into the accounting totals
797
2 \* log N. A 64T database will require 1M ranges, meaning 40 hops
798
worst case. In our experience, accounting tasks over vast ranges of data
799
are most often map/reduce jobs scheduled with coarse-grained
800
periodicity. By contrast, we expect Cockroach to maintain statistics
801
with sub 10s accuracy and with minimal cycles and minimal IOPs.
802
803
Another use for the range-spanning tree is to push accounting, zones and
804
permissions configurations to all ranges. In the case of zones and
805
permissions, this is an efficient way to pass updated configuration
806
information with exponential fan-out. When adding accounting
807
configurations (i.e. specifying a new key prefix to track), the
808
implicated ranges are transactionally scanned and zero-state accounting
809
information is computed as well. Deleting accounting configurations is
810
similar, except accounting records are deleted.
811
812
Last but *not* least, the range-spanning tree provides a convenient
813
mechanism for planning and executing parallel queries. These provide the
814
basis for
815
[Dremel](http://static.googleusercontent.com/media/research.google.com/en/us/pubs/archive/36632.pdf)-like
816
query execution trees and it’s easy to imagine supporting a subset of
817
SQL or even javascript-based user functions for complex data analysis
818
tasks.
819
820
# Raft - Consistency of Range Replicas
821
822
Each range is configured to consist of three or more replicas. The
823
replicas in a range maintain their own instance of a distributed
824
consensus algorithm. We use the [*Raft consensus
825
algorithm*](https://ramcloud.stanford.edu/wiki/download/attachments/11370504/raft.pdf)
826
as it is simpler to reason about and includes a reference implementation
827
covering important details. Every write to replicas is logged twice.
828
Once to RocksDB’s internal log and once to levedb itself as part of the
829
Raft consensus log.
830
[ePaxos](https://www.cs.cmu.edu/~dga/papers/epaxos-sosp2013.pdf) has
831
promising performance characteristics for WAN-distributed replicas, but
832
it does not guarantee a consistent ordering between replicas.
833
834
Raft elects a relatively long-lived leader which must be involved to
835
propose writes. It heartbeats followers periodically to keep their logs
836
replicated. In the absence of heartbeats, followers become candidates
837
after randomized election timeouts and proceed to hold new leader
838
elections. Cockroach weights random timeouts such that the replicas with
839
shorter round trip times to peers are more likely to hold elections
840
first. Although only the leader can propose a new write, and as such
841
must be involved in any write to the consensus log, any replica can
842
service reads if the read is for a timestamp which the replica knows is
843
safe based on the last committed consensus write and the state of any
844
pending transactions.
845
846
Only the leader can propose a new write, but Cockroach accepts writes at
847
any replica. The replica merely forwards the write to the leader.
848
Instead of resending the write, the leader has only to acknowledge the
849
write to the forwarding replica using a log sequence number, as though
850
it were proposing it in the first place. The other replicas receive the
851
full write as though the leader were the originator.
852
853
Having a stable leader provides the choice of replica to handle
854
range-specific maintenance and processing tasks, such as delivering
855
pending message queues, handling splits and merges, rebalancing, etc.
856
857
# Splitting / Merging Ranges
858
859
RoachNodes split or merge ranges based on whether they exceed maximum or
860
minimum thresholds for capacity or load. Ranges exceeding maximums for
861
either capacity or load are split; ranges below minimums for *both*
862
capacity and load are merged.
863
864
Ranges maintain the same accounting statistics as accounting key
865
prefixes. These boil down to a time series of data points with minute
866
granularity. Everything from number of bytes to read/write queue sizes.
867
Arbitrary distillations of the accounting stats can be determined as the
868
basis for splitting / merging. Two sensical metrics for use with
869
split/merge are range size in bytes and IOps. A good metric for
870
rebalancing a replica from one node to another would be total read/write
871
queue wait times. These metrics are gossipped, with each range / node
872
passing along relevant metrics if they’re in the bottom or top of the
873
range it’s aware of.
874
875
A range finding itself exceeding either capacity or load threshold
876
splits. To this end, the range leader computes an appropriate split key
877
candidate and issues the split through Raft. In contrast to splitting,
878
merging requires a range to be below the minimum threshold for both
879
capacity *and* load. A range being merged chooses the smaller of the
880
ranges immediately preceding and succeeding it.
881
882
Splitting, merging, rebalancing and recovering all follow the same basic
883
algorithm for moving data between roach nodes. New target replicas are
884
created and added to the replica set of source range. Then each new
885
replica is brought up to date by either replaying the log in full or
886
copying a snapshot of the source replica data and then replaying the log
887
from the timestamp of the snapshot to catch up fully. Once the new
888
replicas are fully up to date, the range metadata is updated and old,
889
source replica(s) deleted if applicable.
890
891
**Coordinator** (leader replica)
892
896
only after being completed locally, are moved to new target replicas.
897
else if merging
898
Choose new replicas on same servers as target range replicas;
899
add to replica set.
900
else if rebalancing || recovering
901
Choose new replica(s) on least loaded servers; add to replica set.
902
```
903
904
**New Replica**
905
906
*Bring replica up to date:*
907
908
```
909
if all info can be read from replicated log
910
copy replicated log
911
else
912
snapshot source replica
913
send successive ReadRange requests to source replica
914
referencing snapshot
915
916
if merging
917
combine ranges on all replicas
918
else if rebalancing || recovering
919
remove old range replica(s)
920
```
921
922
RoachNodes split ranges when the total data in a range exceeds a
923
configurable maximum threshold. Similarly, ranges are merged when the
924
total data falls below a configurable minimum threshold.
925
926
**TBD: flesh this out**.
927
928
Ranges are rebalanced if a node determines its load or capacity is one
929
of the worst in the cluster based on gossipped load stats. A node with
930
spare capacity is chosen in the same datacenter and a special-case split
931
is done which simply duplicates the data 1:1 and resets the range
932
configuration metadata.
933
934
# Message Queues
935
936
Each range maintains an array of incoming message queues, referred to
937
here as **inboxes**. Additionally, each range maintains and *processes*
938
an array of outgoing message queues, referred to here as **outboxes**.
939
Both inboxes and outboxes are assigned to keys; messages can be sent or
940
received on behalf of any key. Inboxes and outboxes can contain any
941
number of pending messages.
942
943
Messages can be *deliverable*, or *executable.*
944
945
Deliverable messages are defined by Value objects - simple byte arrays -
946
that are delivered to a key’s inbox, awaiting collection by a client
947
invoking the ReapQueue operation. These are typically used by client
948
applications wishing to be notified of changes to an entry for further
949
processing, such as expensive offline operations like sending emails,
950
SMSs, etc.
951
952
Executable messages are *outgoing-only*, and are instances of
953
PutRequest,IncrementRequest, DeleteRequest, DeleteRangeRequest
955
executed when encountered. These are primarily useful when updates that
956
are nominally part of a transaction can tolerate asynchronous execution
957
(e.g. eventual consistency), and are otherwise too busy or numerous to
958
make including them in the original [distributed] transaction efficient.
959
Examples may include updates to the accounting for successive key
960
prefixes (potentially busy) or updates to a full-text index (potentially
961
numerous).
962
963
These two types of messages are enqueued in different outboxes too - see
964
key formats below.
965
966
At commit time, the range processing the transaction places messages
967
into a shared outbox located at the start of the range metadata. This is
968
effectively free as it’s part of the same consensus write for the range
969
as the COMMIT record. Outgoing messages are processed asynchronously by
970
the range. To make processing easy, all outboxes are co-located at the
971
start of the range. To make lookup easy, all inboxes are located
972
immediately after the recipient key. The leader replica of a range is
973
responsible for processing message queues.
974
975
A dispatcher polls a given range’s *deliverable message outbox*
976
periodically (configurable), and delivers those messages to the target
977
key’s inbox. The dispatcher is also woken up whenever a new message is
978
added to the outbox. A separate executor also polls the range’s
979
*executable message outbox* periodically as well (again, configurable),
981
new message is added to the outbox.
982
983
Formats follow in the table below. Notice that inbox messages for a
984
given key sort by the `<outbox-timestamp>`. This doesn’t provide a
985
precise ordering, but it does allow clients to scan messages in an
986
approximate ordering of when they were originally lodged with senders.
987
NTP offers walltime deltas to within 100s of milliseconds. The
988
`<sender-range-key>` suffix provides uniqueness.
989
990
**Outbox**
991
`<sender-range-key>deliverable-outbox:<recipient-key><outbox-timestamp>`
992
`<sender-range-key>executable-outbox:<recipient-key><outbox-timestamp>`
993
994
**Inbox**
995
`<recipient-key>inbox:<outbox-timestamp><sender-range-key>`
996
997
Messages are processed and then deleted as part of a single distributed
998
transaction. The message will be executed or delivered exactly once,
999
regardless of failures at either sender or receiver.
1000
1001
Delivered messages may be read by clients via the ReapQueue operation.
1002
This operation may only be used as part of a transaction. Clients should
1003
commit only after having processed the message. If the transaction is
1004
committed, scanned messages are automatically deleted. The operation
1005
name was chosen to reflect its mutating side effect. Deletion of read
1006
messages is mandatory because senders deliver messages asynchronously
1007
and a delay could cause insertion of messages at arbitrary points in the
1008
inbox queue. If clients require persistence, they should re-save read
1009
messages manually; the ReapQueue operation can be incorporated into
1010
normal transactional updates.
1011
1012
# Node Allocation (via Gossip)
1013
1014
New nodes must be allocated when a range is split. Instead of requiring
1015
every RoachNode to know about the status of all or even a large number
1016
of peer nodes --or-- alternatively requiring a specialized curator or
1017
master with sufficiently global knowledge, we use a gossip protocol to
1018
efficiently communicate only interesting information between all of the
1019
nodes in the cluster. What’s interesting information? One example would
1020
be whether a particular node has a lot of spare capacity. Each node,
1021
when gossiping, compares each topic of gossip to its own state. If its
1022
own state is somehow “more interesting” than the least interesting item
1023
in the topic it’s seen recently, it includes its own state as part of
1024
the next gossip session with a peer node. In this way, a node with
1025
capacity sufficiently in excess of the mean quickly becomes discovered
1026
by the entire cluster. To avoid piling onto outliers, nodes from the
1027
high capacity set are selected at random for allocation.
1028
1029
The gossip protocol itself contains two primary components:
1030
1031
- **Peer Selection**: each node maintains up to N peers with which it
1032
regularly communicates. It selects peers with an eye towards
1033
maximizing fanout. A peer node which itself communicates with an
1034
array of otherwise unknown nodes will be selected over one which
1035
communicates with a set containing significant overlap. Each time
1036
gossip is initiated, each nodes’ set of peers is exchanged. Each
1037
node is then free to incorporate the other’s peers as it sees fit.
1038
To avoid any node suffering from excess incoming requests, a node
1039
may refuse to answer a gossip exchange. Each node is biased
1040
towards answering requests from nodes without significant overlap
1041
and refusing requests otherwise.
1042
1043
Peers are efficiently selected using a heuristic as described in
1044
[Agarwal & Trachtenberg (2006)](https://drive.google.com/file/d/0B9GCVTp_FHJISmFRTThkOEZSM1U/edit?usp=sharing).
1045
1046
**TBD**: how to avoid partitions? Need to work out a simulation of
1047
the protocol to tune the behavior and see empirically how well it
1048
works.
1049
1050
- **Gossip Selection**: what to communicate. Gossip is divided into
1051
topics. Load characteristics (capacity per disk, cpu load, and
1052
state [e.g. draining, ok, failure]) are used to drive node
1053
allocation. Range statistics (range read/write load, missing
1054
replicas, unavailable ranges) and network topology (inter-rack
1055
bandwidth/latency, inter-datacenter bandwidth/latency, subnet
1056
outages) are used for determining when to split ranges, when to
1057
recover replicas vs. wait for network connectivity, and for
1058
debugging / sysops. In all cases, a set of minimums and a set of
1059
maximums is propagated; each node applies its own view of the
1060
world to augment the values. Each minimum and maximum value is
1061
tagged with the reporting node and other accompanying contextual
1062
information. Each topic of gossip has its own protobuf to hold the
1063
structured data. The number of items of gossip in each topic is
1064
limited by a configurable bound.
1065
1066
For efficiency, nodes assign each new item of gossip a sequence
1067
number and keep track of the highest sequence number each peer
1068
node has seen. Each round of gossip communicates only the delta
1069
containing new items.
1070
1071
# Node Accounting
1072
1073
The gossip protocol discussed in the previous section is useful to
1074
quickly communicate fragments of important information in a
1075
decentralized manner. However, complete accounting for each node is also
1076
stored to a central location, available to any dashboard process. This
1077
is done using the map itself. Each node periodically writes its state to
1078
the map with keys prefixed by `\0node`, similar to the first level of
1079
range metadata, but with an ‘`node`’ suffix. Each value is a protobuf
1080
containing the full complement of node statistics--everything
1081
communicated normally via the gossip protocol plus other useful, but
1082
non-critical data.
1083
1084
The range containing the first key in the node accounting table is
1085
responsible for gossiping the total count of nodes. This total count is
1086
used by the gossip network to most efficiently organize itself. In
1087
particular, the maximum number of hops for gossipped information to take
1088
before reaching a node is given by `ceil(log(node count) / log(max
1089
fanout)) + 1`.
1090
1091
# Key-prefix Accounting, Zones & Permissions
1092
1093
Arbitrarily fine-grained accounting and permissions are specified via
1094
key prefixes. Key prefixes can overlap, as is necessary for capturing
1095
hierarchical relationships. For illustrative purposes, let’s say keys
1096
specifying rows in a set of databases have the following format:
1097
1098
`<db>:<table>:<primary-key>[:<secondary-key>]`
1099
1100
In this case, we might collect accounting or specify permissions with
1101
key prefixes:
1102
1103
`db1`, `db1:user`, `db1:order`,
1104
1105
Accounting is kept for the entire map by default.
1106
1107
## Accounting
1108
to keep accounting for a range defined by a key prefix, an entry is created in
1109
the accounting system table. The format of accounting table keys is:
1110
1111
`\0acct<key-prefix>`
1112
1113
In practice, we assume each RoachNode capable of caching the
1114
entire accounting table as it is likely to be relatively small.
1115
1116
Accounting is kept for key prefix ranges with eventual consistency
1117
for efficiency. Updates to accounting values propagate through the
1118
system using the message queue facility if the accounting keys do
1119
not reside on the same range as ongoing activity (true for all but
1120
the smallest systems). There are two types of values which
1121
comprise accounting: counts and occurrences, for lack of better
1122
terms. Counts describe system state, such as the total number of
1123
bytes, rows, etc. Occurrences include transient performance and
1124
load metrics. Both types of accounting are captured as time series
1125
with minute granularity. The length of time accounting metrics are
1126
kept is configurable. Below are examples of each type of
1127
accounting value.
1128
1129
**System State Counters/Performance**
1130
1131
- Count of items (e.g. rows)
1132
- Total bytes
1133
- Total key bytes
1134
- Total value length
1135
- Queued message count
1136
- Queued message total bytes
1137
- Count of values \< 16B
1138
- Count of values \< 64B
1139
- Count of values \< 256B
1140
- Count of values \< 1K
1141
- Count of values \< 4K
1142
- Count of values \< 16K
1143
- Count of values \< 64K
1144
- Count of values \< 256K
1145
- Count of values \< 1M
1146
- Count of values \> 1M
1147
- Total bytes of accounting
1148
1149
1150
**Load Occurences**
1151
1152
Get op count
1153
Get total MB
1154
Put op count
1155
Put total MB
1156
Delete op count
1157
Delete total MB
1158
Delete range op count
1159
Delete range total MB
1160
Scan op count
1161
Scan op MB
1162
Split count
1163
Merge count
1164
1165
Because accounting information is kept as time series and over many
1166
possible metrics of interest, the data can become numerous. Accounting
1167
data are stored in the map near the key prefix described, in order to
1168
distribute load (for both aggregation and storage).
1169
1170
Accounting keys for system state have the form:
1171
`<key-prefix>|acctd<metric-name>*`. Notice the leading ‘pipe’
1172
character. It’s meant to sort the root level account AFTER any other
1173
system tables. They must increment the same underlying values as they
1174
are permanent counts, and not transient activity. Logic at the
1175
RoachNode takes care of snapshotting the value into an appropriately
1176
suffixed (e.g. with timestamp hour) multi-value time series entry.
1177
1178
Keys for perf/load metrics:
1179
`<key-prefix>acctd<metric-name><hourly-timestamp>`.
1180
1181
`<hourly-timestamp>`-suffixed accounting entries are multi-valued,
1182
containing a varint64 entry for each minute with activity during the
1183
specified hour.
1184
1185
To efficiently keep accounting over large key ranges, the task of
1186
aggregation must be distributed. If activity occurs within the same
1187
range as the key prefix for accounting, the updates are made as part
1188
of the consensus write. If the ranges differ, then a message is sent
1189
to the parent range to increment the accounting. If upon receiving the
1190
message, the parent range also does not include the key prefix, it in
1191
turn forwards it to its parent or left child in the balanced binary
1192
tree which is maintained to describe the range hierarchy. This limits
1193
the number of messages before an update is visible at the root to `2*log N`,
1194
where `N` is the number of ranges in the key prefix.
1195
1196
## Zones
1197
zones are stored in the map with keys prefixed by
1198
`\0zone` followed by the key prefix to which the zone
1199
configuration applies. Zone values specify a protobuf containing
1200
the datacenters from which replicas for ranges which fall under
1201
the zone must be chosen.
1202
1203
Please see [proto/config.proto](https://github.com/cockroachdb/cockroach/blob/master/proto/config.proto) for up-to-date data structures used, the best entry point being `message ZoneConfig`.
1204
1205
If zones are modified in situ, each RoachNode verifies the
1206
existing zones for its ranges against the zone configuration. If
1207
it discovers differences, it reconfigures ranges in the same way
1208
that it rebalances away from busy nodes, via special-case 1:1
1209
split to a duplicate range comprising the new configuration.
1210
1211
### Permissions
1212
permissions are stored in the map with keys prefixed by *\0perm* followed by
1213
the key prefix and user to which the specified permissions apply. The format of
1214
permissions keys is:
1215
1216
`\0perm<key-prefix><user>`
1217
1218
Permission values are a protobuf containing the permission details;
1219
please see [proto/config.proto](https://github.com/cockroachdb/cockroach/blob/master/proto/config.proto) for up-to-date data structures used, the best entry point being `message PermConfig`.
1220
1221
A default system root permission is assumed for the entire map
1222
with an empty key prefix and read/write as true.
1223
1224
When determining whether or not to allow a read or a write a key
1225
value (e.g. `db1:user:1` for user `spencer`), a RoachNode would
1226
read the following permissions values:
1227
1228
```
1229
\0perm<db1:user:1>spencer
1230
\0perm<db1:user>spencer
1231
\0perm<db1>spencer
1232
\0perm<>spencer
1233
```
1234
1235
If any prefix in the hierarchy provides the required permission,
1236
the request is satisfied; otherwise, the request returns an
1237
error.
1238
1239
The priority for a user permission is used to order requests at
1240
Raft consensus ranges and for choosing an initial priority for
1241
distributed transactions. When scheduling operations at the Raft
1242
consensus range, all outstanding requests are ordered by key
1243
prefix and each assigned priorities according to key, user and
1244
arrival time. The next request is chosen probabilistically using
1245
priorities to weight the choice. Each key can have multiple
1246
priorities as they’re hierarchical (e.g. for /user/key, one
1247
priority for root ‘/’, and one for ‘/user/key’). The most general
1248
priority is used first. If two keys share the most general, then
1249
they’re compared with the next most general if applicable, and so on.
1250
1251
# Key-Value API
1252
1253
see the protobufs in [proto/](https://github.com/cockroachdb/cockroach/blob/master/proto),
1254
in particular [proto/api.proto](https://github.com/cockroachdb/cockroach/blob/master/proto/api.proto) and the comments within.
1255
1256
# Structured Data API
1257
1258
A preliminary design can be found in the [Go source documentation](http://godoc.org/github.com/cockroachdb/cockroach/structured).