writing correct lock-free and distributed stateful systems in Rust, assisted by TLA+
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tla+rust →

Stable stateful systems through modeling, linear types and simulation.

I like to use things that wake me up at 4am as rarely as possible. Unfortunately, infrastructure vendors don't focus on reliability. Even if a company gives reliability lip service, it's unlikely that they use techniques like modeling or simulation to create a rock-solid core. Let's just build an open-source distributed store that takes correctness seriously at the local storage, sharding, and distributed transactional layers.

My goal: verify core lock-free and distributed algorithms in use with rsdb and rasputin with TLA+. Write an implementation in Rust. Use quickcheck and abstracted RPC/clocks to simulate partitions and test correctness under failure conditions.

table of contents
  1. motivations for doing this at all
  1. introductions to TLA+, PlusCal, quickcheck
  1. lock-free algorithms for efficient local storage
  1. consensus within a shard
  1. sharding operations
  1. distributed transactions

motivations

terminology

Simulation, in this context, refers to writing tests that exercise RPC-related code by simulating a buggy network over time, partitions and all. Many more failures may be tested per unit of compute time using simulation compared to black-box fault injection with something like Namazu, Jepsen, or Blockade.

Modeling, in this context, refers to the use of the TLA+ model checker to ensure the correctness of our lock-free and distributed algorithms.

why rust?

Rust is a new systems programming language that emphasizes memory safety. It is notable for its compiler, which is able to make several types of common memory corruption bugs (and attack vectors for exploits) impossible to create by default, without relying on GC. It is a Mozilla project, and as of this writing, it is starting to be included in their Firefox web browser.

It uses an "ownership" system that ensures an object's destructor will run exactly once, preventing double-frees, dangling pointers, various null pointer related bugs, etc... When an object is created inside a function's scope, it exists as the property of that scope. The object's lifetime is the same as the lifetime of the scope that created it.

When the lifetime of an object is over, the object's destructor is run. When you pass an object to a function as an argument, that object becomes the property of the called function, and when the called function returns, the objects in its posession will be destroyed unless the function is returning them. Objects returned from a function become the property of the calling scope.

In order to pass an object to several functions, you may instead pass a reference. By passing a reference, the object remains the property of the current scope. It is possible to create references that imply sole ownership, called mutable references, which may be used to, you guessed it, mutate the object being referred to. This is useful for using an object with a function that will mutate it, without the object becoming the property of that function, and allowing the object to outlive the mutating function. While only a single mutable reference may be created, infinite immutable references may be created, so long as they do not outlive the object that the reference points to.

Rust does not use GC by default. However, it does have several container types that rely on reference counting for preventing an object's destructor from being called multiple times. These are useful for sharing things with multiple scopes and multiple threads. These objects are generally rare compared to the total number of objects created in a typical Rust program. The lack of GC for every object may be a compelling feature for those creating high-performance systems. Many such systems are currently written in C and C++, which have a long track record of buggy and insecure code, even when written by security-conscious life-long practitioners.

Rust has the potential to make high-performance, widely-deployed systems much more secure and crash less frequently. This means web browsers, SSL libraries, operating systems, networking stacks, toasters and many vital systems that are much harder to hack and more robust against common bugs.

For databases, the memory safety benefits are wonderful, and I'm betting on being able to achieve faster long-term iteration by not spending so much time chasing down memory-related bugs. However, it needs to be noted that when creating lock-free high-performance algorithms, we are going to need to sidestep the safety guarantees of the compiler. Our goal is to create data structures that are mutated using atomic compare-and-swap (CAS) operations by multiple threads simultaneously, and also supporting reads at the same time. We choose not to sacrifice performance by using Mutexes. This means using Rust's Box::into_raw/from_raw, AtomicPtr, unsafe pointers and mem::forget. We are giving up a significant benefit of Rust for certain very high-performance chunks of this system. In place of Rust's compiler, we use the TLA+ model checker to gain confidence in the correctness of our system!

why model?

TLA+ allows us to specify and verify algorithms in very few lines, compared to the programming language that we will use to implement and test it. It is a tool that is frequently mentioned by engineers of stateful distributed systems, but it has been used by relatively few, and has a reputation for being overkill. I believe that this reputation is unfounded for this type of work.

Many systems are not well understood by their creators at the start of the project, which leads to architectural strain as assumptions are invalidated and the project continues to grow over time. Small projects are often cheaper to complete using this approach, as an incorrect initial assumption may have a lower long-term impact. Stateful distributed systems tend to have significant costs associated with unanticipated changes in architecture: reliability, iteration time, and performance can be expected to take hits. For our system, we will specify the core algorithms before implementing them, which will allow us to catch mistakes before they result in bugs or outages.

why simulate?

We want to make sure that our implementation is robust against network partitions, disk failures, NTP issues, etc... So, why not run Namazu, Jepsen, or Blockade? They have great success with finding bugs in databases! However, it is far slower to perform black-box fault injection than simulation. A simulator can artificially advance the clocks of a system to induce a leader election, while a "real" cluster has to wait real time to trigger certain logic. It also takes a lot of time to deploy new code to a "real" cluster, and it is cumbersome to introspect.

Simulation is not a replacement for black-box testing. Simulation will be biased, and it's up to the implementor of the simulator to ensure that all sources of time, IPC, and other interaction are sufficiently encapsulated by the artificial time and interaction logic.

Simulation can allow a contributor working on a more resource-constrained system to test locally, running through thousands or millions of failure situations in the time that it takes to create the RPM/container that is then fed to a black-box fault injection system. A CI/CD pipeline can get far more test coverage per unit of compute time using simulation than with black-box fault injection.

Both simulation and black-box fault injection can be constrained to complete in a certain amount of time, but simulation will likely find a lot more bugs per unit of compute time. Simulation tests may be a reasonable thing to expect to pass for most pull requests, since they can achieve a high bug:compute time ratio. However, black box fault injection is still important, and will probably catch bugs arising from the bias of the simulation authors.

We will also use black-box testing, but we will spend less time talking about it due to its decent existing coverage.

introductions

We want to use TLA+ to model and find bugs in things like:

  • CAS operations on lists, ring buffers, and radix trees for lock-free local systems
  • paxos-like consensus for leadership, replication and shard management systems
  • lock-free distributed transactions

Distributed and concurrent algorithms have many similarities, but there are some key differences in the primitives that we build on in our work. Concurrent algorithms can rely on atomic CAS primitives, as achieving sequentially consistent access semantics is fairly well understood and implemented at this point. The distributed systems world has many databases that provide strong ordering semantics, but it doesn't have such a reliable, standard primitive as CAS that we can simply assume to be present. So we need to initially work in terms of the "asynchronous communication model" in which messages between any two processes can be reordered and arbitrarily delayed, or dropped altogether. After we have proved our own model for achieving consistency, we will build on it in later higher-level models that describe particularly interesting functionality such as lock-free distributed transactions.

In our TLA+ models, we can simply use a fairly short labeled block that performs the duties of compare and swap (or another atomic operation) on shared state when describing a concurrent algorithm, but we will need to build a complete replicated log primitive before we can work at a similar level of abstraction in our models of distributed algorithms.

So, let's learn how to describe some of our primitives and invariants!

here we go... jumping into pluscal

This is a summary of an example from a wonderful primer on TLA+...

The first thing to know is that there are two languages in play: pluscal and TLA. We test models using tlc, which understands most of TLA (not infinite sets, maybe other stuff). TLA started as a specification language, tlc came along later to actually test it, and pluscal is a simpler language that can be transpiled into TLA. Pluscal has two forms, c and p. They are functionally identical, but c form uses braces and p form uses begin and end statements that can be a little easier to spot errors with, in my opinion.

We're writing Pluscal in a TLA comment (block comments are written with (* <comment text> *)), and when we run a translator like pcal2tla it will insert TLA after the comment, in the same file.

------------------------------- MODULE pcal_intro -------------------------------
EXTENDS Naturals, TLC

(* --algorithm transfer
variables alice_account = 10, bob_account = 10,
          account_total = alice_account + bob_account

process TransProc \in 1..2
  variables money \in 1..20;
begin
  Transfer:
    if alice_account >= money then
      A: alice_account := alice_account - money;
      B: bob_account := bob_account + money;
    end if;
C: assert alice_account >= 0;
end process

end algorithm *)

\* this is a TLA comment. pcal2tla will insert the transpiled TLA here

MoneyInvariant == alice_account + bob_account = account_total

=============================================================================

This code specifies 3 global variables, alice_account, bob_account, account_total. It specifies, using process <name> \in 1..2 that it will run in two concurrent processes. Each concurrent process has local state, money, which may take any initial value from 1 to 20, inclusive. It defines steps Transfer, A, B and C which are evaluated as atomic units, although they will be tested against all possible interleavings of execution. All possible values will be tested.

Let's save the above example as pcal_intro.tla, transpile the pluscal comment to TLA, then run it with tlc! (if you want to name it something else, update the MODULE specification at the top)

pcal2tla pcal_intro.tla
tlc pcal_intro.tla

BOOM! This blows up because our transaction code sucks, big time:

The first argument of Assert evaluated to FALSE; the second argument was:
"Failure of assertion at line 16, column 4."
Error: The behavior up to this point is:
State 1: <Initial predicate>
/\ bob_account = 10
/\ money = <<1, 10>>
/\ alice_account = 10
/\ pc = <<"Transfer", "Transfer">>
/\ account_total = 20

State 2: <Action line 35, col 19 to line 40, col 42 of module pcal_intro>
/\ bob_account = 10
/\ money = <<1, 10>>
/\ alice_account = 10
/\ pc = <<"A", "Transfer">>
/\ account_total = 20

State 3: <Action line 35, col 19 to line 40, col 42 of module pcal_intro>
/\ bob_account = 10
/\ money = <<1, 10>>
/\ alice_account = 10
/\ pc = <<"A", "A">>
/\ account_total = 20

State 4: <Action line 42, col 12 to line 45, col 63 of module pcal_intro>
/\ bob_account = 10
/\ money = <<1, 10>>
/\ alice_account = 9
/\ pc = <<"B", "A">>
/\ account_total = 20

State 5: <Action line 47, col 12 to line 50, col 65 of module pcal_intro>
/\ bob_account = 11
/\ money = <<1, 10>>
/\ alice_account = 9
/\ pc = <<"C", "A">>
/\ account_total = 20

State 6: <Action line 42, col 12 to line 45, col 63 of module pcal_intro>
/\ bob_account = 11
/\ money = <<1, 10>>
/\ alice_account = -1
/\ pc = <<"C", "B">>
/\ account_total = 20

Error: The error occurred when TLC was evaluating the nested
expressions at the following positions:
0. Line 52, column 15 to line 52, column 28 in pcal_intro
1. Line 53, column 15 to line 54, column 66 in pcal_intro


9097 states generated, 6164 distinct states found, 999 states left on queue.
The depth of the complete state graph search is 7.

Looking at the trace that tlc outputs, it shows us how alice's account may become negative. Because processes 1 and 2 execute the steps sequentially but with different interleavings, the algorithm will check alice_account >= money before trying to transfer it to bob. By the time one process subtracts the money from alice, however, the other process may have already done so. We can specify that these steps and checks happen atomically by changing:

  Transfer:
    if alice_account >= money then
      A: alice_account := alice_account - money;
      B: bob_account := bob_account + money;
    end if;

to

  Transfer:
    if alice_account >= money then
      \* remove the labels A: and B:
      alice_account := alice_account - money;
      bob_account := bob_account + money;
    end if;

which means that the entire Transfer step is atomic. In reality, maybe this is done by punting this atomicity requirement to a database transaction. Re-running tlc should produce no errors now, because both processes atomically check + deduct + add balances to the bank accounts without violating the assertion.

The invariant, MoneyInvariant, at the bottom is not actually being checked yet. Invariants are specified in TLA, not in the pluscal comment. They can be checked by creating a pcal_intro.cfg file (or replace the one auto-generated by pcal2tla) with the following content:

SPECIFICATION Spec
INVARIANT MoneyInvariant

useful primitives

So, we've seen how to create labels, processes, and invariants. Here are some other useful primitives:

await bags EXTENDS Naturals, FiniteSets, Sequences, Integers, TLC

For a more in-depth TLA+ introduction, refer to the tutorial that this was summarized from and the manual.

lock-free algorithms for efficient local storage

In the interests of achieving a price-performance that is compelling, we need to make this thing sympathetic to modern hardware. Check out Dmitry's wonderful blog for a fast overview of the important ideas in writing scalable code.

lock-free ring buffer

The ring buffer is at the heart of several systems in our local storage system. It serves as the core of our concurrent persistent log IO buffer and the epoch-based garbage collector for our logical page ID allocator.

lock-free list

The list allows us to CAS a partial update to a page into a chain, avoiding the work of rewriting the entire page. To read a page, we traverse its list until we learn about what we sought. Eventually, we need to compact the list of partial updates to improve locality, probably around 4-8.

lock-free stack

The stack allows us to maintain a free list of page identifiers. Our radix tree needs to be very densely populated to achieve a favorable data to pointer ratio, and by reusing page identifiers after they are freed, we are able to keep it dense. Hence this stack. When we free a page, we push its identifier into this stack for reuse.

lock-free radix tree

We use a radix tree for maintaining our in-memory mapping from logical page ID to its list of partial updates. A well-built radix tree can achieve a .92 total size:data ratio when densely populated and using a contiguous key range. This is way better than what we get with B+ trees, which max out between .5-.6. The downside is that with low-density we get extremely poor data:pointer ratios with a radix tree.

lock-free IO buffer

We use a ring buffer to hold buffers for writing data onto the disk, along with associated metadata about where on disk the buffer will end up. This is fraught with peril. We need to avoid ABA problems in the CAS that claims a particular buffer, and later relies on a particular log offset. We also need to avoid creating a stall when all available buffers are claimed, and a write depends on flushing the end of the buffer before the beginning is free. Possible ways of avoiding: fail reservation attempts when the buffer is full of claims, support growing the buffer when necessary. Bandaid: don't seal entire buffer during commit of reservation.

lock-free epoch-based GC

The basic idea for epoch-based GC is that in our lock-free structures, we may end up making certain data inaccessible via a CAS on a node somewhere, but that doesn't mean that there isn't already some thread that is operating on it. We use epochs to track when a structure is marked inaccessible, as well as when threads begin and end operating on shared state. Before reading or mutating the shared state, a thread "enrolls" in an epoch. If the thread makes some state inaccessible, it adds it to the current epoch's free list. The current epoch may be later than the epoch that the thread initially enrolled in. The state is not dropped until there are no threads in epochs before or at the epoch where the state was marked free. When a thread stops reading or mutating the shared state, it leaves the epoch that it enrolled in.

lock-free pagecache

Maintains a radix tree mapping from logical page ID to a list of page updates, terminated by a base page. Uses the epoch-based GC for safely making logical ID's available in a stack. Facilitates atomic splits and merges of pages.

lock-free tree

Uses the pagecache to store B+ tree pages.

consensus within a shard

We use a consensus protocol as the basis of our replication across a shard. Consensus notes:

  1. support OLTP with small replication batch size
  2. support batch loading and analytical jobs with large replication batch size
  3. for max throughput with a single shard, send disparate 1/N of the batch to each other node, and then have them all forward their chunk to everybody else
  4. but this adds complexity, and if each node has several shards, we are already spreading the IO around, so we can just pick the latency-minimizing simple broadcast where the leader sends full batches to all followers.
  5. TCP is already a replicated log, hint hint
  6. UDP may be nice for receiving acks, but it doesn't work in a surprising number of DCs

harpoon consensus

Similar to raft, but uses leader leases instead of a paxos register. The paxos register preemptable election of raft is vulnerable to livelock in the not-unusual case of a network partition between a leader and another node, which triggers a dueling candidate situation. Using leases allows us to make progress as long as a node has connectivity with a majority of its quorum, regardless of interfering nodes. In addition, a node that cannot reach a leader may subscribe to the replication log of any other node which has seen more successful log entries.

sharding operations

Sharding has these ideals:

  1. avoid unnecessary data movement (wait some time before replacing a failed node)
  2. if multiple nodes fail simultaneously, minimize chances of dataloss (chainsets)
  3. minimize MTTR when a node fails (lots of shards per machine, reduce membership overlap)

ideals 2 and 3 are someone at tension, but there is a goldilocks zone.

Sharding introduces the question of "who manages the mapping?" This is sometimes punted to an external consensus-backed system. We will initially create this by punting the metadata problem to such a system. Eventually, we will go single-binary with something like the following:

If we treat shard metadata as just another range, how do we prevent split brain?

General initialization and key metadata:

  1. nodes are configured with a set of "seed nodes" to initially connect to
  2. cluster is initialized when some node is explicitly given permission to do so, either via argv, env var, conf file or admin REST api request
  3. the designated node creates an initial range in an underreplicated state
  4. the metadata range contains a mapping from range to current assigned members
  5. as this node learns of others via the seeds, it assigns peers to the initial range
  6. if the metadata range (or any other range) loses quorum, a particular minority survivor can be manually chosen as a seed for fresh replication. the admin api can also trigger backup dumps for a range, and restoration of a range from a backup file.
  7. nodes each maintain their own monotonic counters, and publish a few basic stats about their ranges and utilization using a shared ORSWOT

shard splitting

Split algorithm:

  1. as operations happen in a range, we keep track of the max and min keys, and keep a running average for the position between max and min of inserts. We then choose a split point around there. If keys are always added to one end, the split should occur at the end.
  2. record split intent in watched meta range at the desired point
  3. record the split intent in the replicated log for the range
  4. all members of the replica set split their metadata when they see the split intent in their replicated log
  5. The half of the split point that contains less density is migrated by changing consensus participants one node at a time.
  6. once the two halves have a balanced placement, the split intent is removed

shard merging

Merge algorithm:

  1. merge intent written to metadata range
  2. the smaller half is to move to the larger's servers
  3. this direction is marked at the time of intent, to prevent flapping
  4. once the ranges are colocated, in the larger range's replicated log, write a merge intent, which causes it to accept keys in the new range
  5. write a merge intent into the less frequently accessed range's replicated log that causes it to redirect reads and writes to the larger range.
  6. update the metadata range to reflect the merge
  7. remove handlers and metadata for old range

distributed transactions

cross-shard lock-free transactions

Relatively simple lock-free distributed transactions:

  1. read all involved data
  2. create txn object somewhere
  3. CAS all involved data to refer to the txn object and the conditionally mutated state
  4. CAS the txn object to successful
  5. (can crash here and the txn is still valid)
  6. CAS all affected data to replace the value, and remove the txn reference

readers at any point will CAS a txn object to aborted if they encounter an in-progress txn on something they are reading. if the txn object is successful, the reader needs to CAS the object's conditionally mutated state to be the present state, and nuke the txn reference, before continuing.

This can be relaxed to just intended writers, but then our isolation level goes from SSI to SI and we are vulnerable to write skew.

Invariants:

  1. must never see any intermediate states, a transaction must be entirely committed or entirely invisible.