protocol-extensions.md

Protocol extensions to the Cassandra Native Protocol

This document specifies extensions to the protocol defined by Cassandra's native_protocol_v4.spec and native_protocol_v5.spec. The extensions are designed so that a driver supporting them can continue to interoperate with Cassandra and other compatible servers with no configuration needed; the driver can discover the extensions and enable them conditionally.

An extension can be discovered by the client driver by using the OPTIONS request; the returned SUPPORTED response will have zero or more options beginning with SCYLLA indicating extensions defined in this document, in addition to options documented by Cassandra. How to use the extension is further explained in this document.

Extending protocol extensions support

As mentioned above, in order to use a protocol extension feature by both server and client, they need to negotiate the used feature set when establishing a connection.

The negotiation procedure has the following steps:

• Client sends the OPTIONS request to the Scylla instance to get a list of protocol extensions that the server understands.
• Server sends the SUPPORTED message in reply to the OPTIONS request. The message body is a string multimap, in which keys describe different extensions and possibly one or more additional values specific to a particular extension (specified as distinct values under a feature key in the following form: ARG_NAME=VALUE).
• The client determines the set of compatible extensions which it is going to use in the current connection by intersecting known capabilities list with what it has received in SUPPORTED response.
• Client driver sends the STARTUP request with additional payload consisting of key-value pairs, each describing a negotiated extension.
• Server determines the set of compatible extensions by intersecting known list of protocol extensions with what it has received in STARTUP request.

Both client and server use the same string identifiers for the keys to determine negotiated extension set, judging by the presence of a particular key in the SUPPORTED/STARTUP messages.

Intranode sharding

This extension allows the driver to discover how Scylla internally partitions data among logical cores. It can then create at least one connection per logical core, and send queries directly to the logical core that will serve them, greatly improving load balancing and efficiency.

To use the extension, send the OPTIONS message. The data is returned in the SUPPORTED message, as a set of key/value options. Numeric values are returned as their base-10 ASCII representation.

The keys and values are:

• SCYLLA_SHARD is an integer, the zero-based shard number this connection is connected to (for example, 3).
• SCYLLA_NR_SHARDS is an integer containing the number of shards on this node (for example, 12). All shard numbers are smaller than this number.
• SCYLLA_PARTITIONER is a the fully-qualified name of the partitioner in use (i.e. org.apache.cassandra.partitioners.Murmur3Partitioner).
• SCYLLA_SHARDING_ALGORITHM is the name of an algorithm used to select how partitions are mapped into shards (described below)
• SCYLLA_SHARDING_IGNORE_MSB is an integer parameter to the algorithm (also described below)
• SCYLLA_SHARD_AWARE_PORT is an additional port number where Scylla is listening for CQL connections. If present, it works almost the same way as port 9042 typically does; the difference is that client-side port number is used as an indicator to which shard client wants to connect. The desired shard number is calculated as: desired_shard_no = client_port % SCYLLA_NR_SHARDS. Its value is a decimal representation of type uint16_t, by default 19042.
• SCYLLA_SHARD_AWARE_PORT_SSL is an additional port number where Scylla is listening for encrypted CQL connections. If present, it works almost the same way as port 9142 typically does; the difference is that client-side port number is used as an indicator to which shard client wants to connect. The desired shard number is calculated as: desired_shard_no = client_port % SCYLLA_NR_SHARDS. Its value is a decimal representation of type uint16_t, by default 19142.

Currently, one SCYLLA_SHARDING_ALGORITHM is defined, biased-token-round-robin. To apply the algorithm, perform the following steps (assuming infinite-precision arithmetic):

• subtract the minimum token value from the partition's token in order to bias it: biased_token = token - (-2**63)
• shift biased_token left by ignore_msb bits, discarding any bits beyond the 63rd: biased_token = (biased_token << SCYLLA_SHARDING_IGNORE_MSB) % (2**64)
• multiply by SCYLLA_NR_SHARDS and perform a truncating division by 2**64: shard = (biased_token * SCYLLA_NR_SHARDS) / 2**64

(this apparently convoluted algorithm replaces a slow division instruction with a fast multiply instruction).

in C with 128-bit arithmetic support, these operations can be efficiently performed in three steps:

    uint64_t biased_token = token + ((uint64_t)1 << 63);
    biased_token <<= ignore_msb;
    int shard = ((unsigned __int128)biased_token * nr_shards) >> 64;

In languages without 128-bit arithmetic support, use the following (this example is for Java):

    private int scyllaShardOf(long token) {
        token += Long.MIN_VALUE;
        token <<= ignoreMsb;
        long tokLo = token & 0xffffffffL;
        long tokHi = (token >>> 32) & 0xffffffffL;
        long mul1 = tokLo * nrShards;
        long mul2 = tokHi * nrShards;
        long sum = (mul1 >>> 32) + mul2;
        return (int)(sum >>> 32);
    }

It is recommended that drivers open connections until they have at least one connection per shard, then close excess connections.

LWT prepared statements metadata mark

This extension allows the driver to discover whether LWT statements have a special bit set in prepared statement metadata flags, which indicates that the driver currently deals with an LWT statement.

Having a designated flag gives the ability to reliably detect LWT statements and remove the need to execute custom parsing logic for each query, which is not only costly but also error-prone (e.g. parsing the prepared query with regular expressions).

The feature is meant to be further utilized by client drivers to use primary replicas consistently when dealing with conditional statements.

Choosing primary replicas in a predefined order ensures that in case of multiple LWT queries that contend on a single key, these queries will queue up at the replica rather than compete: choose the primary replica first, then, if the primary is known to be down, the first secondary, then the second secondary, and so on. This will reduce contention over hot keys and thus increase LWT performance.

The feature is identified by the SCYLLA_LWT_ADD_METADATA_MARK key that is meant to be sent in the SUPPORTED message along with the following additional parameters:

• LWT_OPTIMIZATION_META_BIT_MASK is a 32-bit unsigned integer that represents the bit mask that should be used by the client to test against when checking prepared statement metadata flags to see if the current query is conditional or not.

Rate limit error

This extension allows the driver to send a new type of error in case the operation goes over the allowed per-partition rate limit. This kind of error does not fit other existing error codes well, hence the need for the protocol extension.

On receiving this error, the driver should not retry the request; instead, the error should be propagated to the user so that they can decide what to do with it - sometimes it might make sense to propagate the error, in other cases it might make sense to retry with backoff.

The body of the error consists of the usual error code, error message and then the following fields: <op_type><rejected_by_coordinator>, where:

• op_type is a byte which identifies the operation which is the origin of the rate limit.
  • 0: read
  • 1: write
• rejected_by_coordinator is a byte which is 1 if the operation was rejected on the coordinator and 0 if it was rejected by replicas.

If the driver does not understand this extension and does not enable it, the Config_error will be used instead of the new error code.

In order to be forward compatible with error codes added in the future protocol versions, this extension doesn't reserve a fixed error code - instead, it advertises the integer value used as the error code in the SUPPORTED response.

This extension is identified by the SCYLLA_RATE_LIMIT_ERROR key. The string map in the SUPPORTED response will contain the following parameters:

• ERROR_CODE: a 32-bit signed decimal integer which Scylla will use as the error code for the rate limit exception.

Sending tablet info to the drivers

This extension adds support for sending tablet info to the drivers if the request was routed to the wrong node/shard.

There is a need for sending tablet info to the drivers so they can be tablet aware. For the best performance we want to get this info lazily only when it is needed.

The info is send when driver asks about the information that the specific tablet contains and it is directed to the wrong node/shard so it could use that information for every subsequent query. If we send the query to the wrong node/shard, we want to send the RESULT message with additional information about the tablet in custom_payload:

• tablets-routing-v1 - tablets routing information, which contains info about token range (in format (first_token, last_token]) and tablet replicas, for every replica there is information about the host and shard.

The driver has to be able to receive custom_payload and deserialise its field from bytes to:

• for tablets-routing-v1 - TupleType(LongType, LongType, ListType(TupleType(UUIDType, Int32Type))), two LongType represent first and last token, ListType(TupleType(UUIDType, Int32Type)) contains information about replicas (for every replica there is a tuple with two elements UUIDType and Int32Type representing host and shard ids).

When the driver receives information about the tablet, it has to check if any of the previously received tablets has an overlapping token range. The group of tablets that meets this criterion has to be deleted, and the new tablet should replace them.

Negotiate sending tablets info to the drivers

This extension allows the driver to inform the database that it is aware of tablets and is able to interpret the tablet information sent in custom_payload.

Having a designated flag gives the ability to skip tablet metadata generation (which is quite expensive) if driver is not aware of tablets.

The feature is identified by the TABLETS_ROUTING_V1 key, which is meant to be sent in the SUPPORTED message.