[jira] [Created] (CASSANDRA-16663) Request-Based Native Transport Rate-Limiting

["Caleb Rackliffe (Jira)" <ji...@apache.org>]

Caleb Rackliffe created CASSANDRA-16663:
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             Summary: Request-Based Native Transport Rate-Limiting
                 Key: CASSANDRA-16663
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-16663
             Project: Cassandra
          Issue Type: Improvement
          Components: Messaging/Client
            Reporter: Caleb Rackliffe
            Assignee: Caleb Rackliffe


Together, CASSANDRA-14855, CASSANDRA-15013, and CASSANDRA-15519 added support for a runtime-configurable, per-coordinator limit on the number of bytes allocated for concurrent requests over the native protocol. It supports channel back-pressure by default, and optionally supports throwing OverloadedException if that is requested in the relevant connection’s STARTUP message.

This can be an effective tool to prevent the coordinator from running out of memory, but it may not correspond to how expensive a query is or provide a direct conceptual mapping to how users think about request capacity. I propose adding the option of request-based (or perhaps more correctly message-based) back-pressure, coexisting with (and reusing the logic that supports) the current bytes-based back-pressure.

We can roll this forward in phases, where the server’s cost accounting becomes more accurate, and the client/driver reacts more intelligently to (especially non-back-pressure) overload, but something minimally viable could look like this:

1.) Reuse most of the existing logic in Limits, et al. to support a simple per-coordinator limit only on native transport requests in flight (i.e. where requests have a fixed cost of 1). Under this limit will be CQL reads and writes, but also auth requests, prepare requests, and batches.
 * If the client specifies THROW_ON_OVERLOAD in its STARTUP message at connection time, a breach of the per-node limit will result in an OverloadedException being propagated to the client, and the server will discard the request.
 * If THROW_ON_OVERLOAD is not specified, the server will stop consuming messages from the channel/socket, which should back-pressure the client, while the message continues to be processed.

2.) This limit is infinite by default, and can be enabled via the YAML config or JMX at runtime. (The literal default of -1 corresponding to “no limit” can be used here.)

3.) The current value of the limit is available via JMX, and metrics around coordinator operations/second are already available to compare against it.

4.) Any interaction with existing byte-based limits will intersect. (i.e. A breach of any limit, bytes or request-based, will actuate back-pressure or OverloadedExceptions.)

In this first pass, explicitly out of scope would be any work on the client/driver side.

In terms of validation/testing, our biggest concern with anything that adds overhead on a very hot path is performance. In particular, we want to fully understand how the client and server perform along two axes constituting 4 scenarios. Those are a.) whether or not we are breaching the request limit and b.) whether the server is throwing on overload at the behest of the client. Having said that, query execution should dwarf the cost of limit accounting.



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