[CASSANDRA-10993] Make read and write requests paths fully non-blocking, eliminate related stages

• Type: Sub-task
• Status: Open
• Priority: Normal
• Resolution: Unresolved
• Fix Version/s: 5.x
• Component/s: Legacy/Coordination, Legacy/Local Write-Read Paths
• Assignee: Tom Hobbs
• Reporter: Aleksey Yeschenko

Authors

Tom Hobbs

• Created: 11/Jan/16 01:31
• Updated: 07/Mar/23 10:54

Description

Building on work done by tjake (CASSANDRA-10528), slebresne (CASSANDRA-5239), and others, convert read and write request paths to be fully non-blocking, to enable the eventual transition from SEDA to TPC (CASSANDRA-10989)

Eliminate MUTATION, COUNTER_MUTATION, VIEW_MUTATION, READ, and READ_REPAIR stages, move read and write execution directly to Netty context.

For lack of decent async I/O options on Linux, we’ll still have to retain an extra thread pool for serving read requests for data not residing in our page cache (CASSANDRA-5863), however.

Implementation-wise, we only have two options available to us: explicit FSMs and chained futures. Fibers would be the third, and easiest option, but aren’t feasible in Java without resorting to direct bytecode manipulation (ourselves or using quasar).

I have seen 4 implementations bases on chained futures/promises now - three in Java and one in C++ - and I’m not convinced that it’s the optimal (or sane) choice for representing our complex logic - think 2i quorum read requests with timeouts at all levels, read repair (blocking and non-blocking), and speculative retries in the mix, SERIAL reads and writes.

I’m currently leaning towards an implementation based on explicit FSMs, and intend to provide a prototype - soonish - for comparison with CompletableFuture-like variants.

Either way the transition is a relatively boring straightforward refactoring.

There are, however, some extension points on both write and read paths that we do not control:

• authorisation implementations will have to be non-blocking. We have control over built-in ones, but for any custom implementation we will have to execute them in a separate thread pool
• 2i hooks on the write path will need to be non-blocking
• any trigger implementations will not be allowed to block
• UDFs and UDAs

We are further limited by API compatibility restrictions in the 3.x line, forbidding us to alter, or add any non-default interface methods to those extension points, so these pose a problem.

Depending on logistics, expecting to get this done in time for 3.4 or 3.6 feature release.

Issue Links

• supercedes: CASSANDRA-5239 Asynchronous (non-blocking) StorageProxy