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`serialized_action::join()` is used as a shutdown barrier. After it returns, callers commonly destroy the owning object, and action lambdas often capture that owner by `this`. The previous implementation waited for the internal semaphore once. This handles actions that are already running or triggers already queued before `join()`, because Seastar semaphores serve waiters FIFO. The problematic case is a late `trigger()` after `join()` has started while an older action is still running. Such a trigger can queue behind `join()`, allowing `join()` to return before that late trigger runs. Review also found a separate semaphore bookkeeping bug in `trigger()`. The code manually waited on the semaphore and later signaled it through the caller-visible pending future. If the wait itself completed exceptionally, the signal path could still run and give back a semaphore unit that had never been acquired. Make `join()` a terminal operation for `serialized_action`. Once `join()` starts, new `trigger()` calls fail with `broken_semaphore`. `join()` still waits for work that was accepted before it started, and only then breaks the semaphore so later waiters are rejected. I audited the existing `serialized_action` users. Some callers explicitly remove trigger sources before `join()`, such as audit and topology_coordinator. Others rely on observer destruction or broader shutdown ordering, such as database, compaction_manager, io_throughput_updater, and schema_push. The least locally fenced case is `migration_manager::_group0_barrier`, which is reachable through several external paths, including task status lookup and other services. That makes this better enforced in `serialized_action` itself rather than relying on each caller to prove all trigger entrances are closed. This is generic hardening of the shutdown contract, not a fix for a confirmed topology_coordinator-specific reproducer. Also restore acquire/release ownership in `trigger()` by using `with_semaphore()`. This keeps semaphore release tied to successful acquisition while preserving the existing behavior where action completion and action errors are reported through the shared pending future. Refs SCYLLADB-1904 No backport: this is generic shutdown hardening without a confirmed user-visible reproducer. The semaphore bookkeeping fix closes a latent exceptional wait path noticed during review, not a known production failure. Closes scylladb/scylladb#29991 * github.com:scylladb/scylladb: utils/serialized_action: pair semaphore release with acquisition utils/serialized_action: harden join() against late triggers
Scylla
What is Scylla?
Scylla is the real-time big data database that is API-compatible with Apache Cassandra and Amazon DynamoDB. Scylla embraces a shared-nothing approach that increases throughput and storage capacity to realize order-of-magnitude performance improvements and reduce hardware costs.
For more information, please see the ScyllaDB web site.
Build Prerequisites
Scylla is fairly fussy about its build environment, requiring very recent versions of the C++23 compiler and of many libraries to build. The document HACKING.md includes detailed information on building and developing Scylla, but to get Scylla building quickly on (almost) any build machine, Scylla offers a frozen toolchain. This is a pre-configured Docker image which includes recent versions of all the required compilers, libraries and build tools. Using the frozen toolchain allows you to avoid changing anything in your build machine to meet Scylla's requirements - you just need to meet the frozen toolchain's prerequisites (mostly, Docker or Podman being available).
Building Scylla
Building Scylla with the frozen toolchain dbuild is as easy as:
$ git submodule update --init --force --recursive
$ ./tools/toolchain/dbuild ./configure.py
$ ./tools/toolchain/dbuild ninja build/release/scylla
For further information, please see:
- Developer documentation for more information on building Scylla.
- Build documentation on how to build Scylla binaries, tests, and packages.
- Docker image build documentation for information on how to build Docker images.
Running Scylla
To start Scylla server, run:
$ ./tools/toolchain/dbuild ./build/release/scylla --workdir tmp --smp 1 --developer-mode 1
This will start a Scylla node with one CPU core allocated to it and data files stored in the tmp directory.
The --developer-mode is needed to disable the various checks Scylla performs at startup to ensure the machine is configured for maximum performance (not relevant on development workstations).
Please note that you need to run Scylla with dbuild if you built it with the frozen toolchain.
For more run options, run:
$ ./tools/toolchain/dbuild ./build/release/scylla --help
Testing
See test.py manual.
Scylla APIs and compatibility
By default, Scylla is compatible with Apache Cassandra and its API - CQL. There is also support for the API of Amazon DynamoDB™, which needs to be enabled and configured in order to be used. For more information on how to enable the DynamoDB™ API in Scylla, and the current compatibility of this feature as well as Scylla-specific extensions, see Alternator and Getting started with Alternator.
Documentation
Documentation can be found here. Seastar documentation can be found here. User documentation can be found here.
Training
Training material and online courses can be found at Scylla University. The courses are free, self-paced and include hands-on examples. They cover a variety of topics including Scylla data modeling, administration, architecture, basic NoSQL concepts, using drivers for application development, Scylla setup, failover, compactions, multi-datacenters and how Scylla integrates with third-party applications.
Contributing to Scylla
If you want to report a bug or submit a pull request or a patch, please read the contribution guidelines.
If you are a developer working on Scylla, please read the developer guidelines.
Contact
- The community forum and Slack channel are for users to discuss configuration, management, and operations of ScyllaDB.
- The developers mailing list is for developers and people interested in following the development of ScyllaDB to discuss technical topics.