8ca0f2dd54
Previously, when a snapshot load subsumed a committed entry before apply() was called locally, add_entry would throw commit_status_unknown -- even though the entry was known to be committed and included in the snapshot. This was overly pessimistic. Normal state machine implementations shouldn't care whether an entry was applied via apply() or via a snapshot load. Unnecessary commit_status_unknown caused flakiness of test_frequent_snapshotting and unnecessary retries in group0. Raft groups from strongly consistent tables couldn't hit unnecessary commit_status_unknown's because they use wait_type::committed and `enable_forwarding == false`. Three sites are changed: 1. wait_for_entry (truncation case): the snapshot-term match optimization that proved the entry was committed now applies to both wait_type::committed and wait_type::applied, not just committed. 2. wait_for_entry (snapshot covers entry): instead of throwing commit_status_unknown when the snapshot index >= entry index, return successfully. The entry's effects are included in the state machine's state via the snapshot. 3. drop_waiters: when called from load_snapshot, pass the snapshot term. Waiters whose term matches the snapshot term are resolved successfully (set_value) instead of failing with commit_status_unknown, since the Log Matching Property guarantees they were committed and included. This deflakes test_frequent_snapshotting: the test uses aggressive snapshot settings (snapshot_threshold=1) causing wait_for_entry to occasionally find the snapshot covering its entry. Previously this threw commit_status_unknown, failing the test. With this fix, wait_for_entry returns success. Note that apply() is never actually skipped in this test -- the leader always applies entries locally before taking a snapshot. The nemesis test is updated to handle the new behavior: call() detects when add_entry succeeded but the output channel was not written (apply() skipped locally) and returns apply_skipped instead of hanging. The linearizability checker in basic_generator_test counts skipped applies separately from failures. basic_generator_test exercises this path: skipped_applies > 0 occurs in some runs. Fixes: SCYLLADB-1264 No backport: the changes are quite risky and the test being fixed fails very rarely. Closes scylladb/scylladb#29685 * github.com:scylladb/scylladb: test/raft: fix duplicate check in connected::operator() test/raft: add tests for add_entry snapshot interactions raft: do not throw commit_status_unknown from add_entry when possible raft: change drop_waiters parameter from index to snapshot descriptor raft: server: fix a typo
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.