a2eed4bb45
all_sibling_tablet_replicas_colocated was using committed ti.replicas to decide whether sibling tablets are co-located and merge can be finalized. This caused a false non-co-located window when a co-located pair was moved by the load balancer: as both tablets migrate together, their del_transition commits may land in different Raft rounds. After the first commit, ti.replicas diverge temporarily (one tablet shows the new position, the other the old), causing all_sibling_tablet_replicas_colocated to return false. This clears finalize_resize, allowing the load balancer to start new cascading migrations that delay merge finalization by tens of seconds. Fix this by using the optimistic replica view (trinfo->next when transitioning, ti.replicas otherwise) — the same view the load balancer uses for load accounting — so finalize_resize stays populated throughout an in-flight migration and no spurious cascades are triggered. Steps that lead to the problem: 1. Merge is triggered. The load balancer generates co-location migrations for all sibling pairs that are not yet on the same shard. Some pairs finish co-location before others. 2. Once all pairs are co-located in committed state, all_sibling_tablet_replicas_colocated returns true and finalize_resize is set. Meanwhile the load balancer may have already started a regular LB migration on one co-located pair (both tablets are stable and the load balancer is free to move them). 3. The LB migration moves both tablets together (colocated_tablets). Their two del_transition commits land in separate Raft rounds. After the first commit, ti.replicas[t1] = new position but ti.replicas[t2] = old position. 4. In this window, all_sibling_tablet_replicas_colocated sees the pair as NOT co-located, clears finalize_resize, and the load balancer generates new migrations for other tablets to rebalance the load that the pair move created. 5. Those new migrations can take tens of seconds to stream, keeping the coordinator in handle_tablet_migration mode and preventing maybe_start_tablet_resize_finalization from being called. The merge finalization is delayed until all those cascaded migrations complete. Fixes https://scylladb.atlassian.net/browse/SCYLLADB-821. Fixes https://scylladb.atlassian.net/browse/SCYLLADB-1459. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Closes scylladb/scylladb#29465
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.