1d06ea3a5a
Currently if raft is enabled all nodes are voters in group0. However it is not necessary to have all nodes to be voters - it only slows down the raft group operation (since the quorum is large) and makes deployments with asymmetrical DCs problematic (2 DCs with 5 nodes along 1 DC with 10 nodes will lose the majority if large DC is isolated). The topology coordinator will now maintain a state where there are only limited number of voters, evenly distributed across the DCs and racks. After each node addition or removal the voters are recalculated and rebalanced if necessary. That means: * When a new node is added, it might become a voter depending on the current distribution of voters - either if there are still some voter "slots" available, or if the new node is a better candidate than some existing voter (in which case the existing node voter status might be revoked). * When a voter node is removed or stopped (shut down), its voter status is revoked and another node might become a voter instead (this can also depend on other circumstances, like e.g. changing the number of DCs). * If a node addition or removal causes a change in number of datacenters (DCs) or racks, the rebalance action might become wider (as there are some special rules applying to 1 vs 2 vs more DCs, also changing the number of racks might cause similar effects in the voters distribution) Special conditions for various number of DCs: * 1 DC: Can have up to the maximum allowed number of voters (5 - see below) * 2 DCs: The distribution of the voters will be asymmetric (if possible), meaning that we can tolerate a loss of the DC with the smaller number of voters (if both would have the same number of voters we'd lose the majority if any of the DCs is lost). For example, if we have 2 DCs with 2 nodes each, one of them will only have 1 voter (despite the limit of 5). Also, if one of the 2 DCs has more racks than the other and the node count allows it, the DC with the more racks will have more voters. * 3 and more DCs: The distribution of the voters will be so that every DC has strictly less than half of the total voters (so a loss of any of the DCs cannot lead to the majority loss). Again, DCs with more racks are being preferred in the voter distribution. At the moment we will be handling the zero-token nodes in the same way as the regular nodes (i.e. the zero-token nodes will not take any priority in the voter distribution). Technically it doesn't make much sense to have a zero-token node that is not a voter (when there are regular nodes in the same DC being voters), but currently the intended purpose of zero-token nodes is to form an "arbiter DC" (in case of 2 DCs, creating a third DC with zero-token nodes only), so for that intended purpose no special handling is needed and will work out of the box. If a preference of zero token nodes will eventually be needed/requested, it will be added separately from this PR. Currently the voter limits will not be configurable (we might introduce configurable limits later if that would be needed/requested). The feature is enabled by the `group0_limited_voters` feature flag to avoid issues with cluster upgrade (the feature will be only enabled once all nodes in the cluster are upgraded to the version supporting the feature). Fixes: scylladb/scylladb#18793
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.