fd51d7e448
This patch adds support for recreating group 0 after losing majority. This is the only part of the new Raft-based recovery procedure that touches Scylla core. The following steps are necessary to recreate group 0: 1. Determine the new group 0 members. These are alive nodes that are normal or rebuilding. 2. Choose the recovery leader - the node which will become the new group 0 leader. This must be one of the nodes with the latest persistent group 0 state. 3. Remove `raft_group_id` from `system.scylla_local` and truncate `system.discovery` on each live node. 4. Set the new scylla.yaml parameter - `recovery_leader` - to Host ID of the recovery leader on each live node. 5. Rolling restart all live nodes, but the recovery leader must be restarted first. In the implementation, restarts in step 5 are very similar to normal restarts with the Raft-based topology enabled. The only differences are: 1. Steps 3-4 make the restarting node discover the new group 0 in `join_cluster`. 2. The group 0 server is started in `join_group0`, not `setup_group0_if_exists`. 3. The restarting node joins the new group 0 in `join_topology` using `legacy_handshaker`. There is no reason to contact the topology coordinator since the node has already joined the topology. Unfortunately, this patch creates another execution path for the starting logic. `join_cluster` becomes even messier. However, there is nothing we can do about it. Joining group 0 without joining topology is something completely new. Having a few small changes without touching other execution paths is the best we can do. We will start removing the old stuff soon, after making the Raft-based topology mandatory, and the situation will improve.
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.