7d84007fd5
Replicating `raft_address_map` entries is needed for the following use cases: - the direct failure detector - currently it assumes a static mapping of `raft::server_id`s to `gms::inet_address`es, which is obtained on Raft group 0 configuration changes. To handle dynamic mappings we need to modify the failure detector so it pings `raft::server_id`s and obtains the `gms::inet_address` before sending the message from `raft_address_map`. The failure detector is sharded, so we need the mappings to be available on all shards. - in the future we'll have multiple Raft groups running on different shards. To send messages they'll need `raft_address_map`. Initially I tried to replicate all entries - expiring and non-expiring. The implementation turned out to be very complex - we need to handle dropping expired entries and refreshing expiring entries' timestamps across shards, and doing this correctly while accounting for possible races is quite problematic. Eventually I arrived at the conclusion that replicating only non-expiring entries, and furthermore allowing non-expiring entries to be added only on shard 0, is good enough for our use cases: - The direct failure detector is pinging group 0 members only; group 0 members correspond exactly to the non-expiring entries. - Group 0 configuration changes are handled on shard 0, so non-expiring entries are added/removed on shard 0. - When we have multiple Raft groups, we can reuse a single Raft server ID for all Raft servers running on a single node belonging to different groups; they are 'namespaced' by the group IDs. Furthermore, every node has a server that belongs to group 0. Thus for every Raft server in every group, it has a corresponding server in group 0 with the same ID, which has a non-expiring entry in `raft_address_map`, which is replicated to all shards; so every group will be able to deliver its messages. With these assumptions the implementation is short and simple. We can always complicate it in the future if we find that the assumptions are too strong.
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++20 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 APIs - CQL and Thrift. 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 users mailing list and Slack channel are for users to discuss configuration, management, and operations of the ScyllaDB open source.
- The developers mailing list is for developers and people interested in following the development of ScyllaDB to discuss technical topics.