72bbe75d5b
This PR fixes a problem with replacing a node with tablets when RF=N. Currently, this will fail because tablet replica allocation for rebuild will not be able to find a viable destination, as the replacing node is not considered to be a candidate. It cannot be a candidate because replace rolls back on failure and we cannot roll back after tablets were migrated. The solution taken here is to not drain tablet replicas from replaced node during topology request but leave it to happen later after the replaced node is in left state and replacing node is in normal state. The replacing node waits for this draining to be complete on boot before the node is considered booted. Fixes https://github.com/scylladb/scylladb/issues/17025 Nodes in the left state will be kept in tablet replica sets for a while after node replace is done, until the new replica is rebuilt. So we need to know about those node's location (dc, rack) for two reasons: 1) algorithms which work with replica sets filter nodes based on their location. For example materialized views code which pairs base replicas with view replicas filters by datacenter first. 2) tablet scheduler needs to identify each node's location in order to make decisions about new replica placement. It's ok to not know the IP, and we don't keep it. Those nodes will not be present in the IP-based replica sets, e.g. those returned by get_natural_endpoints(), only in host_id-based replica sets. storage_proxy request coordination is not affected. Nodes in the left state are still not present in token ring, and not considered to be members of the ring (datacanter endpoints excludes them). In the future we could make the change even more transparent by only loading locator::node* for those nodes and keeping node* in tablet replica sets. Currently left nodes are never removed from topology, so will accumulate in memory. We could garbage-collect them from topology coordinator if a left node is absent in any replica set. That means we need a new state - left_for_real. Closes scylladb/scylladb#17388 * github.com:scylladb/scylladb: test: py: Add test for view replica pairing after replace raft, api: Add RESTful API to query current leader of a raft group test: test_tablets_removenode: Verify replacing when there is no spare node doc: topology-on-raft: Document replace behavior with tablets tablets, raft topology: Rebuild tablets after replacing node is normal tablets: load_balancer: Access node attributes via node struct tablets: load_balancer: Extract ensure_node() mv: Switch to using host_id-based replica set effective_replication_map: Introduce host_id-based get_replicas() raft topology: Keep nodes in the left state to topology tablets: Introduce read_required_hosts()
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 community forum 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.