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Strongly consistent reads currently call read_barrier() on whichever replica happens to process the request. When a follower runs read_barrier(), it sends an RPC to the leader to get the current read index, then waits for its local apply index to catch up. If the follower is behind, this wait can be significant. By forwarding linearizable reads to the leader, we don't need an RPC from replica to leader to get the index to wait for apply -- it's available locally. Note that read_barrier() is still required on the leader to confirm it is still the leader and guarantee linearizability. A future optimization would be to implement leases in the raft library, which could eliminate read_barrier() on the leader entirely. The CL-to-behavior mapping is isolated in a single parse_consistency_level() function: - CL=(LOCAL_)QUORUM -> linearizable: forwarded to the raft leader - CL=(LOCAL_)ONE -> non-linearizable: existing behavior (no read_barrier()/forwarding, may return stale results) - All other CLs -> invalid request Read forwarding reuses the same CQL-layer bounce_to_node() mechanism that write forwarding already uses. The transport layer's existing requests_forwarded_* metrics automatically count forwarded reads. Coordinator-level metrics (linearizable_reads, non_linearizable_reads, writes) are added for visibility into the strong consistency workload. Fixes: SCYLLADB-1157 Closes scylladb/scylladb#29575 * github.com:scylladb/scylladb: strong_consistency: test read forwarding to leader strong_consistency: skip read_barrier() for non-linearizable reads strong_consistency: split coordinator-level read latency metrics strong_consistency: forward linearizable reads to raft leader strong_consistency: classify reads by consistency level strong_consistency: add begin_read() to raft_server
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.