40b60e8f09
In commitc82250e0cf(gossip: Allow deferring advertise of local node to be up), the replacing node is changed to postpone the responding of gossip echo message to avoid other nodes sending read requests to the replacing node. It works as following: 1) replacing node does not respond echo message to avoid other nodes to mark replacing node as alive 2) replacing node advertises hibernate state so other nodes knows replacing node is replacing 3) replacing node responds echo message so other nodes can mark replacing node as alive This is problematic because after step 2, the existing nodes in the cluster will start to send writes to the replacing node, but at this time it is possible that existing nodes haven't marked the replacing node as alive, thus failing the write request unnecessarily. For instance, we saw the following errors in issue #8013 (Cassandra stress fails to achieve consistency when only one of the nodes is down) ``` scylla: [shard 1] consistency - Live nodes 2 do not satisfy ConsistencyLevel (2 required, 1 pending, live_endpoints={127.0.0.2, 127.0.0.1}, pending_endpoints={127.0.0.3}) [shard 0] gossip - Fail to send EchoMessage to 127.0.0.3: std::runtime_error (Not ready to respond gossip echo message) c-s: java.io.IOException: Operation x10 on key(s) [4c4f4d37324c35304c30]: Error executing: (UnavailableException): Not enough replicas available for query at consistency QUORUM (2 required but only 1 alive ``` To solve this problem, we can do the replacing operation in multiple stages. One solution is to introduce a new gossip status state as proposed here: gossip: Introduce STATUS_PREPARE_REPLACE #7416 1) replacing node does not respond echo message 2) replacing node advertises prepare_replace state (Remove replacing node from natural endpoint, but do not put in pending list yet) 3) replacing node responds echo message 4) replacing node advertises hibernate state (Put replacing node in pending list) Since we now have the node ops verb introduced in829b4c1438(repair: Make removenode safe by default), we can do the multiple stage without introducing a new gossip status state. This patch uses the NODE_OPS_CMD infrastructure to implement replace operation. Improvements: 1) It solves the race between marking replacing node alive and sending writes to replacing node 2) The cluster reverts to a state before the replace operation automatically in case of error. As a result, it solves when the replacing node fails in the middle of the operation, the repacing node will be in HIBERNATE status forever issue. 3) The gossip status of the node to be replaced is not changed until the replace operation is successful. HIBERNATE gossip status is not used anymore. 4) Users can now pass a list of dead nodes to ignore explicitly. Fixes #8013 Closes #8330 * github.com:scylladb/scylla: repair: Switch to use NODE_OPS_CMD for replace operation gossip: Add advertise_to_nodes gossip: Add helper to wait for a node to be up gossip: Add is_normal_ring_member helper
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.