42fd3704e4
This patch fixes a problem which affects decommission and removenode
which may lead to data consistency problems under conditions which
lead one of the nodes to unliaterally decide to abort the node
operation without the coordinator noticing.
If this happens during streaming, the node operation coordinator would
proceed to make a change in the gossiper, and only later dectect that
one of the nodes aborted during sending of decommission_done or
removenode_done command. That's too late, because the operation will
be finalized by all the nodes once gossip propagates.
It's unsafe to finalize the operation while another node aborted. The
other node reverted to the old topolgy, with which they were running
for some time, without considering the pending replica when handling
requests. As a result, we may end up with consistency issues. Writes
made by those coordinators may not be replicated to CL replicas in the
new topology. Streaming may have missed to replicate those writes
depending on timing.
It's possible that some node aborts but streaming succeeds if the
abort is not due to network problems, or if the network problems are
transient and/or localized and affect only heartbeats.
There is no way to revert after we commit the node operation to the
gossiper, so it's ok to close node_ops sessions before making the
change to the gossiper, and thus detect aborts and prevent later aborts
after the change in the gossiper is made. This is already done during
bootstrap (RBNO enabled) and replacenode. This patch canges removenode
to also take this approach by moving sending of remove_done earlier.
We cannot take this approach with decommission easily, because
decommission_done command includes a wait for the node to leave the
ring, which won't happen before the change to the gossiper is
made. Separating this from decommission_done would require protocol
changes. This patch adds a second-best solution, which is to check if
sessions are still there right before making a change to the gossiper,
leaving decommission_done where it was.
The race can still happen, but the time window is now much smaller.
The PR also lays down infrastructure which enables testing the scenarios. It makes node ops
watchdog periods configurable, and adds error injections.
Fixes #12989
Refs #12969
Closes #13028
* github.com:scylladb/scylladb:
storage_service: node ops: Extract node_ops_insert() to reduce code duplication
storage_service: Make node operations safer by detecting asymmetric abort
storage_service: node ops: Add error injections
service: node_ops: Make watchdog and heartbeat intervals configurable
(cherry picked from commit 2b44631ded)
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.