94650ff482
Consider the following scenario:
1. Let nodes A,B,C form a cluster with RF=3
2. Write query with CL=QUORUM is submitted and is acknowledged by
nodes B,C
3. Follow-up read query with CL=QUORUM is sent to verify the write
from the previous step
4. Coordinator sends data/digest requests to the nodes A,B. Since the
node A is missing data, digest mismatches and data reconciliation
is triggered
5. The node A or B fails, becomes unavailable, etc
6. During reconciliation, data requests are sent to node A,B and fail
failing the entire read query
When the above scenario happens, the tests using `start_writes()` fail
with the following stacktrace:
```
...
> await finish_writes()
test/cluster/test_tablets_migration.py:259:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
test/pylib/util.py:241: in finish
await asyncio.gather(*tasks)
test/pylib/util.py:227: in do_writes
raise e
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
worker_id = 1
...
> rows = await cql.run_async(rd_stmt, [pk])
E cassandra.ReadFailure: Error from server: code=1300 [Replica(s) failed to execute read] message="Operation failed for test_1767777001181_bmsvk.test - received 1 responses and 1 failures from 2 CL=QUORUM." info={'consistency': 'QUORUM', 'required_responses': 2, 'received_responses': 1, 'failures': 1}
```
Note that when a node failure happens before/during a read query,
there is no test failure as the speculative retries are enabled
by default. Hence an additional data/digest read is sent to the third
remaining node.
However, the same speculative read is cancelled the moment, the read
query reaches CL which may trigger a read-repair.
This change:
- Retries the verification read in start_writes() on failure to mitigate
races between reads and node failures
- Adds additional logging to correlate Python exceptions with Scylla logs
Fixes https://github.com/scylladb/scylladb/issues/27478
Fixes https://github.com/scylladb/scylladb/issues/27974
Fixes https://github.com/scylladb/scylladb/issues/27494
Fixes https://github.com/scylladb/scylladb/issues/23529
Note that this change test flakiness observed during tablet transitions.
However, it serves as a workaround for a higher-level issue
https://github.com/scylladb/scylladb/issues/28125
Closes scylladb/scylladb#28140
(cherry picked from commit e07fe2536e)
Closes scylladb/scylladb#28827
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 the ScyllaDB open source.
- The developers mailing list is for developers and people interested in following the development of ScyllaDB to discuss technical topics.