d8cc029a5e
The initial problem:
Some of the tests in test_protocol_exceptions.py started failing. The failure is
on the condition that no more than `cpp_exception_threshold` happened.
Test logic:
These tests assert that specific code paths do not throw an exception anymore.
Initial implementation ran a code path once, and asserted there were 0 exceptions.
Sometimes an exception or several can occur, not directly related to the code paths
the tests check, but those would fail the tests.
The solution was to run the tests multiple times. If there is a regression, there
would be at least as many exceptions thrown as there are test runs. If there is no
regression, a few exceptions might happen, up to 10 per 100 test runs.
I have arbitrarily chosen `run_count = 100` and `cpp_exception_threshold = 10` values.
Note that the exceptions are counted per shard, not per code path.
The new problem:
The occassional exceptions thrown by some parts of the server now throw a bit more
than before. Based on the logs linked on the issues, it is usually 12.
There are possibly multiple ways to resolve the issue. I have considered logging
exceptions and parsing them. I would have to filter exception logs only for wanted
exceptions. However, if a new, different exception is introduced, it might not be
counted.
Another approach is to just increase the threshold a bit. The issue of throwing
more exceptions than before in some other server modules should be addressed by
a set of tests for that module, just like these tests check protocol exceptions,
not caring who used protocol check code paths.
For those reasons, the solution implemented here is to increase `cpp_exception_threshold`
to `20`. It will not make the tests unreliable, because, as mentioned, if there is a
regression, there would be at least `run_count` exceptions per `run_count` test runs
(1 exception per single test run).
Still, to make "background exceptions" occurence a bit more normalized, `run_count` too
is doubled, from `100` to `200`. At the first glance this looks like nothing is changed,
but actually doubling both run count and exception threshold here implies that the
exception burst does not scale as much as run count, it is just that the "jitter" is
bigger than the old threshold.
Fixes #27247
Fixes #27325
(cherry picked from commit 807fc68dc5)
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.