3fda13e20e
We have been seeing rare failures of the cql-pytest (translated from
Cassandra's unit tests) for testing TTL in secondary indexes:
cassandra_tests/validation/entities/secondary_index_test.py::testIndexOnRegularColumnInsertExpiringColumn
The problem is that the test writes an item with 1 second TTL, and then
sleeps *exactly* 1.0 seconds, and expects to see the item disappear
by that time. Which doesn't always happen:
The problem with that assumption stems from Scylla's TTL clock ("gc_clock")
being based on Seastar's lowres clock. lowres_clock only has a 10ms
"granularity": The time Scylla sees when deciding whether an item expires
may be up to 10ms in the past - the arbitrary point when the lowres timer
happened to last run. In rare overload cases, the inaccuracy may be even
grater than 10ms (if the timer got delayed by other things running).
So when Scylla is asked to expire an item in 1 second - we cannot be
sure it will be expired in exactly 1 second or less - the expiration
can be also around 10ms later.
So in this patch we change the test to sleep with more than enough
margin - 1.1 seconds (i.e., 100ms more than 1 second). By that time
we're sure the item must have expired.
Before this patch, I saw the test failing once every few hundred runs,
after this patch I ran if 2,000 times without a single failure.
Fixes #9008
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Message-Id: <20210712100655.953969-1-nyh@scylladb.com>
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.