5007ded2c1
The problematic scenario this patch fixes might happen due to
unfortunate serialization of locks/unlocks between lock_pk and lock_ck,
as follows:
1. lock_pk acquires an exclusive lock on the partition.
2.a lock_ck attempts to acquire shared lock on the partition
and any lock on the row. both cases currently use a fiber
returning a future<rwlock::holder>.
2.b since the partition is locked, the lock_partition times out
returning an exceptional future. lock_row has no such problem
and succeeds, returning a future holding a rwlock::holder,
pointing to the row lock.
3.a the lock_holder previously returned by lock_pk is destroyed,
calling `row_locker::unlock`
3.b row_locker::unlock sees that the partition is not locked
and erases it, including the row locks it contains.
4.a when_all_succeeds continuation in lock_ck runs. Since
the lock_partition future failed, it destroyes both futures.
4.b the lock_row future is destroyed with the rwlock::holder value.
4.c ~holder attempts to return the semaphore units to the row rwlock,
but the latter was already destroyed in 3.b above.
Acquiring the partition lock and row lock in parallel
doesn't help anything, but it complicates error handling
as seen above,
This patch serializes acquiring the row lock in lock_ck
after locking the partition to prevent the above race.
This way, erasing the unlocked partition is never expected
to happen while any of its rows locks is held.
Fixes #12168
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Closes #12208
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.