15f35577ed
In do_execute_cql_with_timeout(), when the prepared statement was not found in the cache, we called qp.prepare() and stored the returned result_message::prepared in a local variable scoped to the 'if' block. We then extracted ps_ptr (a checked_weak_ptr to the prepared statement) from the message, let the message go out of scope at the end of the 'if', and used ps_ptr after a co_await on st->execute().
Since 3ac4e258e8 ("transport/messages: hold pinned prepared entry in PREPARE result"), result_message::prepared owns a strong pinned reference to the prepared cache entry. While qp.prepare() runs it also holds its own pin on the entry, so on return the entry has at least the pin owned by the returned message. As long as that message is alive, the cache entry cannot be purged and the weak handle inside ps_ptr remains promotable.
The lifetime gap manifested only in debug builds. qp.prepare() returns a ready future on the cache-miss path, so in release builds the co_await resumes synchronously: control flows from the assignment of ps_ptr straight into st->execute() with no opportunity for any other task (in particular, prepared cache invalidation triggered by a concurrent schema change) to run in between. Debug builds, however, force a reactor preemption point on every co_await even when the awaited future is ready. With prepared_msg already destroyed at the end of the 'if' block, the only remaining handle on the cache entry was the weak ps_ptr, and the preemption gave a concurrent cache purge
- triggered, for example, by Raft schema changes received during a node restart - the chance to drop the entry. The subsequent execute() then failed when promoting the weak pointer with
checked_ptr_is_null_exception.
The exception propagated out of the Paxos prepare path as a generic std::exception with no type information in the log, surfacing on the coordinator as:
WriteFailure: Failed to prepare ballot ... Replica errors:
host_id ... -> seastar::rpc::remote_verb_error (std::exception)
Hoist the result_message::prepared into the outer scope so the pinned cache entry stays alive across co_await st->execute(...), closing the window in which a concurrent cache purge could invalidate the weak handle.
Fixes SCYLLADB-1173
backport: the patch is simple, we can backport it to all versions with "LWT over tablets" feature. Note that the problem is only in test runs in debug configuration, production is not affected.
Closes scylladb/scylladb#29675
* https://github.com/scylladb/scylladb:
table_helper: retry insert prepare on concurrent cache invalidation
paxos_state: keep prepared message alive across statement execution
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.