892f22f49c
ScyllaDB has special counter columns for which atomic add/subtract operations like `SET a = a + 1` are allowed. Such operations have not been allowed on ordinary non-counter columns, as they would not be properly atomic - the read an the write are separate, and concurrent operations can have incorrect results. This patch makes it allowed to use such atomic add/subtract operations in **LWT** statements. For example UPDATE ... SET a = a - 7 IF a > 0 or UPDATE ... SET a = a + 1 IF a != NULL The row updated in the operation, and the updated column (`a`) should be initialized before the update. The example `SET a = a + 1 IF a != NULL` will fail the condition if `a` is not set. A different request `SET a = a + 1 IF EXISTS` will just leave `a` unset if it's unset (NULL + 1 is NULL, this is SQL's null propagation rules). This add/subtract operations is allowed on any numeric (integer or floating point) column. The ability of LWT to fetch the old values of a column and use it to calculate the new value has long been available in our internal CAS implementation - and has been in use for years in Alternator - but until this patch it was not exposed in CQL's LWT. This series does not add new syntax to CQL - the "SET a = a + b" and "SET a = a - b" syntax already existed for counters, and we just allow the same syntax for non- counters. However, the series does add a bit of machinery that will allow us to easily support more general expressions in the future. In particular, this series implements the addition, subtraction, and unary-minus operators for expressions, and adds the machinery needed to run **any** expression in "SET a = expr()", using existing row values fetched by LWT. This is a new Scylla-only feature that does not exist in Cassandra. Fixes #10568 Refs #22918 ("Support arithmetic operators"), SCYLLADB-1576 ("Decimal arithmetic operations OOM") This is is a new feature, so normally would not be backported. Closes scylladb/scylladb#29939 * github.com:scylladb/scylladb: cql: atomic add/subtract operations with LWT cql3: let constants::setter evaluate expressions using prefetched row data cql3/expr: add NEG unary operator for numeric negation cql3/expr: add SUB binary operator for numeric subtraction cql3/expr: add ADD binary operator for numeric addition types: add is_arithmetic() method for types
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.