7d551f99be
The memtable wants to listen for changes in its `total_memory` in order to decrease its `_flushed_memory` in case some of the freed memory has already been accounted as flushed. (This can happen because the flush reader sees and accounts even outdated MVCC versions, which can be deleted and freed during the flush). Today, the memtable doesn't listen to those changes directly. Instead, some calls which can affect `total_memory` (in particular, the mutation cleaner) manually check the value of `total_memory` before and after they run, and they pass the difference to the memtable. But that's not good enough, because `total_memory` can also change outside of those manually-checked calls -- for example, during LSA compaction, which can occur anytime. This makes memtable's accounting inaccurate and can lead to unexpected states. But we already have an interface for listening to `total_memory` changes actively, and `dirty_memory_manager`, which also needs to know it, does just that. So what happens e.g. when `mutation_cleaner` runs is that `mutation_cleaner` checks the value of `total_memory` before it runs, then it runs, causing several changes to `total_memory` which are picked up by `dirty_memory_manager`, then `mutation_cleaner` checks the end value of `total_memory` and passes the difference to `memtable`, which corrects whatever was observed by `dirty_memory_manager`. To allow memtable to modify its `_flushed_memory` correctly, we need to make `memtable` itself a `region_listener`. Also, instead of the situation where `dirty_memory_manager` receives `total_memory` change notifications from `logalloc` directly, and `memtable` fixes the manager's state later, we want to only the memtable listen for the notifications, and pass them already modified accordingl to the manager, so there is no intermediate wrong states. This patch moves the `region_listener` callbacks from the `dirty_memory_manager` to the `memtable`. It's not intended to be a functional change, just a source code refactoring. The next patch will be a functional change enabled by this.
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.