ea2bdae45a
The primary goal of this change is to reduce the time during which the
Effective Replication Map (ERM) is retained by the mapreduce service.
This ensures that long aggregate queries do not block topology
operations. As ScyllaDB transitions towards tablets, which simplify
work dispatching, the new algorithm is designed specifically for
tablets.
The algorithm divides work so that each `tablet_replica` (a <host,
shard> pair) processes two tablets at a time. After processing of each
`tablet_replica`, the ERM is released and re-acquired.
The new algorithm can be summarized as follows:
1. Prepare a set of exclusive `partition_ranges`, where each range
represents one tablet. This set is called `ranges_left`, because it
contains ranges that still need processing.
2. Loop until `ranges_left` is empty:
I. Create `tablet_replica` -> `ranges` mapping for the current ERM
and `ranges_left`. Store this mapping and the number
representing current ERM version as `ranges_per_replica`.
II. In parallel, for each tablet_replica, iterate through
ranges_per_tablet_replica. Select independently up to two ranges
that are still existing in ranges_left. Remove each range
selected for processing from ranges_left. Before each iteration,
verify that ERM version has not changed. If it has,
return to Step I.
Steps I and II are exclusive to simplify maintaining `ranges_left` and
`ranges_per_replica`:
- Step I iterates through `ranges_left` and creates
`ranges_per_replica`
- Step II iterates through `ranges_per_replica` and remove processed
ranges from `ranges_left`
To maintain the exclusivity, the algorithm uses `parallel_for_each` in
Step II, requiring all ongoing `tablet_replica` processing to finish
before returning to Step I.
Currently, each node can handle any partition range, even if the
mapreduce supercoordinator does not retain the ERM and the range is
absent locally. This is because `execute_on_this_shard` creates a new
pager to coordinate the partition range read, including obtaining its
own ERM. However, absent ranges are handled by shard 0, so proper
routing is necessary to avoid overloading shard 0. Thus, in Step II,
the ERM is retained during each `tablet_replica` processing.
The tablet split scenario is not well-handled in this implementation.
After a split, the entire pre-split range is sent to a node hosting
the `tablet_replica` containing the range's `end_token`. The node
will typically not have other tablets in the range, and as
aforementioned, absent ranges are handled by shard 0. As a result,
in such scenario, shard 0 handles a significant portion of the range.
This issue is addressed later in this patch series by introducing
`shard_id` in `mapreduce_request`.
Ref. scylladb#21831
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.