3155cde9c8
Currently when a node wants to create and broadcast a new CDC generation it performs the following steps: 1. choose the generation's stream IDs and mapping (how this is done is irrelevant for the current discussion) 2. choose the generation's timestamp by taking the current time (according to its local clock) and adding 2 * ring_delay 3. insert the generation's data (mapping and stream IDs) into system_distributed.cdc_generation_descriptions, using the generation's timestamp as the partition key (we call this table the "old internal table" below) 4. insert the generation's timestamp into the "CDC_STREAMS_TIMESTAMP" application state. The timestamp spreads epidemically through the gossip protocol. When nodes see the timestamp, they retrieve the generation data from the old internal table. Unfortunately, due to the schema of the old internal table, where the entire generation data is stored in a single cell, step 3 may fail for sufficiently large generations (there is a size threshold for which step 3 will always fail - retrying the operation won't help). Also the old internal table lies in the system_distributed keyspace that uses SimpleStrategy with replication factor 3, which is also problematic; for example, when nodes restart, they must reach at least 2 out of these 3 specific replicas in order to retrieve the current generation (we write and read the generation data with QUORUM, unless we're a single-node cluster, where we use ONE). Until this happens, a restarting node can't coordinate writes to CDC-enabled tables. It would be better if the node could access the last known generation locally. The commit introduces a new table for broadcasting generation data with the following properties: - it uses a better schema that stores the data in multiple rows, each of manageable size - it resides in the `system_distributed_everywhere` keyspace so the data will be written to every node in the cluster that has a token in the token ring - the data will be written using CL=ALL and read using CL=ONE; thanks to this, restarting node won't have to communicate with other nodes to retrieve the data of the last known generation. Note that writing with CL=ALL does not reduce availability: creating a new generation *requires* all nodes to be available anyway, because they must learn about the generation before their clocks go past the generation's timestamp; if they don't, partitions won't be mapped to stream IDs consistently across the cluster - the partition key is no longer the generation's timestamp. Because it was that way in the old internal table, it forced the algorithm to choose the timestamp *before* the generation data was inserted into the table. What if the inserting took a long time? It increased the chance that nodes would learn about the generation too late (after their clocks moved past its timestamp). With the new schema we will first insert the generation data using a randomly generated UUID as the partition key, *then* choose the timestamp, then gossip both the timestamp and the UUID. The timestamp and the UUID form the "generation identifier" of this new generation; this should explain why we introduced the generation_id_v2 type in previous commits. Observe that after a node learns about a generation broadcasted using this new method through gossip it will retrieve its data very quickly since it's one of the replicas and it can use CL=ONE as it was written using CL=ALL. Note that the node is still using the old method - the actual switch will be done in a later commit.
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.