2b8ce83eea
Scylla list cells are represented internally as a map of timeuuid => value. To append a new value to a list the coordinator generates a timeuuid reflecting the current time as key and adds a value to the map using this key. Before this patch, Scylla always generated a timeuuid for a new value, even if the query had a user supplied or LWT timestamp. This could break LWT linearizability. User supplied timestamps were ignored. This is reported as https://github.com/scylladb/scylla/issues/7611 A statement which appended multiple values to a list or a BATCH generated an own microsecond-resolution timeuuid for each value: BEGIN BATCH UPDATE ... SET a = a + [3] UPDATE ... SET a = a + [4] APPLY BATCH UPDATE ... SET a = a + [3, 4] To fix the bug, it's necessary to preserve monotonicity of timeuuids within a batch or multi-value append, but make sure they all use the microsecond time, as is set by LWT or user. To explain the fix, it's first necessary to recall the structure of time-based UUIDs: 60 bits: time since start of GMT epoch, year 1582, represented in 100-nanosecond units 4 bits: version 14 bits: clock sequence, a random number to avoid duplicates in case system clock is adjusted 2 bits: type 48 bits: MAC address (or other hardware address) The purpose of clockseq bits is as defined in https://tools.ietf.org/html/rfc4122#section-4.1.5 is to reduce the probability of UUID collision in case clock goes back in time or node id changes. The implementation should reset it whenever one of these events may occur. Since LWT microsecond time is guaranteed to be unique by Paxos, the RFC provisioning for clockseq and MAC slots becomes excessive. The fix thus changes timeuuid slot content in the following way: - time component now contains the same microsecond time for all values of a statement or a batch. The time is unique and monotonic in case of LWT. Otherwise it's most always monotonic, but may not be unique if two timestamps are created on different coordinators. - clockseq component is used to store a sequence number which is unique and monotonic for all values within the statement/batch. - to protect against time back-adjustments and duplicates if time is auto-generated, MAC component contains a random (spoof) MAC address, re-created on each restart. The address is different at each shard. The change is made for all sources of time: user, generated, LWT. Conditioning the list key generation algorithm on the source of time would unnecessarily complicate the code while not increase quality (uniqueness) of created list keys. Since 14 bits of clockseq provide us with only 16383 distinct slots per statement or batch, 3 extra bits in nanosecond part of the time are used to extend the range to 131071 values per statement/batch. If the rang is exceeded beyond the limit, an exception is produced. A twist on the use of clockseq to extend timeuuid uniqueness is that Scylla, like Cassandra, uses int8 compare to compare lower bits of timeuuid for ordering. The patch takes this into account and sign-complements the clockseq value to make it monotonic according to the legacy compare function. Fixes #7611 test: unit (dev)
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.