5e8bdf6877
This patch fixes a bug noted in issue #7218 - where PutItem operations sometimes lose part of the item's data - some attributes were lost, and the name of other attributes replaced by empty strings. The problem happened when the write-isolation policy was LWT and there was contention of writes to the same partition (not necessarily the same item). To use CAS (a.k.a. LWT), Alternator builds an alternator::rmw_operation object with an apply() function which takes the old contents of the item (if needed) and a timestamp, and builds a mutation that the CAS should apply. In the case of the PutItem operation, we wrongly assumed that apply() will be called only once - so as an optimization the strings saved in the put_item_operation were moved into the returned mutation. But this optimization is wrong - when there is contention, apply() may be called again when the changed proposed by the previous one was not accepted by the Paxos protocol. The fix is to change the one place where put_item_operation *moved* strings out of the saved operations into the mutations, to be a copy. But to prevent this sort of bug from reoccuring in future code, this patch enlists the compiler to help us verify that it can't happen: The apply() function is marked "const" - it can use the information in the operation to build the mutation, but it can never modify this information or move things out of it, so it will be fine to call this function twice. The single output field that apply() does write (_return_attributes) is marked "mutable" to allow the const apply() to write to it anyway. Because apply() might be called twice, it is important that if some apply() implementation sometimes sets _return_attributes, then it must always set it (even if to the default, empty, value) on every call to apply(). The const apply() means that the compiler verfies for us that I didn't forget to fix additional wrong std::move()s. Additionally, a test I wrote to easily reproduce issue #7218 (which I will submit as a dtest later) passes after this fix. Fixes #7218. Signed-off-by: Nadav Har'El <nyh@scylladb.com> Message-Id: <20200916064906.333420-1-nyh@scylladb.com>
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 in ./docs and on the wiki. There is currently no clear definition of what goes where, so when looking for something be sure to check both. 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.