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The C++ bindings provided by wasmtime are lacking a crucial capability: asynchronous execution of the wasm functions. This forces us to stop the execution of the function after a short time to prevent increasing the latency. Fortunately, this feature is implemented in the native language of Wasmtime - Rust. Support for Rust was recently added to scylla, so we can implement the async bindings ourselves, which is done in this patch. The bindings expose all the objects necessary for creating and calling wasm functions. The majority of code implemented in Rust is a translation of code that was previously present in C++. Types exported from Rust are currently required to be defined by the same crate that contains the bridge using them, so wasmtime types can't be exported directly. Instead, for each class that was supposed to be exported, a wrapper type is created, where its first member is the wasmtime class. Note that the members are not visible from C++ anyway, the difference only applies to Rust code. Aside from wasmtime types and methods, two additional types are exported with some associated methods. - The first one is ValVec, which is a wrapper for a rust Vec of wasmtime Vals. The underlying vector is required by wasmtime methods for calling wasm functions. By having it exported we avoid multiple conversions from a Val wrapper to a wasmtime Val, as would be required if we exported a rust Vec of Val wrappers (the rust Vec itself does not require wrappers if the type it contains is already wrapped) - The second one is Fut. This class represents an computation tha may or may not be ready. We're currently using it to control the execution of wasm functions from C++. This class exposes one method: resume(), which returns a bool that signals whether the computation is finished or not. Signed-off-by: Wojciech Mitros <wojciech.mitros@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 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.