This document is intended to help developers and contributors to Scylla get started. The first part consists of general guidelines that make no assumptions about a development environment or tooling. The second part describes a particular environment and work-flow for exemplary purposes.
This section covers some high-level information about the Scylla source code and work-flow.
Scylla uses Git submodules to manage its dependency on Seastar and other tools. Be sure that all submodules are correctly initialized when cloning the project:
$ git clone https://github.com/scylladb/scylla
$ cd scylla
$ git submodule update --init --recursiveScylla depends on the system package manager for its development dependencies.
Running ./install_dependencies.sh (as root) installs the appropriate packages based on your Linux distribution.
Note: Compiling Scylla requires, conservatively, 2 GB of memory per native thread, and up to 3 GB per native thread while linking.
Scylla is built with Ninja, a low-level rule-based system. A Python script, configure.py, generates a Ninja file (build.ninja) based on configuration options.
To build for the first time:
$ ./configure.py
$ ninja-buildAfterwards, it is sufficient to just execute Ninja.
The full suite of options for project configuration is available via
$ ./configure.py --helpThe most important options are:
-
--mode={release,debug,all}: Debug mode enables AddressSanitizer and allows for debugging with tools like GDB. Debugging builds are generally slower and generate much larger object files than release builds. -
--{enable,disable}-dpdk: DPDK is a set of libraries and drivers for fast packet processing. During development, it's not necessary to enable support even if it is supported by your platform.
Source files and build targets are tracked manually in configure.py, so the script needs to be updated when new files or targets are added or removed.
To save time -- for instance, to avoid compiling all unit tests -- you can also specify specific targets to Ninja. For example,
$ ninja-build build/release/tests/schema_change_testUnit tests live in the /tests directory. Like with application source files, test sources and executables are specified manually in configure.py and need to be updated when changes are made.
A test target can be any executable. A non-zero return code indicates test failure.
Most tests in the Scylla repository are built using the Boost.Test library. Utilities for writing tests with Seastar futures are also included.
Run all tests through the test execution wrapper with
$ ./test.py --mode={debug,release}The --name argument can be specified to run a particular test.
Alternatively, you can execute the test executable directly. For example,
$ build/release/tests/row_cache_test -- -c1 -m1GThe -c1 -m1G arguments limit this Seastar-based test to a single system thread and 1 GB of memory.
All changes to Scylla are submitted as patches to the public mailing list. Once a patch is approved by one of the maintainers of the project, it is committed to the maintainers' copy of the repository at https://github.com/scylladb/scylla.
Detailed instructions for formatting patches for the mailing list and advice on preparing good patches are available at the ScyllaDB website.
Once Scylla has been compiled, executing the (debug or release) target will start a running instance in the foreground:
$ build/release/scyllaThe scylla executable requires a configuration file, scylla.yaml. By default, this is read from $SCYLLA_HOME/conf/scylla.yaml. A good starting point for development is located in the repository at /conf/scylla.yaml.
For development, a directory at $HOME/scylla can be used for all Scylla-related files:
$ mkdir -p $HOME/scylla $HOME/scylla/conf
$ cp conf/scylla.yaml $HOME/scylla/conf/scylla.yaml
$ # Edit configuration options as appropriate
$ SCYLLA_HOME=$HOME/scylla build/release/scyllaThe scylla.yaml file in the repository by default writes all database data to /var/lib/scylla, which likely requires root access. Change the data_file_directories and commitlog_directory fields as appropriate.
Scylla has a number of requirements for the file-system and operating system to operate ideally and at peak performance. However, during development, these requirements can be relaxed with the --developer-mode flag.
Additionally, when running on under-powered platforms like portable laptops, the --overprovisined flag is useful.
On a development machine, one might run Scylla as
$ SCYLLA_HOME=$HOME/scylla build/release/scylla --overprovisioned --developer-mode=yesMultiple release branches are maintained on the Git repository at https://github.com/scylladb/scylla. Release 1.5, for instance, is tracked on the branch-1.5 branch.
Similarly, tags are used to pin-point precise release versions, including hot-fix versions like 1.5.4. These are named scylla-1.5.4, for example.
Most development happens on the master branch. Release branches are cut from master based on time and/or features. When a patch against master fixes a serious issue like a node crash or data loss, it is backported to a particular release branch with git cherry-pick by the project maintainers.
This section describes one possible work-flow for developing Scylla on a Fedora 25 system. It is presented as an example to help you to develop a work-flow and tools that you are comfortable with.
This guide will be written from the perspective of a fictitious developer, Taylor Smith.
Having two Git remotes is useful:
- A public clone of Seastar (
"public") - A private clone of Seastar (
"private") for in-progress work or work that is not yet ready to share
The first step to contributing a change to Scylla is to create a local branch dedicated to it. For example, a feature that fixes a bug in the CQL statement for creating tables could be called ts/cql_create_table_error/v1. The branch name is prefaced by the developer's initials and has a suffix indicating that this is the first version. The version suffix is useful when branches are shared publicly and changes are requested on the mailing list. Having a branch for each version of the patch (or patch set) shared publicly makes it easier to reference and compare the history of a change.
Setting the upstream branch of your development branch to master is a useful way to track your changes. You can do this with
$ git branch -u master ts/cql_create_table_error/v1As a patch set is developed, you can periodically push the branch to the private remote to back-up work.
Once the patch set is ready to be reviewed, push the branch to the public remote and prepare an email to the scylladb-dev mailing list. Including a link to the branch on your public remote allows for reviewers to quickly test and explore your changes.
Scylla includes a CMake file, CMakeLists.txt, for use only with development environments (not for building) so that they can properly analyze the source code.
CLion is a commercial IDE offers reasonably good source code navigation and advice for code hygiene, though its C++ parser sometimes makes errors and flags false issues.
Other good options that directly parse CMake files are KDevelop and QtCreator.
To use the CMakeLists.txt file with these programs, define the FOR_IDE CMake variable or shell environmental variable.
Eclipse is another open-source option. It doesn't natively work with CMake projects, and its C++ parser has many similar issues as CLion.
Scylla's compilations times can be long. Two tools help somewhat:
- ccache caches compiled object files on disk and re-uses them when possible
- distcc distributes compilation jobs to remote machines
A reasonably-powered laptop acts as the coordinator for compilation. A second, more powerful, machine acts as a passive compilation server.
Having a direct wired connection between the machines ensures that object files can be transmitted quickly and limits the overhead of remote compilation.
The coordinator has been assigned the static IP address 10.0.0.1 and the passive compilation machine has been assigned 10.0.0.2.
On Fedora, installing the ccache package places symbolic links for gcc and g++ in the PATH. This allows normal compilation to transparently invoke ccache for compilation and cache object files on the local file-system.
Next, set CCACHE_PREFIX so that ccache is responsible for invoking distcc as necessary:
export CCACHE_PREFIX="distcc"On each host, edit /etc/sysconfig/distccd to include the allowed coordinators and the total number of jobs that the machine should accept.
This example is for the laptop, which has 2 physical cores (4 logical cores with hyper-threading):
OPTIONS="--allow 10.0.0.2 --allow 127.0.0.1 --jobs 4"
10.0.0.2 has 8 physical cores (16 logical cores) and 64 GB of memory.
As a rule-of-thumb, the number of jobs that a machine should be specified to support should be equal to the number of its native threads.
Restart the distccd service on all machines.
On the coordinator machine, edit $HOME/.distcc/hosts with the available hosts for compilation. Order of the hosts indicates preference.
10.0.0.2/16 localhost/2
In this example, 10.0.0.2 will be sent up to 16 jobs and the local machine will be sent up to 2. Allowing for two extra threads on the host machine for coordination, we run compilation with 16 + 2 + 2 = 20 jobs in total: ninja-build -j20.
When a compilation is in progress, the status of jobs on all remote machines can be visualized in the terminal with distccmon-text or graphically as a GTK application with distccmon-gnome.
One thing to keep in mind is that linking object files happens on the coordinating machine, which can be a bottleneck. See the next section speeding up this process.
Linking Scylla can be slow. The gold linker can replace GNU ld and often speeds the linking process. On Fedora, you can switch the system linker using
$ sudo alternatives --config ldSometimes Scylla development is closely tied with a feature being developed in Seastar. It can be useful to compile Scylla with a particular check-out of Seastar.
One way to do this it to create a local remote for the Seastar submodule in the Scylla repository:
$ cd $HOME/src/scylla
$ cd seastar
$ git remote add local /home/tsmith/src/seastar
$ git remote update
$ git checkout -t local/my_local_seastar_branch