

.. _cache_servers:

Configuring Cache Servers
=========================
BuildStream caches the results of builds in a local artifact cache, and will
avoid building an element if there is a suitable build already present in the
local artifact cache. Similarly it will cache sources and avoid pulling them if
present in the local cache. See :ref:`caches <caches>` for more details.

In addition to the local caches, you can configure one or more remote caches and
BuildStream will then try to pull a suitable object from one of the remotes,
falling back to performing a local build or fetching a source if needed.

On the client side, cache servers are declared and configured in
:ref:`user configuration <config_cache_servers>`, and since it is typical
for projects to maintain their own cache servers, it is also possible for
projects to provide recommended :ref:`artifact cache servers <project_artifact_cache>`
and :ref:`source cache servers <project_source_cache>` through project
configuration, so that downstream users can download from services
provided by upstream projects by default.


Setting up a remote cache
-------------------------
BuildStream relies on the `ContentAddressableStorage protocol
<https://github.com/bazelbuild/remote-apis/blob/main/build/bazel/remote/execution/v2/remote_execution.proto>`_
in order to exchange data with remote services, in concert with the `remote asset protocol
<https://github.com/bazelbuild/remote-apis/blob/main/build/bazel/remote/asset/v1/remote_asset.proto>`_
in order to assign symbolic labels (such as :ref:`artifact names <artifact_names>`) to identify
stored content. As such, BuildStream is able to function with any implementations of these
two services.


Known implementations
---------------------
Here are some details about known open source implementations of the required protocols


Buildbarn
~~~~~~~~~
The `Buildbarn <https://github.com/buildbarn>`_ project provides a remote execution
service implementation for use in build tooling such as BuildStream, `Bazel <https://bazel.build/>`_
and `recc <https://gitlab.com/bloomberg/recc>`_, the `bb-storage <https://github.com/buildbarn/bb-storage>`_
and `bb-remote-asset <https://github.com/buildbarn/bb-remote-asset>`_ services are tested
to work as cache service for BuildStream's artifact and source caches.

A simple configuration to spin up the service using `docker compose <https://docs.docker.com/compose/>`_ follows:

.. literalinclude:: ../../.github/compose/ci.buildbarn.yml
    :language: yaml

Visit the `bb-storage <https://github.com/buildbarn/bb-storage>`_ and
`bb-remote-asset <https://github.com/buildbarn/bb-remote-asset>`_ project pages to
find more documentation about setting up services with authentication enabled.