

.. _remote_execution_servers:

Remote Execution Servers
========================
BuildStream supports building remotely using the
`Google Remote Execution API (REAPI). <https://github.com/bazelbuild/remote-apis>`_, which
has various known implementations.

Some of these implementations include:

* `BuildGrid <https://buildgrid.build/>`_
* `BuildBarn <https://github.com/buildbarn>`_
* `Buildfarm <https://github.com/bazelbuild/bazel-buildfarm>`_

These various implementations implement the `Google Remote Execution API (REAPI)
<https://github.com/bazelbuild/remote-apis>`_ to various degrees as these projects
have different priorities.

On the client side, the remote execution service to use can be
specified in the :ref:`user configuration <user_config_remote_execution>`.


BuildStream specific requirements
---------------------------------
In order for BuildStream to work correctly with a remote execution cluster, there
are a couple of requirements that implementation needs to meet.


Implementation of platform properties
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The remote execution service must properly implement `platform properties
<https://github.com/bazelbuild/remote-apis/blob/main/build/bazel/remote/execution/v2/platform.md>`_.

This is crucial because BuildStream needs to be guaranteed the correct operating
system and ISA which it requests from the service.


Staging the input root as the filesystem root
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
BuildStream requires that the *input root* given to the remote execution service
be treated as the absolute *filesystem root*.

This is because BuildStream provides guarantees that all build dependencies, including
the base runtime and compilers, are defined by elements and run within a sandboxed and
isolated build environment, but the `REAPI <https://github.com/bazelbuild/remote-apis>`_
was originally developped without this determinism and control in mind. Instead, typically
it is up to the user to configure a cluster to use a docker image to build payloads
with, rather than allowing the REAPI client to control the entire sandbox.

Unfortunately the ability to dictate that the *input root* be treated as the *filesystem root*
in a container on remote workers in the cluster is not yet standardized in the REAPI protocol.

.. note::

   The *input root* is referred to as the ``input_root_digest`` member of the ``Action`` message
   as defined in the `protocol <https://github.com/bazelbuild/remote-apis/blob/main/build/bazel/remote/execution/v2/remote_execution.proto>`_


Example working configuration
-----------------------------
A simple configuration to spin up the `BuildGrid <https://buildgrid.build/>`_ service using
`docker compose <https://docs.docker.com/compose/>`_ follows:

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