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+:title: Merger
+
+Merger
+======
+
+The Zuul Merger is a separate component which communicates with the
+main Zuul server via Gearman.  Its purpose is to speculatively merge
+the changes for Zuul in preparation for testing.  The resulting git
+commits also must be served to the test workers, and the server(s)
+running the Zuul Merger are expected to do this as well.  Because both
+of these tasks are resource intensive, any number of Zuul Mergers can
+be run in parallel on distinct hosts.
+
+Configuration
+~~~~~~~~~~~~~
+
+The Zuul Merger can read the same zuul.conf file as the main Zuul
+server and requires the ``gearman``, ``gerrit``, ``merger``, and
+``zuul`` sections (indicated fields only).  Be sure the zuul_url is
+set appropriately on each host that runs a zuul-merger.
+
+Zuul References
+~~~~~~~~~~~~~~~
+
+As the DependentPipelineManager may combine several changes together
+for testing when performing speculative execution, determining exactly
+how the workspace should be set up when running a Job can be complex.
+To alleviate this problem, Zuul performs merges itself, merging or
+cherry-picking changes as required and identifies the result with a
+Git reference of the form ``refs/zuul/<branch>/Z<random sha1>``.
+Preparing the workspace is then a simple matter of fetching that ref
+and checking it out.  The parameters that provide this information are
+described in :ref:`launchers`.
+
+These references need to be made available via a Git repository that
+is available to Jenkins.  This is accomplished by serving Zuul's Git
+repositories directly.
+
+Serving Zuul Git Repos
+~~~~~~~~~~~~~~~~~~~~~~
+
+Zuul maintains its own copies of any needed Git repositories in the
+directory specified by ``git_dir`` in the ``merger`` section of
+zuul.conf (by default, /var/lib/zuul/git).  To directly serve Zuul's
+Git repositories in order to provide Zuul refs for Jenkins, you can
+configure Apache to do so using the following directives::
+
+  SetEnv GIT_PROJECT_ROOT /var/lib/zuul/git
+  SetEnv GIT_HTTP_EXPORT_ALL
+
+  AliasMatch ^/p/(.*/objects/[0-9a-f]{2}/[0-9a-f]{38})$ /var/lib/zuul/git/$1
+  AliasMatch ^/p/(.*/objects/pack/pack-[0-9a-f]{40}.(pack|idx))$ /var/lib/zuul/git/$1
+  ScriptAlias /p/ /usr/lib/git-core/git-http-backend/
+
+And set ``push_change_refs`` to ``false`` (the default) in the
+``zuul`` section of zuul.conf.
+
+Note that Zuul's Git repositories are not bare, which means they have
+a working tree, and are not suitable for public consumption (for
+instance, a clone will produce a repository in an unpredictable state
+depending on what the state of Zuul's repository is when the clone
+happens).  They are, however, suitable for automated systems that
+respond to Zuul triggers.