Replication Configuration
=========================

Enabling Replication
--------------------

If replicating over SSH, ensure the host key of the
remote system(s) is already in the Gerrit user's `~/.ssh/known_hosts`
file.  The easiest way to add the host key is to connect once by hand
with the command line:

```
  sudo su -c 'ssh mirror1.us.some.org echo' gerrit2
```

<a name="example_file">
Next, create `$site_path/etc/replication.config` as a Git-style config
file, for example to replicate in parallel to four different hosts:</a>

```
  [remote "host-one"]
    url = gerrit2@host-one.example.com:/some/path/${name}.git

  [remote "pubmirror"]
    url = mirror1.us.some.org:/pub/git/${name}.git
    url = mirror2.us.some.org:/pub/git/${name}.git
    url = mirror3.us.some.org:/pub/git/${name}.git
    push = +refs/heads/*:refs/heads/*
    push = +refs/tags/*:refs/tags/*
    threads = 3
    authGroup = Public Mirror Group
    authGroup = Second Public Mirror Group
```

Then reload the replication plugin to pick up the new configuration:

```
  ssh -p 29418 localhost gerrit plugin reload replication
```

To manually trigger replication at runtime, see
SSH command [start](cmd-start.md).

File `replication.config`
-------------------------

The optional file `$site_path/etc/replication.config` is a Git-style
config file that controls the replication settings for the replication
plugin.

The file is composed of one or more `remote` sections, each remote
section provides common configuration settings for one or more
destination URLs.

Each remote section uses its own thread pool.  If pushing to
multiple remotes, over differing types of network connections
(e.g. LAN and also public Internet), its a good idea to put them
into different remote sections, so that replication to the slower
connection does not starve out the faster local one.  The example
file above does this.

In the keys below, the `NAME` portion is unused by this plugin, but
must be unique to distinguish the different sections if more than one
remote section appears in the file.

gerrit.replicateOnStartup
:	If true, replicates to all remotes on startup to ensure they
	are in-sync with this server.  By default, true.

gerrit.autoReload
:	If true, automatically reloads replication destinations and settings
	after `replication.config` file is updated, without the need to restart
	the replication plugin. When the reload takes place, pending replication
	events based on old settings are discarded. By default, false.

gerrit.defaultForceUpdate
:	If true, the default push refspec will be set to use forced
	update to the remote when no refspec is given.  By default, false.

replication.lockErrorMaxRetries
:	Number of times to retry a replication operation if a lock
	error is detected.

	If two or more replication operations (to the same GIT and Ref)
	are scheduled at approximately the same time (and end up on different
	replication threads), there is a large probability that the last
	push to complete will fail with a remote "failure to lock" error.
	This option allows Gerrit to retry the replication push when the
	"failure to lock" error is detected.

	A good value would be 3 retries or less, depending on how often
	you see lockError collisions in your server logs. A too highly set
	value risks keeping around the replication operations in the queue
	for a long time, and the number of items in the queue will increase
	with time.

	Normally Gerrit will succeed with the replication during its first
	retry, but in certain edge cases (e.g. a mirror introduces a ref
	namespace with the same name as a branch on the master) the retry
	will never succeed.

	The issue can also be mitigated somewhat by increasing the
	replicationDelay.

	Default: 0 (disabled, i.e. never retry)

replication.maxRetries
:	Maximum number of times to retry a push operation that previously
	failed.

	When a push operation reaches its maximum number of retries,
	the replication event is discarded from the queue and the remote
	destinations may remain out of sync.

	Can be overridden at remote-level by setting replicationMaxRetries.

	By default, pushes are retried indefinitely.

remote.NAME.url
:	Address of the remote server to push to.  Multiple URLs may be
	specified within a single remote block, listing different
	destinations which share the same settings.  Assuming
	sufficient threads in the thread pool, Gerrit pushes to all
	URLs in parallel, using one thread per URL.

	Within each URL value the magic placeholder `${name}` is
	replaced with the Gerrit project name.  This is a Gerrit
	specific extension to the otherwise standard Git URL syntax
	and it must be included in each URL so that Gerrit can figure
	out where each project needs to be replicated. `${name}` may
	only be omitted if the remote refers to a single repository
	(i.e.: Exactly one [remote.NAME.projects][3] and that name's
	value is a single project match.).

	See [git push][1] for details on Git URL syntax.

[1]: http://www.git-scm.com/docs/git-push#URLS
[3]: #remote.NAME.projects

remote.NAME.adminUrl
:	Address of the alternative remote server only for repository
	creation.  Multiple URLs may be specified within a single
	remote block, listing different destinations which share the
	same settings.

	The adminUrl can be used as an ssh alternative to the url
	option, but only related to repository creation.  If not
	specified, the repository creation tries to follow the default
	way through the url value specified.

	It is useful when the remote.NAME.url protocols do not allow
	repository creation although their usage is mandatory in the
	local environment.  In that case, an alternative SSH url could
	be specified to repository creation.

	To enable replication to different Gerrit instance use `gerrit+ssh://`
	as protocol name followed by hostname of another Gerrit server eg.

	`gerrit+ssh://replica1.my.org/`

	In this case replication will use Gerrit's SSH API to
	create/remove projects and update repository HEAD references.

	NOTE: In order to replicate project deletion, the
	link:https://gerrit-review.googlesource.com/admin/projects/plugins/delete-project delete-project[delete-project]
	plugin must be installed on the other Gerrit.

remote.NAME.receivepack
:	Path of the `git-receive-pack` executable on the remote
	system, if using the SSH transport.

	Defaults to `git-receive-pack`.

remote.NAME.uploadpack
:	Path of the `git-upload-pack` executable on the remote system,
	if using the SSH transport.

	Defaults to `git-upload-pack`.

remote.NAME.push
:	Standard Git refspec denoting what should be replicated.
	Setting this to `+refs/heads/*:refs/heads/*` would mirror only
	the active branches, but not the change refs under
	`refs/changes/`, or the tags under `refs/tags/`.

	Note that prefixing a source refspec with `+` causes the replication
	to be done with a `git push --force` command.
	Be aware that when you are pushing to remote repositories that may
	have read/write access (e.g. GitHub) you may want to omit the `+`
	to prevent the risk of overwriting branches that have been modified
	on the remote.

	Multiple push keys can be supplied, to specify multiple
	patterns to match against.  In the [example above][2], remote
	"pubmirror" uses two push keys to match both `refs/heads/*`
	and `refs/tags/*`, but excludes all others, including
	`refs/changes/*`.

	Defaults to `refs/*:refs/*` (push all refs) if not specified,
	or `+refs/*:refs/*` (force push all refs) if not specified and
	`gerrit.defaultForceUpdate` is true.

	Note that the `refs/meta/config` branch is only replicated
	when `replicatePermissions` is true, even if the push refspec
	is 'all refs'.

[2]: #example_file

remote.NAME.timeout
:	Number of seconds to wait for a network read or write to
	complete before giving up and declaring the remote side is not
	responding.  If 0, there is no timeout, and the push client
	waits indefinitely.

	A timeout should be large enough to mostly transfer the
	objects to the other side.  1 second may be too small for
	larger projects, especially over a WAN link, while 10-30
	seconds is a much more reasonable timeout value.

	Defaults to 0 seconds, wait indefinitely.

remote.NAME.replicationDelay
:	Time to wait before scheduling a remote push operation. Setting
	the delay to 0 effectively disables the delay, causing the push
	to start as soon as possible.

	This is a Gerrit specific extension to the Git remote block.

	By default, 15 seconds.

remote.NAME.rescheduleDelay
:	Delay when rescheduling a push operation due to an in-flight push
	running for the same project.

	Cannot be set to a value lower than 3 seconds to avoid a tight loop
	of schedule/run which could cause 1K+ retries per second.

	A configured value lower than 3 seconds will be rounded to 3 seconds.

	By default, 3 seconds.

remote.NAME.replicationRetry
:	Time to wait before scheduling a remote push operation previously
	failed due to an offline remote server.

	If a remote push operation fails because a remote server was
	offline, all push operations to the same destination URL are
	blocked, and the remote push is continuously retried unless
	the replicationMaxRetries value is set.

	This is a Gerrit specific extension to the Git remote block.

	By default, 1 minute.

remote.NAME.replicationMaxRetries
:	Maximum number of times to retry a push operation that previously
	failed.

	When a push operation reaches its maximum number of retries
	the replication event is discarded from the queue and the remote
	destinations could be out of sync.

	This is a Gerrit specific extension to the Git remote block.

	By default, use replication.maxRetries.

remote.NAME.threads
:	Number of worker threads to dedicate to pushing to the
	repositories described by this remote.  Each thread can push
	one project at a time, to one destination URL.  Scheduling
	within the thread pool is done on a per-project basis.  If a
	remote block describes 4 URLs, allocating 4 threads in the
	pool will permit some level of parallel pushing.

	By default, 1 thread.

remote.NAME.authGroup
:	Specifies the name of a group that the remote should use to
	access the repositories. Multiple authGroups may be specified
	within a single remote block to signify a wider access right.
	In the project administration web interface the read access
	can be specified for this group to control if a project should
	be replicated or not to the remote.

	By default, replicates without group control, i.e. replicates
	everything to all remotes.

remote.NAME.createMissingRepositories
:	If true, a repository is automatically created on the remote site.
	If the remote site was not available at the moment when a new
	project was created, it will be created if during the replication
	of a ref it is found to be missing.
	
	If false, repositories are never created automatically on this
	remote.

	By default, true, missing repositories are created.

remote.NAME.replicatePermissions
:	If true, permissions-only projects and the refs/meta/config
	branch will also be replicated to the remote site.  These
	projects and branches may be needed to keep a backup or slave
	server current.

	By default, true, replicating everything.

remote.NAME.replicateProjectDeletions
:	If true, project deletions will also be replicated to the
	remote site.

	By default, false, do *not* replicate project deletions.

remote.NAME.replicateHiddenProjects
:	If true, hidden projects will be replicated to the remote site.

	By default, false, do *not* replicate hidden projects.

remote.NAME.mirror
:	If true, replication will remove remote branches that are absent
	locally or invisible to the replication (for example read
	access denied via `authGroup` option).

	By default, false, do not remove remote branches.

remote.NAME.remoteNameStyle
:	Provides possibilities to influence the name of the target
	repository, e.g. by replacing slashes in the `${name}`
	placeholder.

	Github and Gitorious do not permit slashes "/" in repository
	names and will change them to dashes "-" at repository creation
	time.

	If this setting is set to "dash", slashes will be replaced with
	dashes in the remote repository name. If set to "underscore",
	slashes will be replaced with underscores in the repository name.

	Option "basenameOnly" makes `${name}` to be only the basename
	(the part after the last slash) of the repository path on the
	Gerrit server, e.g. `${name}` of `foo/bar/my-repo.git` would
	be `my-repo`.

	By default, "slash", i.e. remote names will contain slashes as
	they do in Gerrit.

<a name="remote.NAME.projects">remote.NAME.projects</a>
:	Specifies which repositories should be replicated to the
	remote. It can be provided more than once, and supports three
	formats: regular expressions, wildcard matching, and single
	project matching. All three formats match case-sensitive.

	Values starting with a caret `^` are treated as regular
	expressions. `^foo/(bar|baz)` would match the projects
	`foo/bar`, and `foo/baz`. Regular expressions have to fully
	match the project name. So the above example would not match
	`foo/bar2`, while `^foo/(bar|baz).*` would.

	Projects may be excluded from replication by using a regular
	expression with inverse match. `^(?:(?!PATTERN).)*$` will
	exclude any project that matches.

	Values that are not regular expressions and end in `*` are
	treated as wildcard matches. Wildcards match projects whose
	name agrees from the beginning until the trailing `*`. So
	`foo/b*` would match the projects `foo/b`, `foo/bar`, and
	`foo/baz`, but neither `foobar`, nor `bar/foo/baz`.

	Values that are neither regular expressions nor wildcards are
	treated as single project matches. So `foo/bar` matches only
	the project `foo/bar`, but no other project.

	By default, replicates without matching, i.e. replicates
	everything to all remotes.

File `secure.config`
--------------------

The optional file `$site_path/secure.config` is a Git-style config
file that provides secure values that should not be world-readable,
such as passwords. Passwords for HTTP remotes can be obtained from
this file.

remote.NAME.username
:	Username to use for HTTP authentication on this remote, if not
	given in the URL.

remote.NAME.password
:	Password to use for HTTP authentication on this remote.

File `~/.ssh/config`
--------------------

If present, Gerrit reads and caches `~/.ssh/config` at startup, and
supports most SSH configuration options.  For example:

```
  Host host-one.example.com
    IdentityFile ~/.ssh/id_hostone
    PreferredAuthentications publickey

  Host mirror*.us.some.org
    User mirror-updater
    IdentityFile ~/.ssh/id_pubmirror
    PreferredAuthentications publickey
```

Supported options:

  * Host
  * Hostname
  * User
  * Port
  * IdentityFile
  * PreferredAuthentications
  * StrictHostKeyChecking

SSH authentication must be by passwordless public key, as there is no
facility to read passphrases on startup or passwords during the SSH
connection setup, and SSH agents are not supported from Java.

Host keys for any destination SSH servers must appear in the user's
`~/.ssh/known_hosts` file, and must be added in advance, before Gerrit
starts.  If a host key is not listed, Gerrit will be unable to connect
to that destination, and replication to that URL will fail.