gitolite/doc/mirroring.mkd

# mirroring using gitolite

<font color="red">**WARNING** existing gitolite mirroring users please note:
**there are [significant changes][g2i-mirroring]** in syntax and usage
compared to g2.  If you're not the kind who reads documentation before doing
serious system admin things, well... good luck!</font>

----

[[TOC]]

----

Mirroring is simple: you have one "master" server and one or more "slave"
servers.  The slaves get updates only from the master; to the rest of the
world they are at best read-only.

Gitolite extends this simple notion in the following ways:

  * different masters and sets of slaves for different repos

    This lets you do things like

      * use the server closest to *most* of its developers as the master for
        that repo
      * not mirror a repo at all to some servers
      * have repos that are purely local to a server (not mirrored at all)
      * negotiate mirroring with servers that are not even under your control
      * push to a slave on demand or via cron (helps deal with bandwidth or
        connectivity constraints).

  * pushes to a slave can be transparently forwarded to the real master

    Your developers need not worry about where a repo's master is -- they just
    write to their local mirror for *all* repos, even if their local mirror is
    only a slave for some.

## caveats

  * mirroring will never *create* a repo on a slave; it has to exist and be
    prepared to receive updates from the master.  (For example, [wild][] repos
    must be created on the slave as well, otherwise they will not propagate).

  * mirroring is only for git repos.  Ancillary files like gl-creator and
    gl-perms in the repo directory are not mirrored; you must do that
    separately.  Files in the admin directory (like log files) are also not
    mirrored.

  * if you ever do a [bypass push][bypass], mirroring will not work.
    Mirroring checks also will not work -- for example, you can push to a
    slave, which is not usually a good idea.  So don't bypass gitolite if the
    repo is mirrored!

## setting up mirroring

This is in two parts: the initial setup and the rc file, followed by the conf
file settings and syntax.

### the initial setup and the rc file

On **each** server:

  * install gitolite normally.  Make clones of the server's 'gitolite-admin'
    repo on your workstation so you can admin them all from one place.

  * give the server a short, simple, "hostname" and set the HOSTNAME in the
    rc file to this name, for example 'mars'.

  * run ssh-keygen if needed and get an ssh key pair for the server.  Copy the
    public key to a common area and name it after the host, but with 'server-'
    prefixed.  So the pubkey for server 'mars' would be stored as
    'server-mars.pub'.

  * copy all keys to all the admin repo clones on your workstation and and add
    them as usual.  This is an `O(N^2)` operation ;-)

    You may have guessed that the prefix 'server-' is special, and
    distinguishes a human user from a mirroring peer.

  * create "host" aliases to refer to all other machines.  See [here][ssh-ha]
    for what/how.

    The host alias for a host (in all other machines' `~/.ssh/config` files)
    MUST be the same as the `HOSTNAME` in the referred host's
    `~/.gitolite.rc`.  Gitolite mirroring **requires** this consistency in
    naming; things will NOT work otherwise.

    Normally you should be able to build one common file and append it to all
    the servers' `~/.ssh/config` files.

  * the following **MUST** work for **each pair** of servers that must talk to
    each other:

        # on server mars
        ssh phobos info
        # the response MUST start with "hello, server-mars..."

    Note the exact syntax used; variations like "ssh git@phobos.example.com
    info" are NOT sufficient.  That is why you need the ssh host aliases.

    Check this command from *everywhere to everywhere else*, and make sure you
    get expected results.  **Do NOT proceed otherwise.**

  * setup the gitolite.conf file on all the servers.  If the slaves are to be
    exact copies of the master, you need to do the complete configuration only
    on the master; the slaves can have just this:

        repo gitolite-admin
            RW+     =   <some local admin>

            option mirror.master    =   mars
            option mirror.slaves    =   phobos

    because on the first push to the master it will update all the slaves
    anyway.

  * when that is all done and tested, **enable mirroring** by going through
    the rc file and uncommenting all the lines mentioning `Mirroring`.

### conf file settings and syntax

Mirroring is defined by the following [options][].  You can have different
settings for different repos, and of course some repos may not have any mirror
options at all -- they are then purely local.

    repo foo
        ...access rules...

        option mirror.master        =   mars
        option mirror.slaves        =   phobos deimos
        option mirror.redirectOK    =   all

The first line is easy, since a repo can have only one master.

The second is a space separated list of hosts that are all slaves.  You can
have several slave lists, as long as the config key starts with
'mirror.slaves' and is unique.  For example.

        option mirror.slaves-1   =   phobos deimos
        option mirror.slaves-2   =   io europa
        option mirror.slaves-3   =   ganymede callisto

Do not repeat a key; then only the last line for that key will be effective.

## redirected pushes

**Please read carefully; there are security implications if you enable this
for mirrors NOT under your control**.

Normally, a master, (and *only* a master), pushes to a slave, and the slaves
are "read-only" to the users.  Gitolite allows a *slave* to receive pushes
from a user and transparently redirect them to the *master*.

This simplifies things for users in complex setups, letting them use their
local mirror for both fetch and push access to all repos.

Just remember that if you do this, **authentication** happens on the slave,
but **authorisation** is on the master.  The master is trusting the slave to
authenticate the user correctly, *and* use the same authentication data (i.e.,
user alice on the slave should be guaranteed to be the same as user alice on
the master).

The syntax for enabling this is one of these:

    option mirror.redirectOK    =   all
    option mirror.redirectOK    =   phobos deimos

The first syntax trusts all valid slaves to redirect user pushes, while the
second one trusts only some slaves.

Note that you cannot redirect gitolite commands (like perms, etc).

## #sync manually synchronising a slave repo

You can use the `gitolite mirror push` command on a master to manually
synchronise any of its slaves.  Try it with `-h` to get usage info.

Tip: if you want to do this to all the slaves, try this:

    for s in `gitolite git-config -r reponame mirror.slave | cut -f3`
    do
        gitolite mirror push $s reponame
    done

This command can also be run remotely; run `ssh git@host mirror -h` for
details.

## #HOSTNAME appendix A: HOSTNAME substitution

Wherever gitolite sees the word `%HOSTNAME`, it will replace it with the
HOSTNAME supplied in the rc file, if one was supplied.  This lets you maintain
configurations for all servers in one repo, yet havethem act differently on
different servers, by saying something like:

    subconf "%HOSTNAME/*.conf"

You can use it in other places also, for example:

    RW+     VREF/NAME/subs/%HOSTNAME/       =   @%HOSTNAME-admins

(you still have to define @mars-admins, @phobos-admins, etc., but the actual
VREF is now one line instead of one for each server!)

## appendix B: efficiency versus paranoia

If you're paranoid enough to use mirrors, you should be paranoid enough to
set this on each server, despite the possible CPU overhead:

    git config --global receive.fsckObjects true