gitolite/doc/mirroring.mkd

# mirroring gitolite servers

Mirroring a repo is simple in git; you just need code like this in a
`post-receive` hook in each repo:

    #!/bin/bash
    git push --mirror slave_user@mirror.host:/path/to/repo.git
    # if running gitolite, the $GL_REPO variable could be useful:
    # git push --mirror slave_user@mirror.host:/repo/base/path/$GL_REPO.git

For a lot of people, though, mirroring is more than just 'backup', and their
needs are complex enough that setup is hard.

----

In this document:

  * <a href="#_why">why</a>
  * <a href="#_RULE_NUMBER_ONE_">RULE NUMBER ONE!</a>
  * <a href="#_IMPORTANT_cautions">IMPORTANT cautions</a>
  * <a href="#_concepts_and_terminology">concepts and terminology</a>
  * <a href="#_setup_and_usage">setup and usage</a>
      * <a href="#_server_level_setup">server level setup</a>
      * <a href="#_repository_level_setup">repository level setup</a>
      * <a href="#_commands_to_re_sync_mirrors">commands to (re-)sync mirrors</a>
  * <a href="#_details">details</a>
      * <a href="#_the_conf_gitolite_conf_file">the `conf/gitolite.conf` file</a>
  * <a href="#_redirecting_pushes">redirecting pushes</a>
  * <a href="#_discussion">discussion</a>
      * <a href="#_problems_with_the_old_mirroring_model">problems with the old mirroring model</a>
      * <a href="#_the_new_mirroring_model">the new mirroring model</a>
  * <a href="#_appendix_A_example_cronjob_based_mirroring">appendix A: example cronjob based mirroring</a>
  * <a href="#_appendix_B_efficiency_versus_paranoia">appendix B: efficiency versus paranoia</a>

----

<a name="_why"></a>

### why

This document is useful if:

  * you have multiple repositories spread across multiple sites around the
    country/world, and would like developers to access their local server
    instead of hitting the WAN, at least for 'fetch' operations.

  * you don't want all your repos mirrored to all the servers for various
    reasons, technical or otherwise (epecially true when some of the mirrors
    don't belong to you).

  * you want some mirrors to be updated only at certain times of the day,
    (with a simple command), instead of every time a push happens.

  * you don't want *one* server being the master server for *all* repos;
    instead you want to choose where a repo gets "mastered" based on where the
    majority of that repo's users are.

  * you might even, if your servers are all in your control, want the
    convenience of them *pushing to a mirror*, and having the push redirect
    transparently to the master server.

As you can see, this is a bit more than a backup solution ;-)

<a name="_RULE_NUMBER_ONE_"></a>

### RULE NUMBER ONE!

**RULE OF GIT MIRRORING: users should push directly to only one server**!  All
the other machines (the slaves) should be updated by the master server.

If a user pushes directly to one of the slaves, those changes will get wiped
out on the next mirror push from the real master server.

Corollary: if the primary went down and you effected a changeover, you must
make sure that the primary does not come up in a push-enabled mode when it
recovers.

<a name="_IMPORTANT_cautions"></a>

### IMPORTANT cautions

  * For reasons given in the 'discussion' section later, the mirroring process
    will never *create* a repo on the receiving side.  It has to exist, and be
    willing to accept pushes from the master.

    In particular, this means that repositories created by end-users ("wild"
    repos) *need to be explicitly created* on the mirror (preferably by the
    same user, assuming his ssh key works there as well).  Once the repo has
    been created on the slave, subsequent pushes will be mirrored correctly.

  * This process will *only* mirror your git repositories, using `git push
    --mirror`.  It will *not* mirror log files, and repo-specific files like
    `gl-creater` and `gl-perms` files, or indeed anything that was manually
    created or added (for example, custom config entries added manually
    instead of via gitolite).

    None of these affect actual repo contents of course, but they could be
    important, (especially the gl-creator, although if your wildcard pattern
    had "CREATOR" in it you can recreate those files easily enough anyway).

  * This document has been tested using a 3-server setup, all installed using
    the *non-root* method (see doc/1-INSTALL.mkd).  However, the process is
    probably not going to be very forgiving of human error -- like anything
    that is this deep in "system admin" territory, errors are likely to be
    costly.  If you're the kind who hits enter first and then thinks about
    what he typed, you're in for some fun times ;-)

    On the plus side, everything we do is done using git commands, so things
    are never *really* lost until you do a `git gc`.

  * Mirroring has *not* been, and will not be, tested with gitolite installed
    using the deprecated 'from-client' method.  Please use one of the other
    methods.

  * Also, this has *not* been tested with smart-http.  I'm not even sure it'll
    work; http is very fiddly to get right.  If you want mirroring, at least
    your server-to-server comms should be over ssh.

  * Finally, this method uses repo-specific `git config` variables to store
    the mirroring information.  Please read the **WARNING** in the
    documentation on [git config commands][rsgc] if you wish to **delete** one
    of those lines.

[rsgc]: http://sitaramc.github.com/gitolite/doc/gitolite.conf.html#_repo_specific_git_config_commands

<a name="_concepts_and_terminology"></a>

### concepts and terminology

Servers can host 3 kinds of repos: master, slave, and local.

  * A repo can be a **master** on one and only one server.  A repo on its
    "master" server is a **native** repo, on slaves it is "non-native".

  * A **slave** repo cannot be pushed to by a user.  It will only accept
    pushes from a master server.  (But see later for an exception).

  * A **local** repo is not involved in mirroring at all, in either direction.

<a name="_setup_and_usage"></a>

### setup and usage

<a name="_server_level_setup"></a>

#### server level setup

To start with, assign each server a short name.  We will use 'frodo', 'sam',
and 'gollum' as examples here.

1.  Generate ssh keys on each machine.  Copy the `.pub` files to all other
    machines with the appropriate names.  I.e., frodo should have sam.pub and
    gollum.pub, etc.

2.  Install gitolite on all servers, under some 'hosting user' (we'll use
    `git` in our examples here).  You need not use the same hosting user on
    all machines.

    It is not necessary to use the same "admin key" on all the machines.
    However, if you do plan to mirror the gitolite-admin repo also, they will
    eventually become the same anyway.  In our example, frodo does mirror the
    admin repo to sam, but not to gollum.  (Can you really see frodo or sam
    trusting gollum?)

3.  Now copy `hooks/common/post-receive.mirrorpush` from the gitolite source,
    and install it as a custom hook called `post-receive`; see [here][ch] for
    instructions.

4.  Edit `~/.gitolite.rc` on each machine and add/edit the following lines.
    The `GL_HOSTNAME` variable **must** have the correct name for that host
    (frodo, sam, or gollum), so that will definitely be different on each
    server.  The other line can be the same, or may have additional patterns
    for other `git config` keys you have previously enabled.  See [here][rsgc]
    and the description for `GL_GITCONFIG_KEYS` in [this][vsi] for details.

        $GL_HOSTNAME = 'frodo';     # will be different on each server!
        $GL_GITCONFIG_KEYS = "gitolite.mirror.*";

    (Remember the "rc" file is NOT mirrored; it is meant to be site-local).

    Note: if `GL_HOSTNAME` is undefined, you cannot push to repos which have
    the 'gitolite.mirror.master' config variable set.  (See 'details' section
    below for more info on this variable).

5.  On each machine, add the keys for all other machines.  For example, on
    frodo you'd run these two commands:

        gl-tool add-mirroring-peer sam.pub
        gl-tool add-mirroring-peer gollum.pub

6.  Create "host" aliases on each machine to refer to all other machines.  See
    [here][ha] for what/why/how.

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

    For example, if machine A's `~/.gitolite.rc` says `$GL_HOSTNAME =
    'frodo';`, then all other machines must use a host alias of "frodo" in
    their `~/.ssh/config` files to refer to machine A.

Once you've done this, each host should be able to reach the other hosts and
get a response back.  For example, running this on sam:

    ssh frodo info

should get you

    Hello sam, I am frodo.

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

<a name="_repository_level_setup"></a>

#### repository level setup

Setting up mirroring at the repository level instead of at the "entire server"
level gives you a lot of flexibility (see "discussion" section below).

The basic idea is to use `git config` variables within each repo (gitolite
allows you to create them from within the gitolite.conf file so that's
convenient), and use these to specify which machine is the master and which
machines are slaves for the repo.

<font color="gray">

>   Side note: if you just want to **simulate the old mirroring scheme**,
>   despite its limitations, it's very easy.  Say frodo is the master for all
>   repos, and the other 2 are slaves.  Just clone the gitolite-admin repos of
>   all servers, add these lines to the top of each:

        repo @all
            config gitolite.mirror.master   =   "frodo"
            config gitolite.mirror.slaves   =   "sam gollum"

>   then commit, and push all 3.  Finally, make a dummy commit on just the
>   frodo clone and push again.  At this point you can do a one-time manual
>   sync (see Appendix A) if you wish but otherwise you're done.

</font>

Let's say frodo and sam are internal servers, while gollum is an external (and
therefore less trusted) server that has agreed to help us out by mirroring one
of our high traffic repos.  We want the following setup:

  * the "gitolite-admin" repo, as well as an internal project repo called
    "ip1", should be mastered on frodo and mirrored to sam.

  * internal project "ip2" has almost all of its developers closer to sam, so
    it should be mastered there, and mirrored on frodo.

  * an open source project we manage, "os1", should be mastered on frodo and
    mirrored on both sam and gollum.

So here's how our example would go:

1.  Clone frodo's and sam's gitolite-admin repos to your workstation, then add
    the following lines to both their gitolite.conf files:

        repo ip1 gitolite-admin
            config gitolite.mirror.master   =   "frodo"
            config gitolite.mirror.slaves   =   "sam"

        repo ip2
            config gitolite.mirror.master   =   "sam"
            config gitolite.mirror.slaves   =   "frodo"

    You also need normal access control lines for ip1 and ip2; I'm assuming
    you already have them elsewhere, at least on frodo.  (What you have on sam
    won't matter in a few minutes, as you will see!)

    Commit and push these changes.

2.  There are a couple of quirks to keep in mind when you make changes to the
    gitolite-admin repo's config.

      * the first push will create the `git config` entries required, but by
        then it is too late to *act* on them; i.e., actually do the mirroring.
        If there were any older values, like a different list of slaves
        perhaps, then those would be in effect.

        This is largely because git invokes post-receive before post-update.
        In theory I can work around this but I do not intend to.

        Anyway, this means that after the 2 pushes, you have to make a dummy
        push from frodo:

            git commit --allow-empty -m empty; git push

        which gets you something like this amidst the other messages:

            remote: (25158&) frodo ==== (gitolite-admin) ===> sam

        telling you that frodo is sending gitolite-admin to sam in the
        background.

      * the second quirk is that your clone of server sam's gitolite-admin
        repo is now completely out of date, since frodo has overwritten it on
        the server.  You have to 'cd' to that clone and do this:

            git fetch
            git reset --hard origin/master

2.  That completes the setup of the gitolite-admin and the internal project
    repos.  We'll now setup things for the open source project, "os1".

    On frodo's gitolite-admin clone, add the following lines to
    `conf/gitolite.conf`, then commit and push:

        repo os1
            config gitolite.mirror.master   =   "frodo"
            config gitolite.mirror.slaves   =   "sam gollum"

    Also, send the same lines to gollum's administrator and ask him to add
    them into his conf/gitolite.conf file, commit, and push.

<a name="_commands_to_re_sync_mirrors"></a>

#### commands to (re-)sync mirrors

You don't have to put all the slaves in `gitolite.mirror.slaves`.  For
example, let's say you have some repos that are very active, and two of your
mirrors that are halfway across the world are getting pushed very frequently.
But you don't need those mirrors to be that closely updated, perhaps *because*
they are halfway across the world and those guys are asleep ;-)

Or maybe there was a network glitch and even the default slaves are now
lagging, so they need to be manually synced.

Or a slave realised that one of its repos is lagging for some reason, and
wants to request an immediate update.

Whatever the reason, you need ways to sync a repo from a command line.  Here
are ways to do that:

1.  On the master server, you can start a **background** job to mirror a repo.
    The command/syntax is

        gl-mirror-shell request-push reponame [list of keys/slaves]

    The list at the end is optional, and can be a mix of slave names or your
    own gitolite mirror config keys.  (Yes, you can have any key, named
    anything you like, as long as it starts with `gitolite.mirror.`).

    If the list is not supplied, the `gitolite.mirror.slaves` key is used.

    Keys can have values that in turn contain a list of keys/slaves.  The list
    is recursively *expanded* but recursion is not *detected*.  Order is
    preserved while duplicates are removed.  If you didn't get that, see the
    example :-)

    **Warning**: the `gitolite.mirror.slaves` key should have only hosts, no
    keys, in it.

    The program exits with a return value of "1" if it found no slaves in the
    list passed, otherwise it fires off the background job, prints an
    informative message, and exits with a return value of "0".

    We'll take an example.  Let's say your gitolite config file has this:

        repo ip1
            config gitolite.mirror.master       =   "frodo"
            config gitolite.mirror.slaves       =   "sam merry pippin"
            config gitolite.mirror.hourly       =   "sam legolas"
            config gitolite.mirror.nightly      =   "gitolite.mirror.hourly gimli"
            config gitolite.mirror.all          =   "gitolite.mirror.nightly gitolite.mirror.hourly gitolite.mirror.slaves"

    Then the following commands have the results described in comments:

        gl-mirror-shell request-push ip1
        # which is the same as:
        gl-mirror-shell request-push ip1 gitolite.mirror.slaves
        # pushes to sam, merry, pippin

        gl-mirror-shell request-push ip1 gollum
        # pushes only to gollum.  Note that gollum is not a member of any of
        # the slave lists we defined.

        gl-mirror-shell request-push ip1 gitolite.mirror.slaves gollum
        # pushes to sam, merry, pippin, gollum

        gl-mirror-shell request-push ip1 gitolite.mirror.slaves gitolite.mirror.hourly
        # pushes to sam, merry, pippin, legolas

        gl-mirror-shell request-push ip1 gitolite.mirror.all
        # pushes to sam, legolas, gimli, merry, pippin

    The last two examples show recursive expansion with order-preserving
    duplicate removal (hey there's now a published conference paper on
    gitolite, so we have to use jargon *somewhere* or they won't accept
    follow-on papers!).

    If you do something like this:

        config gitolite.mirror.nightly      =   "gimli gitolite.mirror.nightly"

    or this:

        config gitolite.mirror.nightly      =   "gimli gitolite.mirror.hourly"
        config gitolite.mirror.hourly       =   "legolas gitolite.mirror.nightly"

    you deserve what you get.

2.  If you want to start a **foreground** job, the syntax is `gl-mirror-shell
    request-push ip1 -fg gollum`.  Foreground mode requires one (and only one)
    slave name -- you cannot send to an implicit list, nor to more than one
    slave.

3.  Cronjobs and custom mirroring schemes are now very easy to do.  Use either
    of the command forms above and write a script around it.  Appendix A
    contains an example setup.

4.  Once in a while a slave will realise it needs an update, and wants to ask
    for one.  It can run this command to do so:

        ssh sam request-push ip2

    If the requesting server is not one of the slaves listed in the config
    variable gitolite.mirror.slaves on the master, it will be rejected.

    This is always a foreground push, reflecting the fact that the slave may
    want to know why their push errored out or didn't work last time or
    whatever.

<a name="_details"></a>

### details

<a name="_the_conf_gitolite_conf_file"></a>

#### the `conf/gitolite.conf` file

One goal I have is to minimise the code changes to "core" gitolite due to
this, so all repo-specific mirror settings are stored as `git config`
variables (you know you can specify git config variables in the gitolite
config file right?).  These are:

  * `gitolite.mirror.master`

    The name of the server which is the master for this repo.  Each server
    will compare this with `$GL_HOSTNAME` (from its own rc file) to
    determine if it's the master or a slave.  Here're the possible values:

      * **undefined** or `local`: this repo is local to this server
      * **same** as `$GL_HOSTNAME`: this server is the "master" for this
        repo.  (The repo is "native" to this server).
      * **not same** as `$GL_HOSTNAME`: this server is a "slave" for the
        repo.  (The repo is a non-native on this server).

  * `gitolite.mirror.slaves`

    Ignored for non-native repos.  For native repos, this is a space-separated
    list of servers to push to from the `post-receive` hook.

    Clearly, you can have different sets of slaves for different repos (again,
    see "discussion" section later for more on this).

  * `gitolite.mirror.redirectOK`

    See the section on "redirecting pushes"

  * In addition, you can create your own slave lists, named whatever you want,
    except they have to start with `gitolite.mirror.`.  The section on
    "commands to (re-)sync mirrors" has some examples.

<a name="_redirecting_pushes"></a>

### redirecting pushes

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

When a user pushes to a non-native repo, it is possible to transparently
redirect the push to the correct master server.  This is a very neat feature,
because now all your users just use one URL (the mirror nearest to them).
They don't need to know where the actual master is, and more importantly, if
you and the other admins change it, they don't need to know it changed!

The `gitolite.mirror.redirectOK` config variable decides where this
redirection is OK.  If it is set to 'true', any valid 'slave' can redirect an
incoming non-native push from a developer.  Otherwise, it contains a list of
slaves that are permitted to redirect pushes (this might happen if you don't
trust some of your slaves enough to accept a redirected push from them).

This check needs to pass on both the master and slave servers; both have a say
in deciding if this is allowed.  (The master may have real reasons not to
allow this; see below.  I cannot think of any real reason for the *slave* to
disable this, but it's there in case some admin doesn't like it).

There are some potential issues that you MUST consider before enabling this:

  * (security) If the slave and master server are so different or autonomous
    that a user, say "alice", on the slave is not guaranteed to be the same
    one as "alice" on the master, then the master admin should NOT enable this
    feature.

    This is because, in this scheme, authentication happens on the slave, but
    authorisation is on the master.  The slave-authenticated userid (alice) is
    passed to the master.

    (If you know ssh well enough, you know that the ssh authentication has
    already happened, so all we can do is ensure authorisation happens with
    whatever username we know so far).

  * If your slave is out of sync with the master for whatever reason, then the
    user will get confusing results.  A `git fetch` may say everything is
    upto-date but the push fails saying it is not a fast-forward push.  (Of
    course there's a way to fix this; see the "commands to (re-)sync mirrors"
    section above).

  * We cannot redirect non-git commands like ADC, setperms, etc because we
    don't really have a way of knowing what repo he's talking about (different
    commands have different syntaxes, some have more than one reponame...).
    Any user who needs to do that should access the end server directly.  It
    should be easy enough to write an ADC to do the forwarding, in case the
    slave server is the only one that can reach the real master due to network
    or firewall setup.

    Ideally, I recommend that ad hoc repos not be mirrored at all.  Keep
    mirroring for "blessed" repos only.

<a name="_discussion"></a>

### discussion

<a name="_problems_with_the_old_mirroring_model"></a>

#### problems with the old mirroring model

The old mirroring model had a single server as the master for *all*
repositories.  Slaves were effectively only for load-balancing reads, or for
failover if the master died.

This is not good enough for corporate setups where the developers are spread
fairly evenly across the world.  Some repos need to be closer to some teams
(NUMA is a good analogy).

A model where different repos are "mastered" in different cities is much more
efficient here.

The old model had other rigidities too, though they're not really *problems*,
as such:

  * the slaves are just slaves; they can't have any "local" repos.

  * a slave had to carry *all* repos; it couldn't choose to carry just a
    subset.

  * it implicitly assumed all the mirrors were under the same admin, and that
    the gitolite-admin repo was itself mirrored too.

<a name="_the_new_mirroring_model"></a>

#### the new mirroring model

In the new model, servers can be much more independent and autonomous than in
the old model.  (Don't miss the side note in the 'repository level setup'
section if you prefer the old model).

The new model has a few pros and cons.  The pros come from the flexibility and
freedom that mirrors servers get, and the cons come from authorisation being
more rigorously checked (for example, a slave will only accept a push if *its*
configuration also says that the sending server is indeed the master for this
repo).

  * A mirroring operation will not *create* a repo on the mirror; it has to
    exist before a push happens on the master.  Typically, the admin on the
    slave must create the repo by adding the appropriate lines in his config.

    If your setup is not autonomous (i.e., you're mirroring the admin repo as
    well) then this happens automatically for normal repos.  However,
    *wildcard repos still won't work as seamlessly as in the old model*; see
    the first bullet in the 'IMPORTANT cautions' section earlier.

  * The gitolite-admin repo (and config) need not be mirrored.  This allows
    the slave server admin to create site-local repos, without forcing him to
    create a second gitolite install for them.

    (Site-local repos are useful for purely local projects that need
    not/should not be mirrored for some reason, or ad-hoc personal repos that
    developers create for themselves, etc.)

  * Servers can choose to mirror a subset of the repos from one of the bigger
    servers.

    In the open source world, you can imagine more popular repos (or more
    popular parts of huge projects like KDE) having more mirrors.  Or
    substitute "more popular" with "larger in size" if you wish
    (FlightGear-data anyone?)

    In the corporate world it could help with jurisdiction issues if the
    mirror is in a different country with different laws.

    I'm sure people will find other uses for this.  And I'm *positive* the
    pros will outweigh the cons.  If you don't like it, follow the suggestion
    in the side note somewhere up above, and just forget this feature exists
    :-)

----

<a name="_appendix_A_example_cronjob_based_mirroring"></a>

### appendix A: example cronjob based mirroring

Let's say you have some repos that are very active.  You're pushing halfway
across the world every few seconds, but those slaves do not need to be that closely
updated, perhaps *because* they are halfway across the world and those guys
are asleep ;-)

You'd like to update them once an hour instead.  Here's how you might do that.

First add this line to the configuration for those repos:

    config gitolite.mirror.hourly = "slave1 slave2 slave3"

Then write a cron job that looks like this (untested).

    #!/bin/bash

    REPO_BASE=`${0%/*}/gl-query-rc REPO_BASE`

    cd $REPO_BASE
    find . -type d -name "*.git" -prune | while read r
    do
        # get reponame as gitolite knows it
        r=${r:2}
        r=${r%.git}

        gl-mirror-shell request-push $r gitolite.mirror.hourly

        # that command backgrounds the push, so you'd best wait a few seconds
        # before hitting the next one, otherwise you'll have all your repos
        # going out at once!
        sleep 10
    done

<a name="_appendix_B_efficiency_versus_paranoia"></a>

### appendix B: efficiency versus paranoia

If you're paranoid enough to use mirrors, you should be paranoid enough to
use the `receive.fsckObjects` setting.  However, informal tests indicate a
40-50% CPU overhead from this.  If you're ok with that, make the appropriate
adjustments to `GL_GITCONFIG_KEYS` in the rc file, then add this to your
gitolite.conf file:

    repo @all
        config receive.fsckObjects = "true"

Personally, I just set `git config --global receive.fsckObjects true`, since
those servers aren't doing anything else anyway, and are idle for long
stretches of time.  It's upto you what you want to do here.

[ch]: http://sitaramc.github.com/gitolite/doc/2-admin.html#_custom_hooks
[ha]: http://sitaramc.github.com/gitolite/doc/ssh-troubleshooting.html#_appendix_4_host_aliases
[rsgc]: http://sitaramc.github.com/gitolite/doc/gitolite.conf.html#_repo_specific_git_config_commands
[vsi]: http://sitaramc.github.com/gitolite/doc/gitolite.rc.html#_variables_with_a_security_impact