git-annex/ tips/ clusters

A cluster is a collection of git-annex repositories which are combined to form a single logical repository.

A cluster is accessed via a gateway repository. The gateway is not itself a node of the cluster.

A cluster's nodes can be any combination of git-annex repositories and special remotes.

## using a cluster

To use a cluster, your repository needs to have its gateway configured as a remote. Clusters can currently only be accessed via ssh or by an annex+http url. This gateway remote is added the same as any other git remote:

    $ git remote add bigserver me@bigserver:annex

The gateway publishes information about the cluster to the git-annex branch. So you may need to fetch from it to learn about the cluster:

    $ git fetch bigserver

That will make available an additional remote for the cluster, eg "bigserver-mycluster", as well as some remotes for each node.

    $ git annex info bigserver
    ...
    gateway to cluster: bigserver-mycluster
    proxying: bigserver-node1 bigserver-node2 bigserver-node3
    ...

You can get files from the cluster without caring which node it comes from:

    $ git-annex get foo --from bigserver-mycluster
    copy foo (from bigserver-mycluster...) ok

And you can send files to the cluster, without caring what nodes they are stored to:

    $ git-annex move bar --to bigserver-mycluster
    move bar (to bigserver-mycluster...) ok

In fact, a single upload like that can be sent to every node of the cluster at once, very efficiently.

    $ git-annex whereis bar
    whereis bar (3 copies)
        acae2ff6-6c1e-8bec-b8b9-397a3755f397 -- [bigserver-mycluster]
        9f514001-6dc0-4d83-9af3-c64c96626892 -- node 1 [bigserver-node1]
        d81e0b28-612e-4d73-a4e6-6dabbb03aba1 -- node 2 [bigserver-node2]
        5657baca-2f11-11ef-ae1a-5b68c6321dd9 -- node 3 [bigserver-node3]

Notice that the file is shown as present in the cluster, as well as on individual nodes. But the cluster itself does not count as a copy of the file, so the 3 copies are the copies on individual nodes.

Most other git-annex commands that operate on repositories can also operate on clusters.

A cluster is not a git repository, and so `git pull bigserver-mycluster` will not work.

## setting up a cluster

A new cluster first needs to be initialized. Run git-annex-initcluster in the repository that will serve as the cluster's gateway. In the example above, this was the "bigserver" repository.

    $ git-annex initcluster mycluster

Once a cluster is initialized, the next step is to add nodes to it. To make a remote be a node of the cluster, configure `git config remote.name.annex-cluster-node`, setting it to the name of the cluster.

In the example above, the three cluster nodes were configured like this:

    $ git remote add node1 /media/disk1/repo
    $ git remote add node2 /media/disk2/repo
    $ git remote add node3 /media/disk2/repo
    $ git config remote.node1.annex-cluster-node mycluster
    $ git config remote.node2.annex-cluster-node mycluster
    $ git config remote.node3.annex-cluster-node mycluster

Finally, run git-annex-updatecluster to record the cluster configuration in the git-annex branch. That tells other repositories about the cluster.

    $ git-annex updatecluster
    Added node node1 to cluster: mycluster
    Added node node2 to cluster: mycluster
    Added node node3 to cluster: mycluster
    Started proxying for node1
    Started proxying for node2
    Started proxying for node3

The cluster will now be accessible over ssh. To also let the cluster be accessed over http, you would need to set up a smart http server.

Operations that affect multiple nodes of a cluster can often be sped up by configuring annex.jobs in the gateway repository. In the example above, the nodes are all disk bound, so operating on more than one at a time will likely be faster.

    $ git config annex.jobs cpus

## preferred content of clusters

The preferred content of the cluster can be configured. This tells users what files the cluster as a whole should contain.

To configure the preferred content of a cluster, as well as other related things like groups and required content, it's easiest to do the configuration in a repository that has the cluster as a remote.

For example:

    $ git-annex wanted bigserver-mycluster standard
    $ git-annex group bigserver-mycluster archive

By default, when a file is uploaded to a cluster, it is stored on every node of the cluster. To control which nodes to store to, the preferred content of each individual node can be configured.

For example, to balance content evenly across nodes:

    $ git-annex groupwanted bigserver-node balanced=bigserver-node
    $ git-annex group bigserver-node1 bigserver-node
    $ git-annex group bigserver-node2 bigserver-node
    $ git-annex group bigserver-node3 bigserver-node
    $ git-annex wanted bigserver-node1 groupwanted
    $ git-annex wanted bigserver-node2 groupwanted
    $ git-annex wanted bigserver-node3 groupwanted

It's also a good idea to configure the preferred content of the cluster's gateway. To avoid files redundantly being stored on the gateway (which remember, is not a node of the cluster), you might make it not want any files:

    $ git-annex wanted bigserver nothing

## special remotes as cluster nodes

Cluster nodes don't have to be regular git remotes. They can also be special remotes.

Even special remotes with `exporttree=yes` can be used as cluster nodes. Those also need to be configured with `annexobjects=yes` though. And, you will also need to configure `remote.name.annex-tracking-branch` to the branch that will trigger an update of the exported tree when it is pushed to the cluster gateway.

Let's set up a directory special remote as cluster node, with the "master" branch exported as a tree:

    $ git-annex initremote node4 type=directory directory=/media/disk3/repo exporttree=yes annexobjects=yes
    $ git config remote.node4.annex-tracking-branch master
    $ git config remote.node4.annex-cluster-node mycluster
    $ git-annex updatecluster

## adding additional gateways to a cluster

A cluster can have more than one gateway. One way to use this is to make a cluster that is distributed across several locations.

Suppose you have a datacenter in AMS, and one in NYC. There will be a gateway in each datacenter which provides access to the nodes there. And the gateways will relay data between each other as well.

Start by setting up the cluster in Amsterdam. The process is the same as in the previous section.

    AMS$ git-annex initcluster mycluster
    AMS$ git remote add node1 /media/disk1/repo
    AMS$ git remote add node2 /media/disk2/repo
    AMS$ git config remote.node1.annex-cluster-node mycluster
    AMS$ git config remote.node2.annex-cluster-node mycluster
    AMS$ git-annex updatecluster
    AMS$ git config annex.jobs cpus

Now in a clone of the same repository in NYC, add AMS as a git remote accessed with ssh:

    NYC$ git remote add AMS me@amsterdam.example.com:annex
    NYC$ git fetch AMS

Setting up the cluster in NYC is different, rather than using `git-annex initcluster` again (which would make a new, different cluster), we ask git-annex to extend the cluster from AMS:

    NYC$ git-annex extendcluster AMS mycluster

The rest of the setup process for NYC is the same, of course different nodes are added.

    NYC$ git remote add node3 /media/disk3/repo
    NYC$ git remote add node4 /media/disk4/repo
    NYC$ git config remote.node3.annex-cluster-node mycluster
    NYC$ git config remote.node4.annex-cluster-node mycluster
    NYC$ git-annex updatecluster
    NYC$ git config annex.jobs cpus

Finally, the AMS side of the cluster has to be updated, adding a git remote for NYC, and extending the cluster to there as well:

    AMS$ git remote add NYC me@nyc.example.com:annex
    AMS$ git-annex sync NYC
    NYC$ git-annex extendcluster NYC mycluster

A user can now add either AMS or NYC as a remote, and will have access to the entire cluster as either `AMS-mycluster` or `NYC-mycluster`.

    user$ git-annex move foo --to AMS-mycluster
    move foo (to AMS-mycluster...) ok

Looking at where files end up, all the nodes are visible, not only those served by the current gateway.

    user$ git-annex whereis foo
    whereis foo (4 copies)
        acfc1cb2-b8d5-8393-b8dc-4a419ea38183 -- cluster mycluster [AMS-mycluster]
        11ab09a9-7448-45bd-ab81-3997780d00b3 -- node4 [AMS-NYC-node4]
        36197d0e-6d49-4213-8440-71cbb121e670 -- node2 [AMS-node2]
        43652651-1efa-442a-8333-eb346db31553 -- node3 [AMS-NYC-node3]
        7fb5a77b-77a3-4032-b3e5-536698e308b3 -- node1 [AMS-node1]
    ok

Notice that remotes for cluster nodes have names indicating the path through the cluster used to access them. For example, "AMS-NYC-node3" is accessed via the AMS gateway, which then relays to NYC where node3 is located.

## considerations for multi-gateway clusters

When a cluster has multiple gateways, nothing keeps the git repositories on the gateways in sync. A branch pushed to one gateway will not be able to be pulled from another one. And gateways only learn about the locations of keys that are uploaded to the cluster via them. So in the example above, after an upload to AMS-mycluster, NYC-mycluster will only know that the key is stored in its nodes, but won't know that it's stored in nodes behind AMS.

So, it's best to have a single git repository that is synced with, or perhaps run git-annex-remotedaemon on each gateway to keep its git repository in sync with the other gateways.

When using special remotes with `exporttree=yes` as nodes, it's particularly important that pushes reach all the gateways, since the exported tree will only get updated when the annex-tracking-branch is pushed.

Clusters can be constructed with any number of gateways, and any internal topology of connections between gateways. But there must always be a path from any gateway to all nodes of the cluster, otherwise a key won't be able to be stored from, or retrieved from some nodes.

It's best to avoid there being multiple paths to a node that go via different gateways, since all paths will be tried in parallel when eg, uploading a key to the cluster.

A breakdown in communication between gateways will temporarily split the cluster. When communication resumes, some keys may need to be copied to additional nodes, and of course the git repositories will need to be pushed as well to get things back in sync.

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