# Praefect

NOTE: **Note:** Praefect is an experimental service, and for testing purposes only at
this time.

## Omnibus

### Architecture

For this document, the following network topology is assumed:

```mermaid
graph TB
  GitLab --> Gitaly;
  GitLab --> Praefect;
  Praefect --> Preafect-Git-1;
  Praefect --> Preafect-Git-2;
  Praefect --> Preafect-Git-3;
```

Where `GitLab` is the collection of clients that can request Git operations.
`Gitaly` is a Gitaly server before using Praefect. The Praefect node has two
storage nodes attached. Praefect itself doesn't store data, but connects to
three Gitaly nodes, `Praefect-Git-1`,  `Praefect-Git-2`, and `Praefect-Git-3`.
There should be no knowledge other than with Praefect about the existence of
the `Praefect-Git-X` nodes.

### Setup

In this setup guide, the Gitaly node will be added first, then Praefect, and
lastly we update the GitLab configuration.

#### Gitaly

In their own machine, configure the Gitaly server as described in the
[gitaly documentation](index.md#3-gitaly-server-configuration).

#### Praefect

Next, Praefect has to be enabled on its own node. Disable all other services,
and add each Gitaly node that will be connected to Praefect. In the example below,
the Gitaly nodes are named `praefect-git-X`. Note that one node is designated as
primary, by setting the primary to `true`:

```ruby
# /etc/gitlab/gitlab.rb

# Avoid running unnecessary services on the Gitaly server
postgresql['enable'] = false
redis['enable'] = false
nginx['enable'] = false
prometheus['enable'] = false
unicorn['enable'] = false
sidekiq['enable'] = false
gitlab_workhorse['enable'] = false
gitaly['enable'] = false

praefect['enable'] = true
praefect['storage_nodes'] = [
  {
    'storage' => 'praefect-git-1',
    'address' => 'tcp://praefect-git-1.internal',
    'primary' => true
  },
  {
    'storage' => 'praefect-git-2',
    'address' => 'tcp://praefect-git-2.internal'
  },
  {
    'storage' => 'praefect-git-3',
    'address' => 'tcp://praefect-git-3.internal'
  }
]
```

Save the file and [reconfigure Praefect](../restart_gitlab.md#omnibus-gitlab-reconfigure).

#### GitLab

When Praefect is running, it should be exposed as a storage to GitLab. This
is done through setting the `git_data_dirs`. Assuming the default storage
configuration is used, there would be two storages available to GitLab:

```ruby
git_data_dirs({
  "default" => {
    "gitaly_address" => "tcp://gitaly.internal"
  },
  "praefect" => {
    "gitaly_address" => "tcp://praefect.internal:2305"
  }
})
```

Restart GitLab using `gitlab-ctl restart` on the GitLab node.