# Gerrit multi-site plugin

This plugin allows to deploy a distributed cluster of multiple Gerrit masters
each using a separate site without sharing any storage. The alignment between
the masters happens using the replication plugin and an external message broker.

Requirements for the Gerrit masters are:

- Gerrit v2.16.5 or later
- Migrated to NoteDb
- Connected to the same message broker
- Accessible via a load balancer (e.g. HAProxy)

**NOTE**: The multi-site plugin will not start if Gerrit is not yet migrated
to NoteDb.

Currently, the only mode supported is one primary read/write master
and multiple read-only masters but eventually the plan is to support multiple
read/write masters. The read/write master is handling any traffic while the
read-only masters are serving the Gerrit GUI assets, the HTTP GET REST API and
git fetch requests (git-upload-pack). The read-only masters are kept synchronized
with the read/write master in order to be always ready to become a read/write master.

For more details on the overall multi-site design and roadmap, please refer
to the [multi-site plugin DESIGN.md document](DESIGN.md)

## License

This plugin is released under the same Apache 2.0 license and copyright holders
as of the Gerrit Code Review project.

## How to build

The multi-site plugin is built like any other Gerrit plugin in tree mode, by cloning
Gerrit and the multi-site plugin code, and checking them out on the desired branch.

Example of cloning Gerrit and multi-site for a stable-2.16 build:

```
git clone -b stable-2.16 https://gerrit.googlesource.com/gerrit
git clone -b stable-2.16 https://gerrit.googlesource.com/plugins/multi-site

cd gerrit/plugins
ln -s ../../multi-site .
rm external_plugin_deps.bzl
ln -s multi-site/external_plugin_deps.bzl .
```

Example of building the multi-site plugin:

```
cd gerrit
bazel build plugins/multi-site
```

The multi-site.jar plugin is generated to `bazel-genfiles/plugins/multi-site/multi-site.jar`.

Example of testing the multi-site plugin:

```
cd gerrit
bazel test plugins/multi-site:multi_site_tests
```

**NOTE**: The multi-site tests include also the use of Docker containers for
instantiating and using a Kafka/Zookeeper broker. Make sure you have a Docker
daemon running (/var/run/docker.sock accessible) or a DOCKER_HOST pointing to
a Docker server.

## How to configure

Install the multi-site plugin into the `$GERRIT_SITE/lib` directory of all
the Gerrit servers that are part of the multi-site cluster.

Add the multi-site module to `$GERRIT_SITE/etc/gerrit.config` as follows:

```
[gerrit]
  installModule = com.googlesource.gerrit.plugins.multisite.Module
```

Create the `$GERRIT_SITE/etc/multi-site.config` on all Gerrit servers with the
following basic settings:

```
[kafka]
  bootstrapServers = <kafka-host>:<kafka-port>

[kafka "publisher"]
  enabled = true

[kafka "subscriber"]
  enabled = true

[ref-database]
  enabled = true

[ref-database "zookeeper"]
  connectString = "localhost:2181"
```

For more details on the configuration settings, please refer to the
[multi-site configuration documentation](src/main/resources/Documentation/config.md).

You also need to setup the Git-level replication between nodes, for more details
please refer to the
[replication plugin documentation](https://gerrit.googlesource.com/plugins/replication/+/refs/heads/master/src/main/resources/Documentation/config.md).