# 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.

Supports multiple read/write masters across multiple sites across different
geographic locations. The Gerrit nodes are kept synchronized
between each other using the replication plugin and a global ref-database in
order to detect and prevent split-brains.

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 can only be built in tree mode, by cloning
Gerrit and the multi-site plugin code, and checking them out on the desired branch.

To build the multi-site plugin in addition to the gerrit core plugins also the
following plugins need to be present in the plugins directory:

- pull-replication
- healthcheck
- events-broker
- global-refdb

Example of cloning Gerrit, the multi-site plugin and the plugins it depends on
for a stable-3.11 build:

```
git clone --recurse-submodules -b stable-3.11 https://gerrit.googlesource.com/gerrit
git clone -b stable-3.11 https://gerrit.googlesource.com/plugins/multi-site
git clone -b stable-3.11 https://gerrit.googlesource.com/modules/events-broker
git clone -b stable-3.11 https://gerrit.googlesource.com/modules/global-refdb
git clone -b stable-3.11 https://gerrit.googlesource.com/plugins/healthcheck
git clone -b stable-3.11 https://gerrit.googlesource.com/plugins/pull-replication

cd gerrit/plugins
ln -s ../../multi-site .
ln -s ../../events-broker .
ln -s ../../global-refdb .
ln -s ../../healthcheck .
ln -s ../../pull-replication .
```

Example of building the multi-site plugin:

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

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

Example of testing the multi-site plugin:

```
cd gerrit
bazelisk test plugins/multi-site/...
```

**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. In addition docker-compose needs to be installed.

## Pre-requisites

Each Gerrit server of the cluster must be identified with a globally unique
[instance-id](https://gerrit-documentation.storage.googleapis.com/Documentation/3.4.5/config-gerrit.html#gerrit.instanceId)
defined in `$GERRIT_SITE/etc/gerrit.config`.
When migrating from a multi-site configuration with Gerrit v3.3 or earlier,
you must reuse the instance-id value stored under `$GERRIT_SITE/data/multi-site`.

Example:

```
[gerrit]
  instanceId = 758fe5b7-1869-46e6-942a-3ae0ae7e3bd2
```

## How to configure

Install the multi-site plugin into the `$GERRIT_SITE/lib` directory of all
the Gerrit servers that are part of the multi-site cluster.
Create a symbolic link from `$GERRIT_SITE/lib/multi-site.jar` into the
`$GERRIT_SITE/plugins`.

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

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

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.
This can be done with either [pull](https://gerrit.googlesource.com/plugins/pull-replication/+/refs/heads/master/src/main/resources/Documentation/config.md) or [push](https://gerrit.googlesource.com/plugins/replication/+/refs/heads/master/src/main/resources/Documentation/config.md) replication plugin.

# HTTP endpoints

For information about available HTTP endpoints please refer to
the [documentation](src/main/resources/Documentation/http-endpoints.md).