src/main/resources/Documentation/about.md

This plugin allows making Gerrit highly available by having redundant Gerrit
active/passive instances.

The Gerrit instances must be:

* sharing the git repositories using a shared file system (e.g. NFS)
* behind a load balancer (e.g. HAProxy)

Currently, the mode supported is one active instance and multiple backup
(passive) instances but eventually the plan is to support `n` active instances.
In the active/passive mode, the active instance is handling all traffic while
the passives are kept updated to be always ready to take over.

Even if git repositories are shared by the instances, there are a few areas
of concern in order to be able to switch traffic between instances in a
transparent manner from the user's perspective. The 4 areas of concern are
things that Gerrit stores either in memory or locally in the review site:

* caches
* secondary indexes
* stream-events
* web sessions

They need either to be shared or kept local to each instances but synchronized.
This plugin needs to be installed in all the instances and it will take care of
sharing or synchronizing them.

#### Caches
Every time a cache eviction occurs in one of the instances, the eviction will be
forwarded the other nodes so their caches do not contain stale entries.

#### Secondary indexes
Every time the secondary index is modified in one of the instances, e.g., a change
is added, updated or removed from the index, the others instances index are updated
accordingly. This way, both indexes are kept synchronized.

#### Stream events
Every time a stream event occurs in one of the instances
(see [more events info](https://gerrit-review.googlesource.com/Documentation/cmd-stream-events.html#events)),
the event is forwarded to the other instances which re-plays it. This way, the output
of the stream-events command is the same, no matter which  instance a client is
connected to.

#### Web session
The built-in Gerrit H2 based web session cache is replaced with a file based
implementation that is shared amongst the instances.

## Setup

Prerequisites:

* Git repositories must be located on a shared file system
* A directory on a shared file system must be available for @PLUGIN@ to use

For the instances:

* Configure gerrit.basePath in gerrit.config to the shared repositories location
* Configure gerrit.serverId in gerrit.config based on [config](config.md)'s introduction
* Install and configure this @PLUGIN@ plugin [further](config.md) or based on below.

Here is an example of the minimal @PLUGIN@.config:

Active instance

```
[main]
  sharedDirectory = /directory/accessible/from/both/nodes

[peerInfo "static"]
  url = http://backupNodeHost1:8081/

[http]
  user = username
  password = password
```

Backup instance

```
[main]
  sharedDirectory = /directory/accessible/from/both/nodes

[peerInfo "static"]
  url = http://primaryNodeHost:8080/

[http]
  user = username
  password = password
```

### HA replica site

It is possible to create a copy of the instance site and configure both
sites to run in HA mode as peers. This is possible when the directory where
the copy will be created is accessible from this machine. Such a replica site
can be created by means of a gerrit [site init](../../../Documentation/pgm-init.html) step,
contributed by the plugin under its init section.

This init step is optional but defaults to creating the replica. If you want to
create the other site manually, or if the other site needs to be created in a
directory not accessible from this machine, then please skip that init step.

For further information and supported options, refer to [config](config.md)
documentation.