Resolve "DR: document process for bring a demoted primary back online"

doc/gitlab-geo/README.md

@@ -191,6 +191,10 @@ Read through the [Geo High Availability documentation](ha.md).
 When you have object storage enabled, please consult the
 [Geo with Object Storage](object_storage.md) documentation.
 
+## Restore demoted primary geo node
+
+Read how to [Bring a demoted primary back](bring-primary-back.md)
+
 ## Replicating the Container Registry
 
 Read how to [replicate the Container Registry](docker_registry.md).

doc/gitlab-geo/bring-primary-back.md

+## Bring a demoted primary back online
+
+After a failover, it is possible to fail back to the demoted primary to restore your original configuration.
+This process consists of two steps: making old primary a secondary and promoting secondary to a primary.
+
+### Configure the former primary to be a secondary
+
+Since the former primary will be out of sync with the current primary, the first
+step is to bring the former primary up to date. There is one downside though, some uploads and repositories
+that have been deleted during an idle period of a primary node, will not be deleted from the disk but the overall sync will be much faster. As an alternative, you can set up a [GitLab instance from scratch](https://docs.gitlab.com/ee/gitlab-geo/#setup-instructions) to workaround this downside.
+
+1. SSH into the former primary that has fallen behind.
+1. Make sure all the services are up by running the command
+
+    ```bash
+    sudo gitlab-ctl start
+    ```
+
+Note: If you [disabled primary permanently](https://docs.gitlab.com/ee/gitlab-geo/disaster-recovery.html#step-2-permanently-disable-the-primary), you need to undo those steps now. For Debian/Ubuntu you just need to run `sudo systemctl enable gitlab-runsvdir`. For CentoOS 6, you need to install GitLab instance from scratch and setup it as a secondary node by following [Setup instructions](https://docs.gitlab.com/ee/gitlab-geo/#setup-instructions). In this case you don't need the step below.
+1. [Setup the database replication](database.md). In this documentation, primary
+   refers to the current primary, and secondary refers to the former primary.
+
+If you have lost your original primary, follow the
+[setup instructions](README.md#setup-instructions) to set up a new secondary.
+
+### Promote the secondary to primary
+
+When initial replication is complete and the primary and secondary are closely in sync you can do a [Planned Failover](planned-failover.md)
+
+### Restore the secondary node
+
+If your objective is to have two nodes again, you need to bring your secondary node back online as well by repeating the first step ([Make primary a secondary](#make-primary-a-secondary)) for the secondary node.

doc/gitlab-geo/database.md

@@ -348,6 +348,7 @@ because we have not yet configured the secondary server. This is the next step.
     ```
 
     For external PostgreSQL instances, [see additional instructions][external postgresql].
+    If you bring a former primary back online to serve as a secondary then you also need to remove `geo_primary_role['enable'] = true`.
 
 1. Reconfigure GitLab for the changes to take effect: