<a id="howto-recover"></a>

# How to recover a MicroCloud cluster

#### NOTE
Each MicroCloud service uses the [Dqlite](https://canonical.com/dqlite) distributed
database for highly-available storage. While the cluster recovery process is
similar for each service, this document only covers cluster recovery for the
`microcloudd` daemon. For cluster recovery procedures for LXD, MicroCeph and
MicroOVN, see:

- [How to recover a LXD cluster](https://canonical.com/microcloud/docs/default/lxd/howto/cluster_recover/#cluster-recover)
- [MicroOVN Launchpad Bug](https://bugs.launchpad.net/microovn/+bug/2072377)
- [MicroCeph Issue](https://github.com/canonical/microceph/issues/380)

MicroCloud requires a majority of the database voters (a quorum) to be
accessible in order to perform database operations. If a cluster has less than
a quorum of voters up and accessible, then database operations will no longer
be possible on the entire cluster.

If the loss of quorum is temporary (for example, some members temporarily lose
power), database operations will be restored when the offline members come back
online.

This document describes how to recover database access if the offline members
have been lost without the possibility of recovery (for example, disk failure).

## Recovery procedure

1. Shut down all cluster members before performing cluster recovery:
   ```default
   sudo snap stop microcloud
   ```
2. Once all cluster members are shut down, determine which Dqlite database is
   most up to date. Look for files in `/var/snap/microcloud/common/state/database`
   whose filenames are two numbers separated by a dash (i.e.
   `0000000000056436-0000000000056501`). The largest second number in that directory
   is the end index of the most recently closed segment (56501 in the example).
   The cluster member with the highest end index is the most up to date.
3. On the most up-to-date cluster member, use the following command to
   reconfigure the Dqlite roles for each member:
   ```default
   sudo microcloud cluster recover
   ```
4. As indicated by the output of the above command, copy
   `/var/snap/microcloud/common/state/recovery_db.tar.gz` to the same path on
   each cluster member.
5. Restart MicroCloud. The recovered database tarball will be loaded on daemon
   startup. Once a quorum of voters have been started, the MicroCloud database
   will become available.
   ```default
   sudo snap start microcloud
   ```

## Backups

MicroCloud creates a backup of the database directory before performing the
recovery operation to ensure that no data is lost. The backup tarball is created
in `/var/snap/microcloud/common/state/`. If the cluster recovery operation
fails, use the following commands to restore the existing database:

```default
cd /var/snap/microcloud/common/state
sudo mv database broken_db
sudo tar -xf db_backup.TIMESTAMP.tar.gz
```