High Availability

Juju High Availability (HA) means that a Juju environment has 3 or more (up to 7) state servers (bootstrap nodes) one of which is the master. Automatic failover occurs should the master lose connectivity.

Juju HA and MongoDB

Juju HA is tightly integrated with the MongoDB database since that is where all environment data is stored. The MongoDB software has a native HA implementation and this is what Juju HA uses. A MongoDB cluster is called a replica set and, in the context of Juju HA, i) each of its members corresponds to a different Juju state server and ii) the MongoDB replica set master corresponds to the Juju state server master.

The number of state servers must be an odd number to prevent ties during voting for master, and the number of state servers cannot exceed 7, due to MongoDB limitations. This means a Juju HA cluster can have 3, 5 or 7 state servers.

Activating and modifying HA

Juju HA is activated and modified with the juju ensure-availability command. As will be shown in the next section, it is also used to recover from failed state servers.

When activating HA, by default, this command sets the number of state servers in the environment to 3. The optional -n switch can modify this number.

When modifying HA, the -n switch can be used to increase the number of state servers. The only way to decrease is to create a backup of your environment and then restore the backup to a new environment, which starts with a single state server. You can then increase to the desired number.

Whenever you run ensure-availability, the command will report the changes it intends to make, which will shortly be implemented.

For complete syntax, see the command reference page.

Recovering from state server failure

In the advent of failed state servers, Juju does not automatically re-spawn new state servers nor remove the failed ones. However, as long as more than half of the original number of state servers remain available you can manually recover. The process is detailed below.

  1. Run juju ensure-availability.
  2. Verify that the output of juju status shows a value of has-vote for the state-server-member-status attribute for each new server and a value of no-vote for each old server. Once confirmed, the new servers are fully operational as cluster members and the old servers have been demoted (no longer part of HA). This process can take between 30 seconds to 20 minutes depending on machine resources and Juju data volume.
  3. Run juju ensure-availability again to have Juju no longer consider the old machines as state servers. The state-server-member-status attribute should disappear from these machines.
  4. Use the juju remove-machine command to remove the old machines entirely.

You cannot repair the cluster as outlined above if fewer than half of the original state servers remain available because the MongoDB replica set will not have the quorum necessary to elect a new master. You must restore from backups in this case.

HA and logging

All Juju units send logs to all state servers in the HA cluster and the user accesses those logs in the usual manner, via juju debug-log or by viewing the logs directly. See Viewing logs.

Logging to a state server begins once it becomes fully operational. One caveat is that past cluster logs are not sent to the new "slave" state server. It should therefore be noted that all state servers are not guaranteed to house the same logs. In particular, this should be understood when using juju debug-log as this triggers a connection to be made to a random state server. This will be corrected in the near future (past logs will be synced).

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