In Juju, an application unit is a running instance of the application. Simple applications may be deployed with a single application unit, but it is possible for an individual application to have multiple units running on different machines. All units for a given application share the same charmed operator code, the same relations, and the same user-provided configuration but one of them called the leader unit is different and can be responsible for managing the lifecycle of the application.

For example, one may deploy a single MongoDB application, and specify that it should run three units (with one machine per unit) so that the replica set is resilient to failures. Internally, even though the replica set shares the same user-provided configuration, each unit may be performing different roles within the replica set, as defined by the charmed operator.

The following diagram represents the scenario described above. For simplicity, the agents have been omitted:


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