A generator is a kind of activity that generates events within an existing workflow instance, even when this activity is not scheduled.

An example of a generator is the Timer activity.
In Elsa 2, the Timer activity will tick only once when used as a blocking activity.

In Elsa 3, the Timer activity will invoke the workflow periodically until the workflow is explicitly transitioned into the Finished state or the activity is deactivated.

Hot vs Cold

By default, generators are "hot". The user should be able to change its initial state as "cold", which means it will become "hot" only once workflow execution enters the generator for the first time (or when marked hot programmatically or via some activity).

When "hot", the generator will continuously (depending on the nature of the generator) generate events / execute the workflow instance, until the generator is deactivated (turning it "cold" again). Or, as mentioned, when the workflow is explicitly completed (e.g. using the Finish activity.