A fiber-based supervisor that monitors the lifecycle of all fibers that are started via its interface. The lifecycles of all these spawned fibers are bound to the lifecycle of the Supervisor itself.

Whereas cats.effect.kernel.GenSpawn.background links the lifecycle of the spawned fiber to the calling fiber, starting a fiber via a Supervisor links the lifecycle of the spawned fiber to the supervisor fiber. This is useful when the scope of some fiber must survive the spawner, but should still be confined within some "larger" scope.

The fibers started via the supervisor are guaranteed to be terminated when the supervisor is terminated. When a supervisor is finalized, all active and queued fibers will be safely finalized before finalization of the supervisor is complete.

The following diagrams illustrate the lifecycle of a fiber spawned via cats.effect.kernel.GenSpawn.start, cats.effect.kernel.GenSpawn.background, and Supervisor. In each example, some fiber A is spawning another fiber B. Each box represents the lifecycle of a fiber. If a box is enclosed within another box, it means that the lifecycle of the former is confined within the lifecycle of the latter. In other words, if an outer fiber terminates, the inner fibers are guaranteed to be terminated as well.

start:

Fiber A lifecycle
+---------------------+
|                 |   |
+-----------------|---+
                  |
                  |A starts B
Fiber B lifecycle |
+-----------------|---+
|                 +   |
+---------------------+

background:

Fiber A lifecycle
+------------------------+
|                    |   |
| Fiber B lifecycle  |A starts B
| +------------------|-+ |
| |                  | | |
| +--------------------+ |
+------------------------+

Supervisor:

Supervisor lifecycle
+---------------------+
| Fiber B lifecycle   |
| +-----------------+ |
| |               + | |
| +---------------|-+ |
+-----------------|---+
                  |
                  | A starts B
Fiber A lifecycle |
+-----------------|---+
|                 |   |
+---------------------+

Supervisor should be used when fire-and-forget semantics are desired.