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Not every process you depend on is a clean OTP gen_server. Sometimes you must supervise a plain Erlang process, a port, or a third-party subsystem. The supervisor_bridge behaviour lets such a process appear as a normal child in a supervision tree.

A standard supervisor only knows how to start, stop and restart OTP-compliant children. A raw process started with spawn_link does not respond to OTP shutdown protocols. The bridge translates between the two worlds.

A supervisor_bridge module implements two callbacks:

• init/1 starts the real subsystem and returns its pid
• terminate/2 cleanly shuts the subsystem down

The bridge is started like a supervisor but behaves like a worker to its parent.

init/1 must start and link the external process, then return {ok, Pid, State}. If start fails, return {error, Reason}.

When the bridge is asked to shut down, it calls terminate/2 so you can stop the subsystem gracefully.

To its parent supervisor, the bridge is just another child spec with type => worker.

Because the bridge links to the external process, a crash there exits the bridge, which the parent supervisor sees as a normal worker crash and restarts according to its strategy.

A real system mixes gen_server workers, sub-supervisors, and bridges. The bridge lets legacy or foreign code participate without weakening the OTP guarantees of the rest of the tree.

If you can wrap the subsystem in a proper gen_server, prefer that — it gives you full OTP introspection. Reserve the bridge for code you cannot rewrite or that has its own process lifecycle.

Because the inner process is not OTP, tools like sys:get_state/1 will not reach it. Add logging inside the subsystem and rely on the bridge state to track the inner pid.

You learned to integrate foreign processes into supervision trees:

• supervisor_bridge wraps non-OTP subsystems
• init/1 starts and links the subsystem; terminate/2 stops it
• It appears as a worker child to its parent supervisor
• Crashes propagate so normal restart strategies apply
• Prefer a real gen_server when you control the code