Top-level engine supervisor. Started by the host (Oban-style) as {GenDurable, opts} in their own supervision tree.

Options

• :name — the instance identity (an atom, default GenDurable). Everything an instance owns — its supervisor registration, task supervisor, config entry, FSM registry — is keyed by it, so several engines (e.g. different repos, or disjoint queue sets) coexist; address API calls with name:. Starting two instances with the same name fails with :already_started.
• :repo — the host's Ecto.Repo (required).
• :fsms — FSM modules to register explicitly (default []). Only needed for a custom :name or to keep an old :version running; otherwise a machine is resolved from the fsm column (its module name). See GenDurable.Registry.
• :queues — keyword list of queue_name => concurrency (default [default: 10]).
• :lease_ttl, :heartbeat_interval, :poll_interval, :reap_interval — timings in ms (Balanced defaults: 60_000 / 20_000 / 1_000 / 30_000).
• :gc_interval — ms between GC sweeps of terminal rows (default 60_000). Set to nil to disable GC entirely (terminal rows then accumulate).
• :gc_retention — ms a done/failed row is kept after it terminates before GC may delete it (default 86_400_000, i.e. 1 day).
• :gc_batch — max rows deleted per GC sweep (default 10_000).
• :prefetch — extra rows each queue claims and buffers beyond its running slots (default 0 ⇒ no over-fetch). See GenDurable.Scheduler.
• :min_demand — batch gate for the picker (default 1).
• :max_poll_interval — idle-backoff ceiling in ms (default 5_000).
• :drain_timeout — on shutdown, how long (ms) each queue waits for its in-flight steps to finish before giving up to the reaper (default 5_000). Buffered (un-started) rows are released immediately regardless.
• :rate_limits — named token-bucket rate limits (default []), e.g. [stripe: [allowed: 100, period: {1, :minute}], emails: [allowed: 5, period: 60, burst: 10]]. allowed/period set the sustained rate; burst (default allowed) the capacity. A step opts into a limit by returning rate_limit: :stripe (or {:stripe, key}).

:prefetch, :min_demand, and :max_poll_interval are the feeder aggressiveness knobs and apply to every queue; see GenDurable.Scheduler for the trade-offs (DB chatter vs. latency vs. cross-node fairness).