# `StatifierOban.Invoke.Delivery.Session`
[🔗](https://github.com/riddler/statifier_oban/blob/v0.17.3/lib/statifier_oban/invoke/delivery/session.ex#L1)

The default `StatifierOban.Invoke.Delivery`: reports a completed
invoke back into a live `Statifier.Session` through
`Statifier.Session.done_invocation/3`, and a permanently failed one
through `Statifier.Session.failed_invocation/3`, both behind the same
two-step liveness check `StatifierOban.Timer.Delivery.Session` runs
for a fired timer.

The two steps run in order because `Statifier.Session.status/1` is a
`GenServer.call` that exits the caller against a dead process - it
cannot itself answer "terminated?":

1. **Terminated?** `Registry.lookup/2` against `Statifier.Registry`
   under the scope (st-ADR-0027 decision 2: sessions register under
   their session id). Empty means discard: the session no longer
   exists, or never registered, and `status/1` is not safe to call.
2. **Halted?** With a live process confirmed, `status/1`. Anything but
   `:running` discards too - "live" is stricter than "not terminated":
   a `:done` session keeps its process but declines to drain, so an
   event fed to it would sit queued forever, not be processed and not
   be discarded. The process exiting between the two steps is caught
   and reported as the same `:terminated` fact step 1 answers.

Delivery is `Statifier.Session.done_invocation/3` - the door st-ADR-0051
decision 5 gives a handler-backed invocation's host. The session builds
`done.invoke.<invoke_id>` from `donedata` itself, stamps `invokeid`, and
runs `<finalize>` off the arriving event; an invocation the session
already popped (a prior cancel, or a redelivered completion) is a
harmless no-op there, which is what makes this deliverable at least
once. `done_invocation/3` is a cast, so `:delivered` means enqueued
onto the session's inbox, not processed.

Failure delivery is `Statifier.Session.failed_invocation/3`, the door
st-ADR-0068 adds beside it, and everything above holds unchanged: the
session builds `error.communication.invoke.<invoke_id>` from the
keyword list itself, stamps the same `invokeid`/`origin`/`origintype`,
pops the same invocation, and casts. Reusing one liveness check for
both doors is deliberate rather than incidental - st-ADR-0068 makes
the failure event travel `done_invocation/3`'s own delivery path
upstream, so a check that diverged here would be this package
contradicting the contract it implements.

## Why only the three-argument doors

This module implements `c:StatifierOban.Invoke.Delivery.deliver/3` and
`c:StatifierOban.Invoke.Delivery.deliver_failure/3`, and deliberately
not their four-argument forms. Those exist to hand a delivery the
invocation's `caller_context` off the job row, for a host that builds
the answer event itself. This one builds nothing: `done_invocation/3`
and `failed_invocation/3` construct the event inside the session and
inherit the slot from the session's own invocation table (st-ADR-0063).
Accepting the row's copy here would mean either ignoring it - a door
that lies about what it does with its argument - or contradicting the
session's own record of the invocation. A process-less host is the one
with no such table, and `StatifierOban.Invoke.Delivery` documents the
wider arities for it.

This module requires `Statifier.Supervisor` (which owns
`Statifier.Registry`) to be running: `Registry.lookup/2` raises when it
is not, and the job retries rather than discards - a missing registry
is an environment fact about the host, not a liveness fact about the
execution. The scope must therefore be the session id the session
registered under (`ctx.session_id`); a host using any other scope owes
its own `StatifierOban.Invoke.Delivery` implementation.

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*Consult [api-reference.md](api-reference.md) for complete listing*