Reneging Customers

Impatient customers leave the queue if not served within their patience window. The reference model for fork — spawning a second puck that shares data with the first — and for racing conditions where two processes compete to determine an entity's outcome.

Problem

Exponential arrivals at a single barber, exponential service, plus a third random variable: per-customer patience, Uniform(2, 8) minutes. If the customer has not started service within that window, it abandons the queue (reneges) and leaves. The model counts served vs reneged, reports average wait among served, and average time-until-abandon among reneged.

This is one of the two SLX-style patterns that cannot be expressed in pure GPSS without extensions — the other being PREEMPT. GPSS transactions don't fork, and there is no clean way to say "wait for A or B". SLX introduced fork and wait until (C₁ or C₂) specifically for this case; odin-des inherits both.

Model in this directory

Two puck types, explicitly linked:

• Customer (reneging.odin:67-136) — the primary puck. Three active states (Arriving / Waiting / In_Service). Carries two boolean flags, reneged and in_service, which are read by the timeout puck.
• Timeout_Puck (reneging.odin:161-206) — the forked offspring. One-shot: sleeps patience, fires once, terminates. At fire time it either finds the customer already in service (stands down silently) or sets customer.reneged = true and reactivates the customer.

Shared state lives on the Customer struct; the Timeout_Puck holds a ^Customer pointer and reads those fields directly. No Notify_Var, no channel — the fork pattern is fields + a pointer.

Gen_Process (reneging.odin:222-254) is the generator. Two separate pools — customer_pool and timeout_pool — keep the heterogeneous puck types in sized-right slabs.

Why this shape

Two pucks, one decision, one winner. The customer and its timeout are racing: whichever event fires first wins. The pattern:

• Customer tries sim.seize. If it succeeds immediately, no fork is needed — the customer is already winning. Fast path out.
• If seize fails, customer is suspended in the facility's wait set. The customer forks a timeout puck and returns.
• Two things can now happen:
  1. Barber releases → facility wakes customer (now owning barber, in_service not yet true). Customer falls through .Waiting and calls begin_service, which sets in_service = true before any yield.
  2. Timeout fires first → sets reneged = true, reactivates customer. Customer wakes in .Waiting, sees the flag, removes itself from the facility's wait set, records stats, terminates.

The timeout puck, on fire, checks in_service — if the customer has already started, the timeout is moot and stands down. This clean-stand-down path is the critical correctness property: without it, a served customer could be hit by a stale timeout and marked reneged after the fact.

Shared fields, not channels. The reneged and in_service flags live on the Customer struct and are read/written across two pucks without synchronization. This is safe because the engine is single-threaded: between any two puck ticks there is no interleaving. The flags work as plain data; fork does not need locks, queues, or signals.

sim.seize is used here, unlike the barbershop. The customer goes into the facility's internal wait set (reneging.odin:92). The reneging logic needs to remove a suspended puck from that set on timeout (reneging.odin:105) — sim.set_remove on barber.waiters directly. A model-owned wait list (barbershop style) would work equally, but the example is short enough that reaching into the facility's waiters is both acceptable and illustrative of when such access is legitimate: when you're modeling leaving the queue, you have to be able to remove yourself from it, and the facility's set is the canonical place to do that.

The timeout is a separate puck, not a scheduled event. One could imagine adding a "deadline" field to the customer puck and having the engine fire a special callback at that time. We don't — making the timeout a real puck means:

• It appears in engine.processes and on the heap like any other work.
• The engine's stop-on-quiescence check accounts for it.
• Tracing and inspection treat it uniformly.
• The pattern generalizes: a customer can fork any number of timeout, heartbeat, or watchdog pucks with the same machinery.

Patience is currently deterministic-pseudo-random. The model computes patience as 2.0 + 6.0 * f64(id % 100) / 100.0 (reneging.odin:176) — a note-to-self placeholder for a proper Uniform(2, 8) draw from an sdidb stream. This is the kind of detail per-example READMEs are good at flagging: the code works and the model demonstrates the pattern, but the RNG hygiene isn't production-grade. A one-line swap to db.d_uniform(&patience_rng, 5.0, 3.0) would clean it up and match the RNG discipline of the other examples.

Alternatives considered

sim.wait_until with a compound predicate

SLX writes reneging as:

wait until (barber available or reneged);

We could mirror that with a Notify_Var on barber availability and another on reneged-flag, and a sim.wait_until predicate over both. It works, but the Notify_Var/predicate machinery is heavier than the fork pattern for a one-shot race: the predicate allocator runs, the Waiter registers on two vars, and on signal the waiter chain is walked. Fork-with-flags is one advance, one reactivate, and two field reads. For this model, fork is the cleaner hammer.

The wait_until variant becomes the right shape when the predicate is naturally compound — "queue non-empty AND shift active AND not on break" — because then you'd otherwise fork N timeouts, one per condition, and have to join them manually.

One combined puck with a post-advance check

We could forgo the fork and have the customer advance(patience) after joining the wait list, then check on wake-up whether it landed in service or timed out. This is almost right and fails subtly: if the barber frees while the customer is mid-advance, the release mechanism would need to cancel the advance and wake early. eng.advance has no cancel; the engine would deliver a stale tick at arrival_time + patience regardless. Fork is the way to get a cancelable timeout.

Timeout as a calendar event, no puck

The engine exposes a calendar — raw scheduled events with a handler proc, no puck involved. A reneging timeout could be a calendar event: cheaper allocation, no per-timeout pool. engine/calendar.odin supports this. We don't use it here because the example is deliberately showing Henriksen's fork vocabulary — fork_timeout is pedagogically the right name and shape. A production variant aiming for millions of short-lived timeouts might well use the calendar directly.

Patience per-class

If reneging thresholds varied by customer class (VIP waits longer, walk-ins leave faster), patience would be drawn from a class-keyed distribution. One Random_Spec per class, stored on the shared Shop struct, is all that's needed. The fork pattern itself doesn't change.

What this example teaches

This is the reference for:

• The fork pattern — a second puck sharing data with a first via fields + pointer, no channels or signals
• Racing to determine outcome (seize race vs patience timer) and the in_service / reneged flag protocol that disambiguates
• Heterogeneous puck pools (customer_pool + timeout_pool) sized-right per type
• Cancelable delays expressed as "timeout puck + check-on-fire" rather than engine-level cancel
• When to use sim.seize (and reach into barber.waiters) vs the model-owned wait list pattern

CLI

./reneging [options]

Add --json to emit the uniform envelope (metadata, execution_stats, metrics, details) instead of the default text output.

Flag	Type	Default	Description
--json	bool	false	Emit uniform JSON envelope instead of text.

Example runs:

./reneging           # default text run
./reneging --json    # uniform envelope

Inter-arrival and service distributions come from examples-db-text-files/barbershop.txt; edit that file to vary them. Patience is hardcoded — see the note above.

Running it

odin run examples/reneging

Default output is a per-event trace (verbose by design for pedagogy) and a summary block: served %, reneged %, average time in system for each cohort.

See also

• Modeling guide §2 — Henriksen's vocabulary (fork is the non-trivial one this example exercises)
• Modeling guide §4.5 — wait_until as the alternative compound-condition shape
• coffee_shop — uses wait_until for a compound hold; contrast with fork-with-flags here
• docs/wait-until-and-helpers.md — the full technical reference on Notify_Var and wait_until
• Henriksen, An Introduction to SLX — original exposition of the fork pattern and wait until compound waits