# T6.1 — Challenger allocation

| Field | Value |
|---|---|
| Phase | P6 — Learning loop |
| Size | M — 1 to 3 days |
| Status | Not started |
| Flags | — |
| Spec | inlined below |
| Blocks | T6.3 |

## Goal

One current version, at most one challenger, 95/5 traffic split, guarded by a
generation counter with compare-and-swap.

## Facts (inlined — no spec read needed)

- **One current version, at most one challenger.** The loop converges toward a
  single state rather than maintaining a population. That is the change that
  makes everything else affordable: no N-way traffic split, no per-variant
  aggregation across groups, no allocation state to contend over, one comparison
  per episode instead of a bracket.
- The cost is exploration. A single challenger at a time is hill-climbing — it
  finds improvements more slowly than a population would, and it can sit in a
  local optimum indefinitely with nothing in the loop able to say so. The
  held-out report (T6.6) is the only instrument that will notice, which makes it
  **more** important here, not less.
- **The decide stage is single-writer per `(TenantId, WorkflowId)`.** Two
  schedulers adjusting traffic allocation concurrently produce an allocation
  neither holds. A compare-and-swap on a generation counter is sufficient; no
  lock service is needed at this size.
- The slow generate loop fires when the fast loop **rejects a challenger without
  finding a replacement** — a trigger, not a timer.
- Multi-variant selection stays available for deployments that enable the
  tournament and allow N challengers; the machinery is the same.

## Steps

1. Define the allocation record: `(TenantId, WorkflowId) → { current: WorkflowVersion,
   challenger: Option<WorkflowVersion>, split, generation: u64 }`.
2. Registration of a challenger is rejected when one is already live — the error
   **names the incumbent challenger**, so the operator knows what to retire.
3. Every update is a CAS on `generation`. The loser retries against the winner's
   generation rather than overwriting.
4. Route spawns by the split: 95% current, 5% challenger. Record which version a
   run was allocated to on the spawn event.
5. Emit the "rejected without replacement" trigger as an event so the generate
   loop can subscribe. Do not add a timer.
6. Test two concurrent schedulers committing allocations.

## Acceptance

- Registering a second challenger while one is live is **rejected, naming the
  incumbent challenger**.
- Two concurrent schedulers cannot both commit an allocation; the loser retries
  and observes the winner's generation.

## Verify

**Harness:** two scheduler instances against one backend — real concurrency, not
a simulated race.

**Integration test** — `tests/it_challenger_allocation.rs`:
1. Register challenger A. Register challenger B while A is live; assert
   **rejected**, and that the error **names A**.
2. **Concurrent CAS:** two schedulers commit different allocations
   simultaneously, in a loop of 100 rounds. Assert exactly one wins per round,
   the loser retries, and the final generation equals the number of successful
   commits — no lost updates.
3. Traffic split: spawn 1000 runs; assert the challenger share is within
   tolerance of 5% and that each run's spawn event **records which version it was
   allocated to**.
4. Assert control appears in every allocation, so T6.4's anchor exists.
5. Trigger: reject a challenger with no replacement; assert the generate-loop
   trigger event fires **once**. Assert no timer path exists that would fire it
   otherwise — run 60s of simulated time with no rejection and assert zero
   triggers.
6. Restart mid-decision; assert the allocation and generation survive.

**Command:** `cargo test -p loop allocation -- --test-threads=1`

**False pass:**
- Step 2 simulated by calling the update function twice sequentially. Last-write-
  wins passes that and loses an allocation under real concurrency. Use two live
  schedulers.
- Step 3 asserting the split ratio without asserting the per-run recorded
  version. Runs allocated correctly but recorded wrongly are unattributable, and
  the ratio still looks right.
- Step 5's negative half omitted, so a timer-driven generate loop passes.

## Traps

- Last-write-wins on the allocation record. The result is a split neither
  scheduler intended and no error anywhere.
- A timer-driven generate loop. It proposes challengers when nothing has been
  learned.

---

Background (not required to do this task):
[rust-agentic-sys.md](../../../rust-agentic-sys.md) §9.1, §12.1 ·
[rust-agentic-task.md](../../../rust-agentic-task.md)