# m4p2 — Session Signal Engine (✅ COMPLETE 2026-02-21)

Phase spec and acceptance criteria: [ROADMAP · Milestone 4 · Phase 2](../ROADMAP.md).
Milestone index: [README.md](README.md). Backfilled record.

## What shipped

1. **`session_signal()` write path** (`tidal/src/db/sessions.rs`).
   `db.session_signal(&handle, signal_type, entity_id, weight, timestamp,
   Option<annotation>)` validates the session is open, evaluates policy
   (m4p3), folds the weight into the per-signal-type accumulator, bumps the
   session counters, and journals `WalCommand::SessionSignal`
   (`tidal/src/wal/writer.rs:109`).
2. **`SessionHotState` — reuse, not a second decay implementation**
   (`tidal/src/session/signal_state.rs`). Session scores delegate the running
   arithmetic to the canonical `forward_decay_step` kernel, the same kernel
   `HotSignalState` uses, so the two tiers cannot drift
   (`CODING_GUIDELINES.md` §3). Session decay is deliberately aggressive:
   `DEFAULT_SESSION_LAMBDA = ln(2)/300s` — a five-minute half-life, matching
   session timescales rather than the multi-day content half-lives.
   Score/timestamp/count are three atomics updated by a CAS loop; the struct is
   intentionally *not* cache-line padded, because sessions are not on the
   200-entity hot ranking path.
3. **Windowed counters per session signal type.** Each
   `SessionSignalState` owns a `BucketedCounter::with_start_time(now_ns)`, so a
   snapshot reports a 1-hour window count alongside the decayed score.
4. **Snapshot read model** (`tidal/src/session/snapshot.rs`).
   `db.session_snapshot(session_id) -> SessionSnapshot` carries
   `signals_written`, `signals_rejected`, `overrides_rejected`, `duration_ms`,
   `metadata`, timestamped `annotations`, `reward_velocity`,
   `signaled_entities`, the `audit_log` + `audit_truncated` flag, per-signal-type
   `signals: HashMap<String, SignalSnapEntry>`, and `started_at_ns` /
   `closed_at_ns`. Active sessions decay lazily to wall-clock read time;
   archived sessions are frozen at `close_session`.
5. **Isolation by construction.** Session signals never touch the global item
   ledger, the user preference vector, or the interaction-weight ledger. A
   session's influence is read-time only, via the m4p4 `FOR SESSION` path.
6. **Bounded memory, stated in constants** (`tidal/src/session/audit.rs`):
   `MAX_ANNOTATIONS = 100`, `MAX_AUDIT_ENTRIES = 10_000`,
   `MAX_CLOSED_SESSIONS = 10_000` with `EVICT_BATCH_SIZE = 1_000`, plus a
   `MAX_SIGNALED_ENTITIES` structural cap on the distinct-entity set. The
   entity cap is independent of `max_signals_per_session` (which may be `0` =
   unlimited) so an adversarial session cannot grow the boost set without bound;
   past the cap, new distinct entities are dropped and the boost degrades
   gracefully instead of the process growing.

## Evidence

| Criterion | Proof |
|-----------|-------|
| Accepted writes update counts and audit | `m4_uat.rs::step3_session_signal_and_audit` |
| Annotations captured, entity recorded in `signaled_entities` | `m4_uat.rs::step6_session_annotations_and_snapshot` |
| Archived snapshot readable after close | `m4_uat.rs::step7_closed_session_snapshot` |
| Archived snapshot readable after close **and reopen** | `session_durability.rs::archived_session_readable_after_close_and_reopen` |
| Per-signal windowed counts surface in the snapshot | `session_durability.rs::per_signal_snapshot_shows_windowed_counts` |
| Annotation timestamps preserved; annotations survive crash | `session_durability.rs::annotation_timestamps_preserved`, `annotations_survive_crash` |
| WAL replay reproduces the accumulators exactly | `session_durability.rs::wal_replay_restores_signal_counts_exactly`, `wal_replay_reproduces_identical_window_1h` |
| Two sessions never see each other's entities | `m4_uat.rs::step9_session_isolation` |

## Divergence from the plan

- **Where the decay constant lives.** The ROADMAP framed session decay as
  per-signal-type schema decay reused verbatim. Shipped behaviour is a single
  session-tier lambda (`DEFAULT_SESSION_LAMBDA`, 5-minute half-life) captured at
  `SessionSignalState` construction — one knob for the whole session tier rather
  than per-type specs.
- **The performance criteria have a harness but no recorded numbers.** The
  ROADMAP asserts `session_signal` < 200 µs, `session_snapshot` < 50 µs, and
  50,000 session signals/s "(benchmarked)". The harness is real —
  `tidal/benches/session.rs` (added one commit later by `192c473`) benches
  exactly `session_signal`, `session_snapshot_100_signals`, and
  `retrieve_1k_items/{without_session,with_session}` — but no run of it is
  recorded anywhere in `docs/`, unlike the M11/M12 numbers in
  `docs/profiling/`. So the functional criteria are proven by the tests above;
  the three µs/throughput figures are asserted, measurable on demand
  (`cargo bench -p tidaldb --bench session`), and **not currently evidenced**.

## Fixed later, worth recording

The out-of-order arm of the session decay update originally clamped `dt` to zero
and folded a late weight in at full value, silently over-crediting stale
activity. That is why `SessionHotState::on_signal` now delegates to
`forward_decay_step`: the kernel folds the *pre-decayed* weight
(`weight × exp(-λ·age)`) and refuses to regress `last_update_ns`. The
divergence, and the reason the kernel exists, are documented in the doc comment
at `tidal/src/session/signal_state.rs`.