# Propagation pipeline runbook

The hourly propagation grid flows across two runtimes and a shared
NFS volume. This doc names every failure mode at that seam and says
how the system recovers.

```
prop-grid-rs (worker nodes)      Postgres (10.0.15.30)            prop pods (node1/2/3)
 ├─ fetch GRIB2 (NOMADS)         ├─ grid_tasks (SKIP LOCKED)     ├─ ScoreCache (ETS, capped LRU)
 ├─ decode                       ├─ LISTEN/NOTIFY channel        ├─ NotifyListener
 ├─ score 22 bands × 92k         │    "propagation_ready"        ├─ Lazy disk read on miss
 ├─ write /data/scores/.../.prop │                               └─ PubSub → LiveView
 └─ NOTIFY propagation_ready ────┘
```

## Primary refresh path (happy case)

1. Rust completes `{band, valid_time}` compute and writes the `.prop`
   file via atomic rename. (Readers also accept legacy `.ntms` files
   with `NTMS` magic during a rolling deploy.)
2. Rust emits `NOTIFY propagation_ready '<iso valid_time>'`.
3. `NotifyListener` on each Elixir pod receives it, prunes
   ScoreCache entries outside the active forecast window, and
   broadcasts `propagation_updated` over PubSub.
4. LiveView clients see the broadcast and re-request scores. The
   first request per `{band, valid_time}` lazy-fills `ScoreCache`
   from the on-disk `.prop` file (~50–100 ms); subsequent requests
   hit ETS until the LRU evicts.

Latency budget end-to-end: < 2 s on the warm path; < 200 ms added
on the first lazy fill.

## Failure modes

### FM1 — NOTIFY dropped on reconnect

**Symptom.** `{band, valid_time}` files land on disk but
LiveView clients don't auto-refresh until the user interacts.

**Why.** Postgres queues LISTEN notifications per-connection and
discards any that are unread on disconnect. A DB restart, a network
blip, or the Postgrex.Notifications connection hitting its idle
timeout all drop whatever queued notifies were in flight.

**Recovery.** Lazy fill: the next `/map` request reads the on-disk
`.prop` file directly via `Propagation.scores_at_fresh/3`. Stale
ETS entries get evicted by NotifyListener's `prune_outside_window`
on the next NOTIFY that does land.

### FM2 — NFS stale read racing Rust write

**Symptom.** `ScoresFile.read/2` returns `{:error, reason}` when
the reader opens the file exactly between Rust's `rename(2)` of the
`.tmp.<uniq>` file and the `fsync` landing on the NFS server.

**Why.** The rename is atomic on the same NFSv4 filesystem, but the
NFS client cache can briefly serve a stale directory listing or a
no-longer-existing path.

**Recovery.** LiveView callers retry on the next interaction; the
read settles after NFS consistency converges (sub-second).

### FM3 — Cluster partition

**Symptom.** Pods on one side of a cluster partition don't see
the other side's `propagation_updated` PubSub messages.

**Why.** Phoenix.PubSub fans messages over the BEAM cluster mesh.
A libcluster partition stops the message reaching partitioned peers.

**Recovery.** Each pod reads scores from the shared NFS mount on
demand, so partitions only delay LiveView refresh — they never cause
divergent answers.

### FM4 — Rust worker dead

**Symptom.** `grid_tasks` rows with `status = 'queued'` accumulate;
no new `.prop` files land; `/map` serves the last successful run's
scores.

**Why.** Deployment bug, Rust panic, OOM on a worker node, or
disk-full on `/data`.

**Recovery.** Manual. `kubectl -n prop logs deploy/prop-grid-rs` for
cause. The Elixir side is read-only against the grid — it does not
regenerate scores from within the BEAM since the extraction in
`65693ed`. `lib/microwaveprop/workers/propagation_grid_worker.ex`
still exists as a seed worker: the hourly cron fires it with empty
args and it inserts 19 `grid_tasks` rows (f00 + f01..f18) for Rust
to drain. All fetch/decode/score helpers moved to
`rust/prop_grid_rs/src/pipeline.rs`; the Elixir module is retained
only as the cron entry point, not as a fallback compute path.

## Monitoring signals

- `avg_over_time(beam_memory_total_bytes[1h])` on prop pods — steady
  climb above 1 GiB means the ScoreCache LRU cap isn't holding (see
  `ScoreCache.max_entries/0`).
- `microwaveprop.propagation.scores_at.cache.count{hit=...}` — if the
  miss rate is consistently above ~30 % the LRU cap may be too small
  for the active session count.
- `grid_tasks` queue depth in Prometheus — should drain to 0 between
  hourly cron ticks.

## Test coverage

- `test/microwaveprop/propagation/score_cache_test.exs` covers the
  LRU eviction path.
- Partition recovery (FM3) is untested in CI — requires multi-node
  cluster and libcluster orchestration.
