prop/docs/plans/2026-04-14-duckdb-scores-storage.md

# DuckDB / Flat-File Propagation Scores Storage Plan

**Goal:** Replace Postgres-backed `propagation_scores` with per-`(band_mhz, valid_time)` on-disk files (DuckDB or raw binary), so the hot write path becomes a single file write instead of 475k Postgres rows through indexes + WAL.

**Status:** Speculative architectural alternative. Do not execute until the already-shipped Postgres wins (DELETE+COPY, skip-factors, UNLOGGED) have been measured in prod. If those three drop the scoring+upsert phase below ~60s per forecast hour, this rewrite is likely unnecessary.

---

## Why consider this

Each forecast-hour write to `propagation_scores` is:

- 475k rows (95k grid points × 5 bands)
- Passes through 4 indexes (PK UUID, unique tuple, valid_time, (band_mhz, valid_time))
- Even with DELETE+COPY and UNLOGGED, Postgres overhead — toast pages, page locks, vacuum state, MVCC tuple housekeeping — adds per-row cost that a flat array layout avoids entirely
- Shapes a dense numeric grid into a row-oriented representation the map never needs

The map render path also doesn't want individual rows. It wants a dense `(lat, lon) → score` array for a single `(band, valid_time)`. SQL is the wrong shape.

## Approach options

### Option A — Parquet files, read via DuckDB

One Parquet file per `(band_mhz, valid_time)`:

```
/srtm/scores/10000/2026-04-14T18-00-00Z.parquet
/srtm/scores/24000/2026-04-14T18-00-00Z.parquet
...
```

- Columns: `lat`, `lon`, `score` (plus `factors` only on f00 files)
- ~95k rows × 3 numeric columns + JSONB on f00 ≈ 1-2 MB per f00 file, ~400 KB per fh≥1 file
- Written once per forecast hour via Elixir → DuckDB WAL-less writer (`COPY ... TO 'file.parquet'`) or a Rust/Elixir native Parquet encoder
- Read on the map side via [`duckdbex`](https://hex.pm/packages/duckdbex) (Elixir NIF wrapper) or a Rust sidecar

Pros:
- Columnar compression: a single Parquet file per hour is ~300-500 KB compressed
- DuckDB can query across files: `SELECT score FROM 'scores/10000/*.parquet' WHERE lat BETWEEN ... AND ...`
- Files are portable — easy to debug, copy, inspect with `duckdb-cli`
- Pruning is `rm` — instant

Cons:
- Add a new dependency (`duckdbex` or a Rust NIF)
- Concurrent writers across multiple pods need file coordination (atomic rename, or a single writer pod)
- `factors` for f00 still needs somewhere to live — either embedded in the Parquet or a separate lookup file

### Option B — Raw binary mmap files

One file per `(band_mhz, valid_time)`, laid out as:

```
header: 16 bytes (magic + lat_min, lon_min, step, n_rows, n_cols)
scores: n_rows × n_cols × 1 byte (score 0-100)
```

- ~95k × 1 byte ≈ 95 KB per file uncompressed
- Written in Elixir via `File.write/2` on a binary blob
- Read on the map side via `:ets.new(:mmap)` / `File.read/1` cached per `(band, vt)`

Pros:
- Simplest possible layout
- Writes are ~20 ms per file
- No dependencies at all
- Matches the canvas heatmap's consumption shape exactly

Cons:
- Point-detail (single-cell lookup) needs an index by (lat, lon) — easy with the grid step but less ergonomic than SQL
- `factors` needs a separate store (another binary file, or keep it in Postgres)
- No ad-hoc query capability (can't ask "what was the score at 35.0/-97.0 two hours ago" without loading the file)

### Option C — DuckDB on persistent disk

Single `.duckdb` file under `/srtm/scores.duckdb`:

- One table with the same schema as `propagation_scores`
- DuckDB is columnar, compressed, and serializes writes via a single database file
- Writes via `INSERT INTO scores VALUES ...` batched
- Reads via direct SQL from Elixir

Pros:
- Least migration friction — keeps a SQL-ish API
- Columnar storage naturally compresses `score` columns
- DuckDB writes are dramatically faster than Postgres for bulk inserts (no WAL, no per-row overhead)

Cons:
- DuckDB is single-writer. Multi-pod clusters need to funnel writes through one pod (the propagation grid worker pod — which is exactly the worker-pod-isolation idea discussed earlier in this session)
- File corruption on crash: DuckDB has improved crash recovery but it's not Postgres-grade
- Requires `duckdbex` NIF

## Recommendation

If the already-shipped Postgres wins aren't enough, start with **Option B** (raw binary files). It's the lowest-risk, highest-impact change because:

1. The map's canvas heatmap *already* wants a dense score array — a binary file is a closer shape match than SQL rows
2. No new dependencies — `File.write/2` and `File.read/1` are enough
3. Concurrent write safety is a solved problem: write to a temp file, `File.rename/2` atomically
4. Reverting is easy: keep `propagation_scores` populated from the flat file asynchronously during a migration period

The `factors` storage question is the only thing that forces picking between Option A and Option B:

- If we're fine with **no factors for forecast hours** (already the case after the skip-factors change), factors for f00 stay in Postgres and the hot path is file-only. Option B works as-is.
- If we want factors for all hours, Option A (Parquet) is cleaner because factors can ride on the same file.

## Architectural prerequisites

Before this is worth doing:

1. **Separate worker pod**. Multi-writer coordination on a single file is painful; a single "propagation" pod that owns all writes is much simpler. The isolation idea from earlier in this session dovetails here.
2. **Shared volume or single-node**. Files need to live on a PVC shared between the worker and web pods, or the web pods need a cached-read path that pulls the file from the worker pod via HTTP. The existing `/srtm` NFS mount is already shared, which makes `/srtm/scores` free.
3. **Baseline perf measurements** from the already-shipped Postgres changes. Don't do this if the map already feels fast.

## Read-path changes

### Map render (bulk)

Current:
```elixir
Propagation.scores_at(band_mhz, valid_time, bounds)
# → list of %{lat, lon, score} maps from Postgres
```

New:
```elixir
Propagation.ScoresFile.load(band_mhz, valid_time)
# → %{lat_min, lon_min, step, rows, cols, data: <<binary>>}
```

The JS hook receives the binary directly (base64 or bytestring in an assign) and draws the canvas without an intermediate list traversal.

### Point detail (single-cell)

Current:
```elixir
Propagation.point_detail(band, lat, lon, valid_time)
# → %{score, factors, ...}
```

New:
```elixir
Propagation.ScoresFile.point(band_mhz, valid_time, lat, lon)
# → %{score}
Propagation.FactorsStore.for_point(lat, lon, valid_time)
# → %{factors} (still Postgres-backed, only f00)
```

### Scorer diff

Current: reads latest f00 rows with factors from Postgres. Unchanged — factors stay in Postgres.

### ScoreCache

Current: ETS cache of `(band, vt) → list-of-scores`. Unchanged — can wrap the file-load path the same way.

## Write-path changes

`PropagationGridWorker.run_chain_step/2` currently calls:

```elixir
scores = compute_scores(grid_data, valid_time, forecast_hour)
Propagation.replace_scores(scores, valid_time)
```

Replace with:

```elixir
scores = compute_scores(grid_data, valid_time, forecast_hour)
Propagation.ScoresFile.write!(band_mhz, valid_time, scores)
# Still write factors to Postgres for f00 only
if forecast_hour == 0, do: Propagation.replace_factors(factor_rows, valid_time)
```

Expected timings: ~10-50 ms per band per fh vs 4-5 seconds per band per fh today. The scoring CPU cost stays the same; only the storage phase collapses.

## Pruning

Current: 15-minute cron running `DELETE WHERE valid_time < now - 2h`.

New: 15-minute cron running `find /srtm/scores -mmin +120 -delete` (or Elixir equivalent).

Zero vacuum pressure, zero WAL, zero index maintenance.

## Migration path (if this gets greenlit)

1. **Commit 1**: Introduce `Propagation.ScoresFile` module. Dual-write: `replace_scores` still writes Postgres as today, but also writes the flat file. Map render still reads from Postgres. Verify file shape matches what the JS side wants.

2. **Commit 2**: Switch map-render read path to load from the flat file. Keep Postgres dual-write as a fallback. Measure wall-time improvement.

3. **Commit 3**: Stop writing to Postgres. Drop the `propagation_scores` table (or keep it empty for rollback). Point detail for forecast hours reads score from file; factors for f00 still hits a Postgres table (or a separate file).

Each commit is independently reversible.

## Risk / pushback

- **Added complexity** of a second storage system. Today: Postgres only. After: Postgres for QSOs + ASOS + weather + factors, plus a flat-file store for grid scores. The cognitive overhead is real.
- **Shared-volume assumptions** couple web and worker pods to the same PVC. Worth revisiting the separate-worker-pod idea first; if that's not on the table, the flat-file approach is harder because every web pod needs a consistent view of the files.
- **Queryability loss**. Today, ad-hoc SQL against `propagation_scores` works for debugging. After: need to boot a Python/DuckDB shell to query files.
- **Opportunity cost**. If the Postgres tuning already lands us in under 90 min per chain, this rewrite is 2-3 weeks of work for perhaps a 2× improvement on a phase that's already fast enough. The plan survives regardless — we can always pick it up if scale demands it (finer grid resolution, more forecast hours, more bands).

## When to revisit

Trigger conditions for picking this up:

- A single forecast-hour chain step still exceeds 5 min after all three Postgres wins are deployed
- A new feature requires 100m grid resolution (~8× more points → 4M rows per fh), which will blow through the Postgres wins immediately
- The Postgres pool becomes a bottleneck for other workers because `propagation` queue writes keep it saturated

Until one of those trips, the existing `propagation_scores` table + the shipped tuning is the right place to be.