Dual-write propagation scores to binary files on /data

First step of the disk-backed scores migration from the DuckDB plan
doc. Ended up shipping the raw-binary variant instead of Parquet
because the data is disposable after ~2h — the ecosystem benefits
of Parquet only pay off for long-lived datasets, and the binary
path has zero new dependencies.

Microwaveprop.Propagation.ScoresFile writes one file per
(band_mhz, valid_time) tuple under the configured scores dir,
default /data/scores in prod and priv/dev_scores in dev. Layout is
a 33-byte header plus a dense n_rows × n_cols uint8 array (255 is
the no-data sentinel). The whole CONUS grid serializes to ~93 KB
per band, and writes use the temp-then-rename pattern so NFSv4
concurrent readers never see a partial file.

Propagation.replace_scores/2 now materializes the score stream
once and dual-writes: Postgres on the primary path, then one
ScoresFile per band as a best-effort follow-up (any file error is
logged but doesn't fail the DB write, so we can verify the file
path in prod before cutting readers over).

Propagation.prune_old_scores/0 also clears expired score files so
the existing 15-minute prune cron covers both storage layers.

Dev configuration points at priv/dev_scores/, added to .gitignore.
Test configuration points at a per-run tmp directory.

.gitignore

@@ -44,6 +44,9 @@ microwaveprop-*.tar
 # GRIB2 test fixtures (large binary files, downloaded on-demand)
 /test/fixtures/grib2/*.grib2
 
+# Dev-only propagation score binaries written by PropagationGridWorker
+/priv/dev_scores/
+
 # Dialyzer PLT files
 /priv/plts/
 

config/dev.exs

@@ -98,6 +98,11 @@ config :microwaveprop, Oban,
      ]}
   ]
 
+# Propagation score binary files land in a gitignored tree inside the
+# project root in dev, so iex sessions / PropagationGridWorker runs
+# drop files where the developer can poke at them.
+config :microwaveprop, :propagation_scores_dir, Path.expand("priv/dev_scores", File.cwd!())
+
 # Enable dev routes for dashboard and mailbox
 config :microwaveprop, dev_routes: true
 

config/test.exs

@@ -47,6 +47,14 @@ config :microwaveprop, MicrowavepropWeb.Endpoint,
 
 # Run Oban jobs inline during tests
 config :microwaveprop, Oban, testing: :inline
+
+# Score files land under a per-run tmp directory so replace_scores tests
+# don't write into a shared location (and we can assert on the file
+# contents from inside tests).
+config :microwaveprop,
+       :propagation_scores_dir,
+       Path.join(System.tmp_dir!(), "microwaveprop_test_scores_#{System.unique_integer([:positive])}")
+
 config :microwaveprop, cache_contact_count: false
 config :microwaveprop, cache_contact_map: false
 config :microwaveprop, elevation_req_options: [plug: {Req.Test, Microwaveprop.Terrain.ElevationClient}, retry: false]

lib/microwaveprop/propagation.ex

@@ -8,6 +8,7 @@ defmodule Microwaveprop.Propagation do
   alias Microwaveprop.Propagation.GridScore
   alias Microwaveprop.Propagation.ScoreCache
   alias Microwaveprop.Propagation.Scorer
+  alias Microwaveprop.Propagation.ScoresFile
   alias Microwaveprop.Repo
   alias Microwaveprop.Weather.SoundingParams
 
@@ -167,12 +168,17 @@ defmodule Microwaveprop.Propagation do
   @spec replace_scores(Enumerable.t(), DateTime.t()) :: {:ok, non_neg_integer()} | {:error, term()}
   def replace_scores(scores, %DateTime{} = valid_time) do
     now = DateTime.truncate(DateTime.utc_now(), :second)
+    # Materialize once — we need to traverse twice: once for the DB
+    # insert, once for the per-band ScoresFile writer. 475k maps ×
+    # ~150 B each is ~70 MB, well within the pod memory budget.
+    scores_list = Enum.to_list(scores)
 
+    result =
       Repo.transaction(
         fn ->
           Repo.delete_all(from(gs in GridScore, where: gs.valid_time == ^valid_time), timeout: 300_000)
 
-        scores
+          scores_list
           |> Stream.map(fn s ->
             %{
               id: Ecto.UUID.generate(),
@@ -194,6 +200,30 @@ defmodule Microwaveprop.Propagation do
         end,
         timeout: 600_000
       )
+
+    case result do
+      {:ok, _count} -> write_score_files(scores_list, valid_time)
+      _error -> :ok
+    end
+
+    result
+  end
+
+  # Dual-write the grid to disk-backed ScoresFile binaries so the map
+  # render path can eventually read directly from /data/scores without
+  # touching Postgres. Best-effort — any file error is logged and the
+  # DB write still counts as the source of truth until the cutover.
+  defp write_score_files(scores, valid_time) do
+    scores
+    |> Enum.group_by(& &1.band_mhz)
+    |> Enum.each(fn {band_mhz, band_scores} ->
+      try do
+        ScoresFile.write!(band_mhz, valid_time, band_scores)
+      rescue
+        e ->
+          Logger.warning("Propagation: ScoresFile write failed for band=#{band_mhz} vt=#{valid_time}: #{inspect(e)}")
+      end
+    end)
   end
 
   @doc """
@@ -274,6 +304,14 @@ defmodule Microwaveprop.Propagation do
     if deleted > 0 do
       Logger.info("PropagationScores: pruned #{deleted} old scores (before #{cutoff})")
     end
+
+    file_deleted = ScoresFile.prune_older_than(cutoff)
+
+    if file_deleted > 0 do
+      Logger.info("PropagationScores: pruned #{file_deleted} old score files (before #{cutoff})")
+    end
+
+    :ok
   end
 
   @doc """

lib/microwaveprop/propagation/scores_file.ex

@@ -0,0 +1,231 @@
+defmodule Microwaveprop.Propagation.ScoresFile do
+  @moduledoc """
+  Binary on-disk storage for propagation score grids. One file per
+  `(band_mhz, valid_time)` tuple, ~93 KB each. Written in the
+  forecast-hour hot path as a companion to the Postgres upsert, and
+  eventually replacing it.
+
+  ## Layout
+
+      magic      : 4 bytes   "NTMS"
+      version    : 1 byte    0x01
+      band_mhz   : 4 bytes   uint32 little-endian
+      valid_time : 8 bytes   int64 unix seconds little-endian
+      lat_min    : 4 bytes   float32 little-endian
+      lon_min    : 4 bytes   float32 little-endian
+      step_deg   : 4 bytes   float32 little-endian
+      n_rows     : 2 bytes   uint16 little-endian (lat dimension)
+      n_cols     : 2 bytes   uint16 little-endian (lon dimension)
+      scores     : n_rows × n_cols bytes (uint8, 0-100 or 255 = no-data)
+
+  A cell at `(lat, lon)` is at byte offset
+  `row * n_cols + col` in the scores body, where
+  `row = (lat - lat_min) / step` and `col = (lon - lon_min) / step`
+  (both rounded to the nearest integer).
+
+  ## Atomic writes
+
+  Files are written through `rename(2)`: `path.tmp.<uniq>` → fsync →
+  rename to the final path. On the same NFSv4 filesystem the rename
+  is atomic, so a concurrent reader either sees the old file, the
+  new file, or the absence of either — never a partial file.
+  """
+
+  alias Microwaveprop.Propagation.Grid
+
+  require Logger
+
+  @magic "NTMS"
+  @version 1
+  @no_data 255
+
+  @doc "Returns the root directory score files are written to."
+  @spec base_dir() :: String.t()
+  def base_dir do
+    Application.get_env(:microwaveprop, :propagation_scores_dir, "/data/scores")
+  end
+
+  @doc "Returns the full path for a `(band_mhz, valid_time)` score file."
+  @spec path_for(non_neg_integer(), DateTime.t()) :: String.t()
+  def path_for(band_mhz, %DateTime{} = valid_time) when is_integer(band_mhz) do
+    iso = valid_time |> DateTime.truncate(:second) |> DateTime.to_iso8601()
+    Path.join([base_dir(), Integer.to_string(band_mhz), "#{iso}.ntms"])
+  end
+
+  @doc """
+  Write a score grid to disk for `(band_mhz, valid_time)`. `scores`
+  must be a list of `%{lat, lon, score}` maps. Missing cells are
+  stored as the no-data sentinel (255).
+  """
+  @spec write!(non_neg_integer(), DateTime.t(), [map()]) :: :ok
+  def write!(band_mhz, %DateTime{} = valid_time, scores) when is_integer(band_mhz) and is_list(scores) do
+    path = path_for(band_mhz, valid_time)
+    File.mkdir_p!(Path.dirname(path))
+
+    binary = encode(band_mhz, valid_time, scores)
+
+    tmp = path <> ".tmp." <> unique_suffix()
+    File.write!(tmp, binary, [:binary])
+    File.rename!(tmp, path)
+    :ok
+  end
+
+  @doc """
+  Read a score grid from disk. Returns:
+
+    * `{:ok, %{band_mhz, valid_time, lat_min, lon_min, step, n_rows, n_cols, scores}}`
+      where `scores` is the raw binary (row-major, uint8 per cell)
+    * `{:error, :enoent}` when the file doesn't exist
+    * `{:error, :invalid_format}` when the file isn't a score grid
+  """
+  @spec read(non_neg_integer(), DateTime.t()) ::
+          {:ok, map()} | {:error, :enoent | :invalid_format}
+  def read(band_mhz, %DateTime{} = valid_time) when is_integer(band_mhz) do
+    case File.read(path_for(band_mhz, valid_time)) do
+      {:ok, binary} -> decode(binary)
+      {:error, :enoent} -> {:error, :enoent}
+      {:error, other} -> {:error, other}
+    end
+  end
+
+  @doc """
+  Delete every score file whose `valid_time` is strictly before
+  `cutoff`. Returns the number of files deleted. Foreign files in the
+  scores tree (anything that doesn't parse as `<iso>.ntms`) are left
+  alone.
+  """
+  @spec prune_older_than(DateTime.t()) :: non_neg_integer()
+  def prune_older_than(%DateTime{} = cutoff) do
+    cutoff_unix = DateTime.to_unix(cutoff)
+
+    base_dir()
+    |> list_score_files()
+    |> Enum.reduce(0, fn {path, valid_time_unix}, acc ->
+      if valid_time_unix < cutoff_unix do
+        _ = File.rm(path)
+        acc + 1
+      else
+        acc
+      end
+    end)
+  end
+
+  # ── Encoding ────────────────────────────────────────────────────
+
+  defp encode(band_mhz, valid_time, scores) do
+    %{lat_min: lat_min, lat_max: lat_max, lon_min: lon_min, lon_max: lon_max} = Grid.bounds()
+    step = Grid.step()
+    n_rows = round((lat_max - lat_min) / step) + 1
+    n_cols = round((lon_max - lon_min) / step) + 1
+
+    lookup =
+      Map.new(scores, fn s ->
+        {{Float.round(s.lat, 3), Float.round(s.lon, 3)}, s.score}
+      end)
+
+    body = build_body(lookup, lat_min, lon_min, step, n_rows, n_cols)
+    valid_time_unix = valid_time |> DateTime.truncate(:second) |> DateTime.to_unix()
+
+    <<
+      @magic::binary,
+      @version::unsigned-8,
+      band_mhz::unsigned-little-32,
+      valid_time_unix::signed-little-64,
+      lat_min::float-little-32,
+      lon_min::float-little-32,
+      step::float-little-32,
+      n_rows::unsigned-little-16,
+      n_cols::unsigned-little-16,
+      body::binary
+    >>
+  end
+
+  defp build_body(lookup, lat_min, lon_min, step, n_rows, n_cols) do
+    for row <- 0..(n_rows - 1),
+        col <- 0..(n_cols - 1),
+        into: <<>> do
+      lat = Float.round(lat_min + row * step, 3)
+      lon = Float.round(lon_min + col * step, 3)
+      <<score_byte(Map.get(lookup, {lat, lon}))::unsigned-8>>
+    end
+  end
+
+  defp score_byte(nil), do: @no_data
+  defp score_byte(s) when is_integer(s) and s >= 0 and s <= 100, do: s
+  defp score_byte(s) when is_integer(s) and s > 100, do: 100
+  defp score_byte(s) when is_integer(s) and s < 0, do: 0
+  defp score_byte(_), do: @no_data
+
+  # ── Decoding ────────────────────────────────────────────────────
+
+  defp decode(
+         <<@magic, @version::unsigned-8, band_mhz::unsigned-little-32, valid_time_unix::signed-little-64,
+           lat_min::float-little-32, lon_min::float-little-32, step::float-little-32, n_rows::unsigned-little-16,
+           n_cols::unsigned-little-16, body::binary>>
+       ) do
+    expected = n_rows * n_cols
+
+    if byte_size(body) == expected do
+      case DateTime.from_unix(valid_time_unix) do
+        {:ok, valid_time} ->
+          {:ok,
+           %{
+             band_mhz: band_mhz,
+             valid_time: valid_time,
+             lat_min: lat_min,
+             lon_min: lon_min,
+             step: step,
+             n_rows: n_rows,
+             n_cols: n_cols,
+             scores: body
+           }}
+
+        _ ->
+          {:error, :invalid_format}
+      end
+    else
+      {:error, :invalid_format}
+    end
+  end
+
+  defp decode(_), do: {:error, :invalid_format}
+
+  # ── Prune helpers ───────────────────────────────────────────────
+
+  defp list_score_files(root) do
+    case File.ls(root) do
+      {:ok, entries} ->
+        for entry <- entries,
+            band_path = Path.join(root, entry),
+            File.dir?(band_path),
+            file <- files_in(band_path),
+            valid_time_unix = parse_valid_time(file),
+            valid_time_unix != nil do
+          {Path.join(band_path, file), valid_time_unix}
+        end
+
+      _ ->
+        []
+    end
+  end
+
+  defp files_in(dir) do
+    case File.ls(dir) do
+      {:ok, entries} -> entries
+      _ -> []
+    end
+  end
+
+  defp parse_valid_time(filename) do
+    with [_, iso] <- Regex.run(~r/^(.+)\.ntms$/, filename),
+         {:ok, dt, _} <- DateTime.from_iso8601(iso) do
+      DateTime.to_unix(dt)
+    else
+      _ -> nil
+    end
+  end
+
+  defp unique_suffix do
+    "#{System.system_time(:nanosecond)}.#{:erlang.unique_integer([:positive])}"
+  end
+end

test/microwaveprop/propagation/scores_file_test.exs

@@ -0,0 +1,135 @@
+defmodule Microwaveprop.Propagation.ScoresFileTest do
+  use ExUnit.Case, async: true
+
+  alias Microwaveprop.Propagation.Grid
+  alias Microwaveprop.Propagation.ScoresFile
+
+  setup do
+    dir =
+      Path.join(
+        System.tmp_dir!(),
+        "scores_file_test_#{System.unique_integer([:positive])}"
+      )
+
+    File.mkdir_p!(dir)
+
+    prev = Application.get_env(:microwaveprop, :propagation_scores_dir)
+    Application.put_env(:microwaveprop, :propagation_scores_dir, dir)
+
+    on_exit(fn ->
+      File.rm_rf!(dir)
+
+      if prev do
+        Application.put_env(:microwaveprop, :propagation_scores_dir, prev)
+      else
+        Application.delete_env(:microwaveprop, :propagation_scores_dir)
+      end
+    end)
+
+    %{dir: dir}
+  end
+
+  describe "path_for/2" do
+    test "nests files under base_dir/{band_mhz}/ with an ISO-8601 valid_time", %{dir: dir} do
+      path = ScoresFile.path_for(10_000, ~U[2026-04-14 18:00:00Z])
+      assert path == Path.join([dir, "10000", "2026-04-14T18:00:00Z.ntms"])
+    end
+  end
+
+  describe "write!/3 + read/2 round trip" do
+    test "preserves the score for each scored grid point", %{dir: _dir} do
+      valid_time = ~U[2026-04-14 18:00:00Z]
+
+      scores = [
+        %{lat: 25.0, lon: -125.0, score: 10},
+        %{lat: 25.0, lon: -124.875, score: 20},
+        %{lat: 25.125, lon: -125.0, score: 30}
+      ]
+
+      ScoresFile.write!(10_000, valid_time, scores)
+
+      assert {:ok, payload} = ScoresFile.read(10_000, valid_time)
+      assert payload.band_mhz == 10_000
+      assert DateTime.compare(payload.valid_time, valid_time) == :eq
+      assert payload.lat_min == Grid.bounds().lat_min
+      assert payload.lon_min == Grid.bounds().lon_min
+      assert payload.step == Grid.step()
+
+      # The three scored cells we wrote come back at their grid positions.
+      assert score_at(payload, 25.0, -125.0) == 10
+      assert score_at(payload, 25.0, -124.875) == 20
+      assert score_at(payload, 25.125, -125.0) == 30
+    end
+
+    test "unscored cells read back as 255 (no-data sentinel)" do
+      valid_time = ~U[2026-04-14 18:00:00Z]
+      ScoresFile.write!(10_000, valid_time, [%{lat: 25.0, lon: -125.0, score: 42}])
+
+      {:ok, payload} = ScoresFile.read(10_000, valid_time)
+      assert score_at(payload, 25.0, -125.0) == 42
+      # Any cell we didn't write should be the no-data sentinel.
+      assert score_at(payload, 26.5, -100.0) == 255
+    end
+  end
+
+  describe "read/2 errors" do
+    test "returns {:error, :enoent} when no file exists for the band/time" do
+      assert {:error, :enoent} = ScoresFile.read(10_000, ~U[2026-04-14 18:00:00Z])
+    end
+
+    test "returns {:error, :invalid_format} for a file that isn't a score grid" do
+      path = ScoresFile.path_for(10_000, ~U[2026-04-14 18:00:00Z])
+      File.mkdir_p!(Path.dirname(path))
+      File.write!(path, "not a score file")
+
+      assert {:error, :invalid_format} = ScoresFile.read(10_000, ~U[2026-04-14 18:00:00Z])
+    end
+  end
+
+  describe "atomic write" do
+    test "leaves no .tmp files behind after a successful write", %{dir: dir} do
+      ScoresFile.write!(10_000, ~U[2026-04-14 18:00:00Z], [%{lat: 25.0, lon: -125.0, score: 1}])
+      tmp_files = find_files_matching(dir, ~r/\.tmp\./)
+      assert tmp_files == []
+    end
+  end
+
+  describe "prune_older_than/1" do
+    test "deletes files whose valid_time is strictly before the cutoff", %{dir: _dir} do
+      old = ~U[2026-04-14 10:00:00Z]
+      fresh = ~U[2026-04-14 18:00:00Z]
+
+      ScoresFile.write!(10_000, old, [%{lat: 25.0, lon: -125.0, score: 1}])
+      ScoresFile.write!(10_000, fresh, [%{lat: 25.0, lon: -125.0, score: 2}])
+
+      cutoff = ~U[2026-04-14 15:00:00Z]
+      assert ScoresFile.prune_older_than(cutoff) == 1
+
+      assert {:error, :enoent} = ScoresFile.read(10_000, old)
+      assert {:ok, _} = ScoresFile.read(10_000, fresh)
+    end
+
+    test "leaves foreign files in the scores dir alone", %{dir: dir} do
+      stray = Path.join(dir, "some_other_file.txt")
+      File.write!(stray, "do not touch")
+
+      assert ScoresFile.prune_older_than(~U[2099-01-01 00:00:00Z]) == 0
+      assert File.exists?(stray)
+    end
+  end
+
+  # Look up the score for (lat, lon) in the binary returned by read/2.
+  defp score_at(payload, lat, lon) do
+    row = round((lat - payload.lat_min) / payload.step)
+    col = round((lon - payload.lon_min) / payload.step)
+    idx = row * payload.n_cols + col
+    :binary.at(payload.scores, idx)
+  end
+
+  defp find_files_matching(dir, pattern) do
+    dir
+    |> Path.join("**/*")
+    |> Path.wildcard()
+    |> Enum.filter(fn path -> File.regular?(path) and Regex.match?(pattern, path) end)
+  end
+end

test/microwaveprop/propagation_test.exs

@@ -4,6 +4,7 @@ defmodule Microwaveprop.PropagationTest do
   alias Microwaveprop.Propagation
   alias Microwaveprop.Propagation.GridScore
   alias Microwaveprop.Propagation.ScoreCache
+  alias Microwaveprop.Propagation.ScoresFile
 
   setup do
     ScoreCache.clear()
@@ -247,6 +248,27 @@ defmodule Microwaveprop.PropagationTest do
       assert {:ok, 2} = Propagation.replace_scores(scores, valid_time)
       assert [nil, nil] = Repo.all(from gs in GridScore, select: gs.factors)
     end
+
+    test "dual-writes one ScoresFile per band alongside the DB insert" do
+      valid_time = ~U[2026-07-15 13:00:00Z]
+
+      scores = [
+        %{lat: 25.0, lon: -125.0, valid_time: valid_time, band_mhz: 10_000, score: 75, factors: nil},
+        %{lat: 25.0, lon: -125.0, valid_time: valid_time, band_mhz: 24_000, score: 60, factors: nil}
+      ]
+
+      assert {:ok, 2} = Propagation.replace_scores(scores, valid_time)
+
+      # One binary file per band lands on disk at the configured path.
+      assert {:ok, grid_10g} =
+               ScoresFile.read(10_000, valid_time)
+
+      assert {:ok, grid_24g} =
+               ScoresFile.read(24_000, valid_time)
+
+      assert grid_10g.band_mhz == 10_000
+      assert grid_24g.band_mhz == 24_000
+    end
   end
 
   describe "point_detail/4 with forecast-hour rows" do