Persist telemetry data in local sqlite database

compose.yaml

@@ -24,6 +24,7 @@ services:
       - POOL_SIZE
       - PORT
       - SECRET_KEY_BASE
+      - TELEMETRY_DATABASE_PATH
       - TIMEZONE
     volumes:
       - /data/coolify/applications/music-library:/mnt/music_library

config/config.exs

@@ -14,7 +14,7 @@ config :elixir, :time_zone_database, TimeZoneInfo.TimeZoneDatabase
 config :time_zone_info, update: :daily
 
 config :music_library,
-  ecto_repos: [MusicLibrary.BackgroundRepo, MusicLibrary.Repo],
+  ecto_repos: [MusicLibrary.BackgroundRepo, MusicLibrary.Repo, MusicLibrary.TelemetryRepo],
   generators: [timestamp_type: :utc_datetime, binary_id: true]
 
 config :music_library, default_timezone: "Europe/London"
@@ -112,6 +112,10 @@ config :oban_met, sketch_time_unit: :millisecond
 
 config :music_library, MusicLibrary.BackgroundRepo, priv: "priv/background_repo"
 
+config :music_library, MusicLibrary.TelemetryRepo, priv: "priv/telemetry_repo", log: false
+
+config :music_library, MusicLibraryWeb.Telemetry.Storage, buffer_size: 1024
+
 config :swoosh, :api_client, Swoosh.ApiClient.Req
 
 config :music_library, MusicLibrary.Mailer,

config/dev.exs

@@ -14,6 +14,12 @@ config :music_library, MusicLibrary.BackgroundRepo,
   stacktrace: true,
   show_sensitive_data_on_connection_error: true
 
+config :music_library, MusicLibrary.TelemetryRepo,
+  database: Path.expand("../data/music_library_telemetry_dev.db", __DIR__),
+  pool_size: 3,
+  stacktrace: true,
+  show_sensitive_data_on_connection_error: true
+
 # For development, we disable any cache and enable
 # debugging and code reloading.
 #

config/runtime.exs

@@ -117,6 +117,19 @@ if config_env() == :prod do
     pool_size: String.to_integer(System.get_env("POOL_SIZE") || "5"),
     show_sensitive_data_on_connection_error: false
 
+  telemetry_database_path =
+    System.get_env("TELEMETRY_DATABASE_PATH") ||
+      raise """
+      environment variable TELEMETRY_DATABASE_PATH is missing.
+      For example: /etc/music_library/music_library_telemetry.db
+      """
+
+  config :music_library, MusicLibrary.TelemetryRepo,
+    database: telemetry_database_path,
+    cache_size: -4000,
+    pool_size: 2,
+    show_sensitive_data_on_connection_error: false
+
   # The secret key base is used to sign/encrypt cookies and other secrets.
   # A default value is used in config/dev.exs and config/test.exs but you
   # want to use a different value for prod and you most likely don't want

config/test.exs

@@ -21,6 +21,15 @@ config :music_library, MusicLibrary.BackgroundRepo,
   pool_size: 32,
   pool: Ecto.Adapters.SQL.Sandbox
 
+config :music_library, MusicLibrary.TelemetryRepo,
+  database:
+    Path.expand(
+      "../data/music_library_telemetry_test#{System.get_env("MIX_TEST_PARTITION")}.db",
+      __DIR__
+    ),
+  pool_size: 5,
+  pool: Ecto.Adapters.SQL.Sandbox
+
 # We don't run a server during test. If one is required,
 # you can enable the server option below.
 config :music_library, MusicLibraryWeb.Endpoint,

docs/architecture.md

@@ -7,8 +7,8 @@
 ## Overview
 
 Phoenix LiveView application for managing a personal music collection and wishlist.
-Uses SQLite (via `ecto_sqlite3`) with two databases: one for the app, one for background
-jobs (Oban). All schemas use `binary_id` primary keys.
+Uses SQLite (via `ecto_sqlite3`) with three databases: one for the app, one for background
+jobs (Oban), and one for telemetry metrics. All schemas use `binary_id` primary keys.
 
 Key capabilities:
 - Browse/search collected and wishlisted records
@@ -31,8 +31,9 @@ MusicLibrary.Application (one_for_one)
 ├── MusicLibrary.Vault           # Cloak encryption vault
 ├── MusicLibrary.Repo            # Main SQLite repo
 ├── MusicLibrary.BackgroundRepo  # Oban SQLite repo (separate DB)
+├── MusicLibrary.TelemetryRepo   # Telemetry metrics SQLite repo
 ├── MusicLibraryWeb.Telemetry    # Telemetry supervisor
-│   ├── Telemetry.Storage        # Metrics storage (circular buffer)
+│   ├── Telemetry.Storage        # Metrics storage (SQLite-backed, persistent)
 │   └── :telemetry_poller        # 30s periodic measurements
 ├── Oban                         # Background job engine
 ├── Ecto.Migrator                # Auto-migration on boot
@@ -52,6 +53,7 @@ MusicLibrary.Application (one_for_one)
 |------|---------------|---------|
 | `MusicLibrary.Repo` | `data/music_library_dev.db` | All application data |
 | `MusicLibrary.BackgroundRepo` | `data/music_library_background_dev.db` | Oban job queue |
+| `MusicLibrary.TelemetryRepo` | `data/music_library_telemetry_dev.db` | Telemetry metrics history (persistent across restarts) |
 
 SQLite extensions loaded at runtime: `unicode`, `vec0` (vector search).
 

lib/music_library/application.ex

@@ -17,6 +17,7 @@ defmodule MusicLibrary.Application do
       MusicLibrary.Vault,
       MusicLibrary.Repo,
       MusicLibrary.BackgroundRepo,
+      MusicLibrary.TelemetryRepo,
       MusicLibraryWeb.Telemetry,
       {Oban, Application.fetch_env!(:music_library, Oban)},
       {Ecto.Migrator,

lib/music_library/telemetry_repo.ex

@@ -0,0 +1,5 @@
+defmodule MusicLibrary.TelemetryRepo do
+  use Ecto.Repo,
+    otp_app: :music_library,
+    adapter: Ecto.Adapters.SQLite3
+end

lib/music_library_web/telemetry/storage.ex

@@ -1,7 +1,11 @@
 defmodule MusicLibraryWeb.Telemetry.Storage do
   use GenServer
 
-  @history_buffer_size 50
+  @buffer_size Application.compile_env(
+                 :music_library,
+                 [__MODULE__, :buffer_size],
+                 50
+               )
 
   def metrics_history(metric) do
     GenServer.call(__MODULE__, {:data, metric})
@@ -15,14 +19,11 @@ defmodule MusicLibraryWeb.Telemetry.Storage do
   def init(metrics) do
     Process.flag(:trap_exit, true)
 
-    metric_histories_map =
-      metrics
-      |> Map.new(fn metric ->
-        attach_handler(metric)
-        {metric, CircularBuffer.new(@history_buffer_size)}
-      end)
+    for metric <- metrics do
+      attach_handler(metric)
+    end
 
-    {:ok, metric_histories_map}
+    {:ok, metrics}
   end
 
   @impl true
@@ -51,15 +52,68 @@ defmodule MusicLibraryWeb.Telemetry.Storage do
 
   @impl true
   def handle_cast({:telemetry_metric, data, metric}, state) do
-    {:noreply, update_in(state[metric], &CircularBuffer.insert(&1, data))}
+    key = metric_key(metric)
+    label = if is_map(data), do: Map.get(data, :label)
+    measurement = if is_map(data), do: Map.get(data, :measurement, 0), else: 0
+
+    time =
+      if is_map(data),
+        do: Map.get(data, :time, System.system_time(:microsecond)),
+        else: System.system_time(:microsecond)
+
+    insert_and_prune(key, label, measurement, time)
+
+    {:noreply, state}
   end
 
   @impl true
   def handle_call({:data, metric}, _from, state) do
-    if history = state[metric] do
-      {:reply, CircularBuffer.to_list(history), state}
-    else
-      {:reply, [], state}
+    key = metric_key(metric)
+    datapoints = fetch_datapoints(key)
+    {:reply, datapoints, state}
+  end
+
+  defp metric_key(metric) do
+    "#{inspect(metric.__struct__)}:#{Enum.join(metric.name, ".")}"
+  end
+
+  defp insert_and_prune(key, label, measurement, time) do
+    MusicLibrary.TelemetryRepo.query(
+      "INSERT INTO telemetry_datapoints (metric_key, label, measurement, time) VALUES (?1, ?2, ?3, ?4)",
+      [key, label, measurement, time]
+    )
+
+    MusicLibrary.TelemetryRepo.query(
+      """
+      DELETE FROM telemetry_datapoints
+      WHERE metric_key = ?1
+        AND id NOT IN (
+          SELECT id FROM telemetry_datapoints
+          WHERE metric_key = ?1
+          ORDER BY id DESC
+          LIMIT ?2
+        )
+      """,
+      [key, @buffer_size]
+    )
+  catch
+    _, _ -> :ok
+  end
+
+  defp fetch_datapoints(key) do
+    case MusicLibrary.TelemetryRepo.query(
+           "SELECT label, measurement, time FROM telemetry_datapoints WHERE metric_key = ?1 ORDER BY time ASC",
+           [key]
+         ) do
+      {:ok, %{rows: rows}} ->
+        Enum.map(rows, fn [label, measurement, time] ->
+          %{label: label, measurement: measurement, time: time}
+        end)
+
+      _ ->
+        []
     end
+  catch
+    _, _ -> []
   end
 end

mix.exs

@@ -44,8 +44,6 @@ defmodule MusicLibrary.MixProject do
     [
       # Low-level tooling
       {:jason, "~> 1.2"},
-      # Version 0.4.2 and 1.0 are identical, so it's safe to override
-      {:circular_buffer, "~> 1.0"},
 
       # Translations
       {:gettext, "~> 1.0"},

priv/telemetry_repo/migrations/20260305115929_create_telemetry_datapoints.exs

@@ -0,0 +1,15 @@
+defmodule MusicLibrary.TelemetryRepo.Migrations.CreateTelemetryDatapoints do
+  use Ecto.Migration
+
+  def change do
+    create table(:telemetry_datapoints) do
+      add :metric_key, :text, null: false
+      add :label, :text
+      add :measurement, :real, null: false
+      add :time, :integer, null: false
+    end
+
+    # Serves read queries (ORDER BY time) and prune subselect (ORDER BY id DESC)
+    create index(:telemetry_datapoints, [:metric_key, :time])
+  end
+end

test/test_helper.exs

@@ -1,3 +1,4 @@
 ExUnit.start()
 Ecto.Adapters.SQL.Sandbox.mode(MusicLibrary.Repo, :manual)
 Ecto.Adapters.SQL.Sandbox.mode(MusicLibrary.BackgroundRepo, :manual)
+Ecto.Adapters.SQL.Sandbox.mode(MusicLibrary.TelemetryRepo, :auto)