AshMetrics

Status: pre-release, not on Hex. Counters, distributions, gauges with their polling from either a timer or Oban, the emission API, the behaviours and the test helpers work. The OpenTelemetry backend does not exist yet; with the default Backend.Noop the host application's own reporter ships the metrics.

What it is

AshMetrics is an Ash resource extension that adds a metrics do block for declarative business metrics: counts and rates of business events (emails sent, invoices captured, syncs completed), the current depth of state-machine backlogs (how many records are pending right now), and latency distributions. Declarations sit next to the action they describe, are validated at compile time, carry a uniform set of tags without every call site repeating them, and enforce a tag allowlist that keeps high-cardinality values out.

Those declarations compile into a list of Telemetry.Metrics structs, which the host application's existing reporter ships to whatever backend it already uses — OTLP, StatsD, Prometheus, AppSignal. Counter emission is a synchronous :telemetry.execute/3; periodic polling is only needed for gauges.

Primitives

• counter — "How many events? How fast?" Emitted manually at the business moment the outcome becomes known.
• gauge — "How many right now?" Filled by a package-managed periodic poll over the resource.
• distribution — "What's the spread?" Observed manually.

Installation

Once the package is on Hex, one command does the whole installation:

mix igniter.install ash_metrics

It adds the dependency, writes prefix and otp_app to config/config.exs, appends ++ AshMetrics.metrics() to the metrics/0 of the module that imports Telemetry.Metrics — MyAppWeb.Telemetry in a generated Phoenix application — and adds AshMetrics.Supervisor to your application's children after the repositories. It never overwrites a value you have already chosen, prints the optional configuration keys with their defaults, and prints a reporter snippet to add by hand when it finds no telemetry module. See mix ash_metrics.install.

The package is not on Hex yet. Until it is, and for a project that would rather not run an installer, install by hand: add the dependency, write the configuration block below, and wire the metrics into your reporter and supervision tree as Wiring into your reporter describes.

Configuration:

config :ash_metrics,
  prefix: "myapp",                                  # REQUIRED
  otp_app: :my_app,                                 # REQUIRED
  name_builder: AshMetrics.NameBuilder.Default,
  tag_extractor: AshMetrics.TagExtractor.Default,
  backend: AshMetrics.Backend.Noop,
  poller: AshMetrics.Poller.GenServer,
  tenant_source: MyApp.Tenants                      # per-tenant gauges only

# Only when the Oban poller is chosen; see "Polling with Oban".
config :ash_metrics, AshMetrics.Poller.AshOban,
  queue: :default,
  max_attempts: 1

prefix must be set in compile-time configuration — config/config.exs, not config/runtime.exs alone — and is never derived from otp_app. A compile-time verifier rejects a resource that declares metrics unless it is configured; the metric names themselves are built when AshMetrics.metrics/0 runs.

otp_app is the application whose Ash domains AshMetrics.metrics/0 searches to find the resources that declare metrics.

Usage

Declare metrics on the resource:

defmodule MyApp.Mailings.TemplatedDelivery do
  use Ash.Resource,
    domain: MyApp.Mailings,
    extensions: [AshMetrics]

  metrics do
    # Optional; defaults to the resource short name.
    name :templated_delivery

    # → myapp.mailings.templated_delivery.delivery.count
    #   tags: provider, template, status, tenant
    counter :delivery,
      tags: [:provider, :template, status: [:queued, :sent, :bounced, :delivered, :error]],
      description: "Templated deliveries by status"

    # → myapp.mailings.templated_delivery.backlog.gauge
    #   tags: status, provider
    gauge :backlog,
      filter: expr(status in [:pending, :processing]),
      group_by: [:status, :provider],
      period: :timer.minutes(1),
      description: "Deliveries waiting to be sent"

    # → myapp.mailings.templated_delivery.send_latency.duration
    #   tags: provider, tenant
    distribution :send_latency,
      unit: {:native, :millisecond},
      buckets: [10, 50, 100, 250, 500, 1_000, 5_000],
      tags: [provider: [:ses, :smtp]]
  end
end

A tag entry written key: [value, ...] is a closed tag: every emission must carry it, with one of the listed values, and the whole enumeration lives in one metric name rather than one name per value. An entry written key is open, and a call site may pass any value or none at all. A distribution's value is measured by the call site, not by the package.

The declarations themselves are checked while the resource compiles: metric names must be unique, tag keys must be unique and must not collide with the keys the tag extractor adds, a closed tag needs at least one value and no duplicates, and buckets must be strictly ascending positive numbers. Spark reports a failed check as a compiler warning pointing at the offending declaration, so compile with mix compile --warnings-as-errors in CI if a bad declaration should fail the build.

See the DSL reference for every option.

Emit them where the outcome becomes known: an action returning {:ok, _} does not mean the email was delivered, so nothing is emitted from the action lifecycle unless the action says so.

AshMetrics.increment(MyApp.Mailings.TemplatedDelivery, :delivery,
  tags: %{status: :sent, provider: "ses", template: "welcome_v2"},
  metadata: changeset.context
)

AshMetrics.observe(MyApp.Mailings.TemplatedDelivery, :send_latency, 142,
  tags: %{provider: :ses},
  metadata: changeset.context
)

metadata: is the bridge to the AshMetrics.TagExtractor. Pass anything shaped like Ash event metadata — a changeset's context will do — and the extractor pulls the tags that belong on every emission. The default pulls the tenant, and refuses to stringify a struct into a tag value.

A missing closed tag, a value that tag does not declare, or a tag key that was not declared at all raises rather than emitting.

Emitting from actions

When the fact is written by an Ash action, that action can emit it:

update :update_status do
  accept [:status, :delivered_at]
  require_atomic? false

  change AshMetrics.increment_on_change(:delivery, :status)

  change AshMetrics.observe_elapsed(:delivery_time,
           from: :inserted_at,
           to: :delivered_at
         ),
         where: [attribute_equals(:status, :delivered)]
end

increment_on_change/2 counts one delivery after the transaction whenever the action leaves status holding a value the record did not have before, tagged with that value and with every other declared tag of the counter that names an attribute of the record. observe_elapsed/2 records the time between two timestamps of the written record into delivery_time, in that distribution's unit; where: narrows it to the one transition that means delivered. Both take their extractor metadata from the changeset, emit nothing when the action fails, and never alter its result. See AshMetrics.Changes.IncrementOnChange and AshMetrics.Changes.ObserveElapsed.

Two caveats. A value outside a closed tag's declared set is skipped, so a status the counter does not enumerate is not counted and nothing is raised. And both changes refuse to run atomically: the action needs require_atomic? false, and Ash.bulk_update/4 needs :stream among its strategies, or it emits nothing and returns Ash.Error.Invalid.NoMatchingBulkStrategy. Ash.bulk_create/4 needs nothing extra.

Gauges

A gauge is the one primitive you never emit. AshMetrics polls it every period and emits one value per group, so the declaration above publishes myapp.mailings.templated_delivery.backlog.gauge with a status and a provider tag and one timeseries per combination that exists.

A poll costs queries: the default :count strategy runs 1 + groups of them per period, again per tenant for a resource that is polled per tenant, and runs them with authorize?: false. See AshMetrics.Gauge.Strategy.Count. When that is too expensive, declare strategy: MyApp.Stats.Backlog — any module implementing AshMetrics.Gauge.Strategy — and compute the number however you like.

Sub-minute periods are usually wasted resolution: most collectors flush on a ten second interval anyway, and every poll costs the queries above.

A group that disappears is emitted once as a zero, so a drained backlog does not keep reporting its last value; see AshMetrics.Gauge.Runner.

Polling with Oban

The default AshMetrics.Poller.GenServer runs a timer on every node. AshMetrics.Poller.AshOban polls from Oban's cron instead: one job per period for the whole cluster, a failed poll as a failed job with its error in Oban Web, and one worker and queue entry per gauge.

# In config/config.exs: the choice is read while resources compile.
config :ash_metrics, poller: AshMetrics.Poller.AshOban

config :ash_metrics, AshMetrics.Poller.AshOban,
  queue: :default,
  max_attempts: 1

Like prefix, the poller has to be compile-time configuration: the schedules are generated while the resource compiles, and a poller that differs between compile time and runtime leaves the gauges with no poller at all.

The poller can also be chosen per resource, for example one expensive backlog on the queue and the remaining gauges on the timer:

metrics do
  poller AshMetrics.Poller.AshOban

  gauge :backlog, filter: expr(status == :pending), period: :timer.minutes(5)
end

Such a resource must use the AshOban extension, must declare gauge periods cron can express exactly, and needs its queue and its AshOban.config/2 crontab in the host application's Oban configuration; the first two are compile errors naming the gauge. Zeroing a drained group is weaker than with the timer, since an Oban job has no state between runs. See AshMetrics.Poller.AshOban for all of it, and for the private action and schedule it generates per gauge.

Multitenancy

A gauge is tagged with tenant under either of Ash's multitenancy strategies. A resource Ash will not read without a tenant — :context, or :attribute without global? true — is polled once per tenant of the configured tenant_source, a module implementing AshMetrics.TenantSource, and a compile-time verifier rejects such a resource that declares a gauge while tenant_source is unset. :attribute with global? true needs no configuration.

Note the multiplier: a gauge polled per tenant costs its queries once per tenant per period. See AshMetrics.Gauge.Runner for how each strategy is polled.

Wiring into your reporter

AshMetrics.metrics/0 returns the Telemetry.Metrics definitions of every resource that declares metrics. Splice it into whatever reporter you already run:

defmodule MyApp.Telemetry do
  use Supervisor

  def start_link(arg), do: Supervisor.start_link(__MODULE__, arg, name: __MODULE__)

  @impl true
  def init(_arg) do
    children = [
      {Telemetry.Metrics.ConsoleReporter, metrics: my_own_metrics() ++ AshMetrics.metrics()},
      AshMetrics.Supervisor
    ]

    Supervisor.init(children, strategy: :one_for_one)
  end
end

The same list works for a Prometheus reporter:

{TelemetryMetricsPrometheus, metrics: AshMetrics.metrics()}

AshMetrics.Supervisor starts whatever the configured backend needs, followed by every poller in use. Put it after your repository, since a gauge is answered by a query. AshMetrics.child_specs/1 returns the same children as a list, for a tree that would rather splice them in than add a supervisor. With the default AshMetrics.Backend.Noop the backend adds nothing. Pass an explicit resource list to AshMetrics.metrics_for/1 if domain discovery is not what you want.

With the default poller every node polls, so in a cluster each gauge is computed and emitted once per node per period; see AshMetrics.Poller.GenServer, polling with Oban, and AshMetrics.Poller for implementing anything else.

Testing

defmodule MyApp.MailingsTest do
  use ExUnit.Case, async: false
  use AshMetrics.Test

  test "a delivery emits a sent counter" do
    MyApp.Mailings.deliver!(...)

    assert_metric_emitted "myapp.mailings.templated_delivery.delivery",
      tags: %{status: :sent, provider: "ses"}

    refute_metric_emitted "myapp.mailings.templated_delivery.delivery",
      tags: %{status: :bounced}
  end
end

use AshMetrics.Test attaches a handler for the duration of each test and imports the assertions. Name the metric as the declaration produces it, without the .count, .gauge or .duration suffix a reporter adds. :telemetry handlers are global, so keep such modules async: false — see AshMetrics.Test for the details.

Development

mix test runs the whole suite except the Postgres integration tests, and needs no database and no container. The tests that do need one are tagged :postgres, excluded by default, and run against the container this repository's docker-compose.yml defines:

docker compose up -d
mix test.integration
docker compose stop

mix test.integration creates the database, migrates it and runs everything with --include postgres. The container is named ash_metrics-postgres-1 and publishes Postgres on ${ASH_METRICS_PG_PORT:-54329}; set that variable if 54329 is taken. Only ever drive it through docker compose from the repository root, so that no container outside this project is touched.

The rest of the checks:

mix format --check-formatted
mix credo --strict
mix dialyzer
mix docs

mix docs regenerates the DSL cheat sheet in documentation/dsls, which is checked in; commit it with whatever DSL change produced it.

Non-goals

• Not a new emit/aggregate/export pipeline. AshMetrics produces Telemetry.Metrics structs and lets the existing reporter ecosystem ship them.
• Not an APM or tracing tool. ash_appsignal and opentelemetry_ash cover that.
• Not a replacement for Oban.Telemetry, which already emits job queue_time and duration.

License

MIT. See the LICENSE file in the repository root.