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# Monitoring

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Monitoring in Ash has two primary components, `Ash.Tracer` and `:telemetry`. Monitoring might also be referred to as observability and instrumentation.

## Packages

If you want to integrate with Appsignal, use the AshAppsignal package, which is maintained by the core team. We believe that Appsignal is a great way to get started quickly, is relatively cost effective, and provides a great user experience.

## Telemetry

Ash emits the following telemetry events, suffixed with `:start` and `:stop`. Start events have `system_time` measurements, and stop events have `system_time` and `duration` measurements. All times will be in the native time unit.

### Important

Note the mention of `:start` and `:stop` suffixes. The event below `[:ash, (domain_short_name), :create]`, is actually referring to two events, `[:ash, (domain_short_name), :create, :start]` and `[:ash, (domain_short_name), :create, :stop]`.

_Replace `(domain_short_name)` with your domain short name, from `Ash.Domain.Info.short_name`.

### Events

- `[:ash, (domain_short_name), :create]` - The execution of a create action. Use `resource_short_name` and `action` metadata to break down measurements.
- `[:ash, (domain_short_name), :update]` - The execution of a update action. Use `resource_short_name` and `action` metadata to break down measurements.
- `[:ash, (domain_short_name), :read]` - The execution of a read action. Use `resource_short_name` and `action` metadata to break down measurements.
- `[:ash, (domain_short_name), :destroy]` - The execution of a destroy action. Use `resource_short_name` and `action` metadata to break down measurements.
- `[:ash, (domain_short_name), :action]` - The execution of a generic action. Use `resource_short_name` and `action` metadata to break down measurements.
- `[:ash, :changeset]` - A changeset being processed for a given action, i.e with `Ash.Changeset.for_create`. Use `resource_short_name` metadata to break down measurements.
- `[:ash, :query]` - A query being processed for an action, with `Ash.Query.for_read`. Use `resource_short_name` metadata to break down measurements.
- `[:ash, :validation]` - A validation being run on a changeset. Use `resource_short_name` and `validation` metadata to break down measurements.
- `[:ash, :change]` - A change being run on a changeset. Use `resource_short_name` and `change` metadata to break down measurements.
- `[:ash, :calculation]` - A calculation being computed in the app. Use `resource_short_name` and `calculation` metadata to break down measurements.
- `[:ash, :before_action]` - A before_action being run on a changeset. Use `resource_short_name` to break down measurements.
- `[:ash, :after_action]` - An after_action being run on a changeset. Use `resource_short_name` to break down measurements.
- `[:ash, :preparation]` - A preparation being run on a changeset. Use `resource_short_name` and `preparation` metadata to break down measurements.
- `[:ash, :request_step]` - The resolution of an internal request. Ash breaks up its operations internally into multiple requests, this can give you a high resolution insight onto the execution of those internal requests resolution. Use `name` metadata to break down measurements.
- `[:ash, :flow]` - The execution of an Ash flow. Use `flow_short_name` to break down measurements.
- `[:ash, :flow, :custom_step]` - The execution of a custom flow step (only if using the built in runner, which is currently the only runner). Use `flow_short_name` and `name` metadata to break down measurements.

## Tracing

Tracing is very similar to telemetry, but gives you some additional hooks to `set_span_context()` and `get_span_context()`. This allows you to "move" some piece of context between two processes. Ash will call this whenever it starts a new process to do anything. What this means is that if you are using a tracing tool or library you can ensure that any processes spawned by Ash are properly included in the trace. Additionally, you should be able to integrate a tracing library to include Ash actions/spans relatively easily by implementing the other callbacks.

A tracer can be configured globally in application config.

    config :ash, :tracer, MyApp.Tracer

Additionally, one can be provide when creating changesets or calling an action, i.e

    Resource
    # better to put it here, as changesets are traced as well. It will be carried over to the domain action
    |> Ash.Changeset.for_create(:create, %{}, tracer: MyApp.Tracer)
    # but you can also pass it here.
    |> Ash.create!(tracer: MyApp.Tracer)

For customizing the names created for each span, see:

- `Ash.Domain.Dsl.execution.trace_name`
- `Ash.Resource.Dsl.resource.trace_name`

### Trace types

These are the list of trace types.

- :custom
- :action
- :changeset
- :validation
- :change
- :calculation
- :before_transaction
- :before_action
- :after_transaction
- :after_action
- :request_step
- :custom_flow_step
- :flow
- :query
- :preparation

## After/Before Action Hooks

Due to the way before/after action hooks run, their execution time won't be included in the span created for the change. In practice, before/after action hooks are where the long running operations tend to be. We start a corresponding `span` and emit a telemetry event for before and after hooks, but they are only so useful. In a trace, they can highlight that "some hook" took a long time. In telemetry metrics they are of even less use. The cardinality of the metric would be extremely high, and we don't have a "name" or anything to distinguish them. To that end, you can use the macros & functions available in `Ash.Tracer` to create custom spans and/or emit custom telemetry events from your hooks. They automatically handle cases where the provided tracer is `nil`, for convenience. For example:

    defmodule MyApp.CustomChange do
      use Ash.Resource.Change

      require Ash.Tracer

      def change(changeset, _, _) do
        changeset
        |> Ash.Changeset.before_action(fn changeset ->
          Ash.Tracer.span(:custom, "custom name", changeset.context[:private][:tracer]) do
            # optionally set some metadata
            metadata = %{...}
            Ash.Tracer.set_metadata(changeset.context[:private][:tracer], :custom, metadata)
            # will get `:start` and `:stop` suffixed events emitted
            Ash.Tracer.telemetry_span([:telemetry, :event, :name], metadata) do
              ## Your logic here
            end
          end
        end)
      end
    end

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