opentelemetry-erlang-contrib/utilities/opentelemetry_telemetry/lib/opentelemetry_telemetry.ex

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defmodule OpentelemetryTelemetry do
@moduledoc """
`OpentelemetryTelemetry` provides conveniences for leveraging `telemetry`
events for `OpenTelemetry` bridge libraries.
## OpenTelemetry Contexts
`opentelemetry` does not automatically set current span context when ending
another span. Since `telemetry` events are executed in separate handlers with
no shared context, correlating individual events requires a mechanism to do so.
Additionally, when ending telemetry-based spans, the user must set the correct
parent context back as the current context. This ensures sibling spans are
correctly correlated to the shared parent span.
This library provides helper functions to manage contexts automatically with
`start_telemetry_span/4`, `set_current_telemetry_span/2`, and `end_telemetry_span/2`
to give bridge library authors a mechanism for working with these challenges. Once
`start_telemetry_span/4` or `set_current_telemetry_span/2` are called, users
can use all of `OpenTelemetry` as normal. By providing the application tracer id
and the event's metadata, the provided span functions will identify and manage
span contexts automatically.
### Example Telemetry Event Handlers
```
def handle_event(_event,
%{system_time: start_time},
metadata,
%{type: :start, tracer_id: tracer_id, span_name: name}) do
start_opts = %{start_time: start_time}
OpentelemetryTelemetry.start_telemetry_span(tracer_id, name, metadata, start_opts)
:ok
end
def handle_event(_event,
%{duration: duration},
metadata,
%{type: :stop, tracer_id: tracer_id}) do
OpentelemetryTelemetry.set_current_telemetry_span(tracer_id, metadata)
OpenTelemetry.Tracer.set_attribute(:duration, duration)
OpentelemetryTelemetry.end_telemetry_span(tracer_id, metadata)
:ok
end
def handle_event(_event,
%{duration: duration},
%{kind: kind, reason: reason, stacktrace: stacktrace} = metadata,
%{type: :exception, tracer_id: tracer_id}) do
ctx = OpentelemetryTelemetry.set_current_telemetry_span(tracer_id, metadata),
status = Opentelemetry.status(:error, to_string(reason, :utf8))
OpenTelemetry.Span.record_exception(ctx, kind, reason, stacktrace, [duration: duration])
OpenTelemetry.Tracer.set_status(status)
OpentelemetryTelemetry.end_telemetry_span(tracer_id, metadata)
:ok
end
def handle_event(_event, _measurements, _metadata, _config), do: :ok
```
### Limitations
Span contexts are currently stored in the process dictionary, so spans can only
be correlated within a single process at this time. This covers the primary use
case where library authors have implemented `telemetry:with_span` or the pattern
established in said function. Non-library authors should use opentelemetry directly
wherever possible.
If the `event_metadata` includes a `telemetry_span_context` (introduced in telemetry
`v0.4.3`), contexts are correlated by the `telemetry_span_context` id to guarantee
the correct otel span context. Span events in earlier versions of `telemetry` are stored
in a stack by `tracer_id` to lessen the likelihood of inadvertently closing the wrong
span.
"""
@typedoc """
A span ctx for a telemetry-based span.
"""
@type telemetry_span_ctx() :: :opentelemetry.span_ctx()
@typedoc """
The parent span ctx for a telemetry-based span. This is what the current span ctx was
at the time of starting a telemetry-based span.
"""
@type parent_span_ctx() :: :opentelemetry.span_ctx()
@type ctx_set() :: {parent_span_ctx(), telemetry_span_ctx()}
@typep tracer_id() :: atom()
@doc """
Start a telemetry-based span.
"""
@spec start_telemetry_span(
tracer_id(),
:opentelemetry.span_name(),
:telemetry.event_metadata(),
OpenTelemetry.Span.start_opts()
) :: OpenTelemetry.span_ctx()
defdelegate start_telemetry_span(tracer_id, span_name, event_metadata, start_opts),
to: :otel_telemetry
@doc """
Set the current span ctx based on the tracer_id and telemetry event metadata.
"""
@spec set_current_telemetry_span(tracer_id(), :telemetry.event_metadata()) ::
OpenTelemetry.span_ctx()
defdelegate set_current_telemetry_span(tracer_id, event_metadata), to: :otel_telemetry
@doc """
End a telemetry-based span based on the `tracer_id` and telemetry event metadata
and restore the current ctx to the span's parent ctx.
"""
@spec end_telemetry_span(tracer_id(), :telemetry.event_metadata()) :: :ok
defdelegate end_telemetry_span(tracer_id, event_metadata), to: :otel_telemetry
@doc false
defdelegate trace_application(app), to: :otel_telemetry
@doc false
defdelegate trace_application(app, opts), to: :otel_telemetry
end