docs: dedicated tracing.md (#172)

README.md

@@ -142,6 +142,10 @@ possible to dynamically modify the logging at runtime.
 - `http://$RAINBOW_CTL_LISTEN_ADDRESS/mgr/log/level?subsystem=<system name or * for all system>&level=<level>` will set the logging level for a subsystem
 - `http://$RAINBOW_CTL_LISTEN_ADDRESS/mgr/log/ls` will return a comma separated list of available logging subsystems
 
+## Tracing
+
+See [docs/tracing.md](docs/tracing.md).
+
 ## Deployment
 
 Suggested method for self-hosting is to run a [prebuilt Docker image](#docker).

docs/environment-variables.md

@@ -300,30 +300,20 @@ Etag: "bafkreicysg23kiwv34eg2d7qweipxwosdo2py4ldv42nbauguluen5v6am"
 
 ## Tracing
 
-Tracing across the stack follows, as much as possible, the [Open Telemetry]
-specifications. Configuration environment variables are specified in the
-[OpenTelemetry Environment Variable Specification] where possible. The
-[Boxo Tracing] documentation is the basis for tracing here.
-
-A major distinction from the more general tracing enabled in boxo is that when
-tracing is enabled it is restricted to flows through HTTP Gateway requests, rather
-than also included background processes.
-
-Note: requests are also traced when there is a `Traceparent` header passed that is valid
-According to the [Trace Context] specification, even if the sampling fraction is set to 0.
+See [tracing.md](tracing.md).
 
 ### `RAINBOW_TRACING_AUTH`
 
+Optional, setting to non-empty value enables on-demand tracing per-request.
+
 The ability to pass `Traceparent` or `Tracestate` headers is guarded by an
 `Authorization` header. The value of the `Authorization` header should match
 the value in the `RAINBOW_TRACING_AUTH` environment variable.
 
 ### `RAINBOW_SAMPLING_FRACTION`
 
+Optional, set to 0 by default.
+
 The fraction (between 0 and 1) of requests that should be sampled.
 This is calculated independently of any Traceparent based sampling.
 
-[Boxo Tracing]: https://github.com/ipfs/boxo/blob/main/docs/tracing.md
-[Open Telemetry]: https://opentelemetry.io/
-[OpenTelemetry Environment Variable Specification]: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/sdk-environment-variables.md
-[Trace Context]: https://www.w3.org/TR/trace-context

docs/tracing.md

@@ -0,0 +1,41 @@
+## Tracing
+
+Tracing across the stack follows, as much as possible, the [Open Telemetry]
+specifications. Configuration environment variables are specified in the
+[OpenTelemetry Environment Variable Specification] where possible. The
+[Boxo Tracing] documentation is the basis for tracing here.
+
+> [!NOTE]
+> A major distinction from the more [general tracing enabled in boxo][Boxo Tracing] is that when
+> tracing is enabled it is restricted to flows through HTTP Gateway requests, rather
+> than also included background processes.
+
+### Fractional Sampling
+
+To sample a % of requests set [`RAINBOW_SAMPLING_FRACTION`](environment-variables.md#rainbow_sampling_fraction) to a value between `0` and `1`.
+
+### Per Request
+
+Per-request tracing is possible when a non-empty [`RAINBOW_TRACING_AUTH`](environment-variables.md#rainbow_tracing_auth) is set in Rainbow and when there are both valid
+[Authorization](headers.md#authorization) and [`Traceparent`](headers.md#traceparent) HTTP headers passed in the request.
+
+When tracing, it is often useful to skip local blockcache by setting [`Rainbow-No-Blockcache`](headers.md#rainbow-no-blockcache) header to `true`.
+
+Per-request tracing exmaple:
+
+```console
+$ export RAINBOW_TRACING_AUTH=CHANGEME-tracing-auth-secret # use value from Rainbow config
+$ export CID=bafybeigdyrzt5sfp7udm7hu76uh7y26nf3efuylqabf3oclgtqy55fbzdi
+$ curl -H "Authorization: $RAINBOW_TRACING_AUTH" -H "Traceparent: 00-$(openssl rand -hex 16)-00$(openssl rand -hex 7)-01" -H "Rainbow-No-Blockcache: true" http://127.0.0.1:8090/ipfs/$CID -v -o /dev/null
+...
+> Authorization: CHANGEME-tracing-auth-secret
+> Traceparent: 00-b617dc6b6e302ccbabe0115eac80320b-00033792c7de8fc6-01
+...
+````
+
+Now you can search for `trace_id = b617dc6b6e302ccbabe0115eac80320b` to find the trace.
+
+[Boxo Tracing]: https://github.com/ipfs/boxo/blob/main/docs/tracing.md
+[Open Telemetry]: https://opentelemetry.io/
+[OpenTelemetry Environment Variable Specification]: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/sdk-environment-variables.md
+[Trace Context]: https://www.w3.org/TR/trace-context