Propagation

With context propagation, Signals can be correlated with each other, regardless of where they are generated. Although not limited to tracing, context propagation allows traces to build causal information about a system across services that are arbitrarily distributed across process and network boundaries.

For the vast majority of use cases, libraries that natively support OpenTelemetry or instrumentation libraries will automatically propagate trace context across services for you. It is only in rare cases that you will need to propagate context manually.

To learn more, see Context propagation.

Automatic context propagation

Distributed traces extend beyond a single service, meaning some context must be propagated across services to create the parent-child relationship between Spans. This requires cross service context propagation, a mechanism where identifiers for a Trace are sent to remote processes.

Instrumentation libraries for HTTP frameworks and servers like Phoenix, Cowboy, Elli and clients like Tesla automatically inject or extract context using the globally registered propagators. By default the global propagators used are the W3C Trace Context and Baggage formats.

You can configure global propagators using the the OTP application environment variable text_map_propagators:

• Erlang
• Elixir

%% sys.config
...
{text_map_propagators, [baggage,
                        trace_context]},
...

## runtime.exs
...
text_map_propagators: [:baggage, :trace_context],
...

You can also pass a comma separated list using the environment variable OTEL_PROPAGATORS. Both forms of configuration accept the values trace_context, baggage, b3 and b3multi.

Manual context propagation

To manually inject or extract context, you can use the otel_propagator_text_map module:

• Erlang
• Elixir

%% uses the context from the process dictionary to add to an empty list of headers
Headers = otel_propagator_text_map:inject([]),

%% creates a context in the process dictionary from Headers
otel_propagator_text_map:extract(Headers),

# uses the context from the process dictionary to add to an empty list of headers
headers = :otel_propagator_text_map.inject([])

# creates a context in the process dictionary from headers
:otel_propagator_text_map.extract(headers)

otel_propagator_text_map:inject/1 and otel_propagator_text_map:extract/1 use globally registered propagators. To use a specific propagator otel_propagator_text_map:inject/2 and otel_propagator_text_map:extract/2 can be used with the first argument being the name of the propagator module to call.

Next steps

To learn more about propagation, read the Propagators API specification.