Propagation

Cross Service Propagators

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 will automatically inject or extract context using the globally registered propagators. By default the global propagators used are the W3C Trace Context and Baggage formats.

These global propagators can be configured by the Application environment variable text_map_propagators:

• Erlang
• Elixir

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

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

Or through a comma separated list with the environment variable OTEL_PROPAGATORS. Both forms of configuration accept the values trace_context, baggage, b3 and b3multi.

To manually inject or extract context the otel_propagator_text_map module can be used:

• 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.