# Propagation With context propagation, [Signals](/docs/concepts/signals/) can be correlated with each other, regardless of where they are generated. Although not limited to tracing, context propagation allows [traces](/docs/concepts/signals/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](../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](/docs/concepts/context-propagation). {{__hugo_ctx/}} ## 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_](/docs/specs/otel/overview/#context-propagation), a mechanism where identifiers for a Trace are sent to remote processes. Instrumentation libraries for HTTP frameworks and servers like [Phoenix](https://github.com/open-telemetry/opentelemetry-erlang-contrib/tree/main/instrumentation/opentelemetry_phoenix), [Cowboy](https://github.com/open-telemetry/opentelemetry-erlang-contrib/tree/main/instrumentation/opentelemetry_cowboy), [Elli](https://github.com/open-telemetry/opentelemetry-erlang-contrib/tree/main/instrumentation/opentelemetry_elli) and clients like [Tesla](https://github.com/open-telemetry/opentelemetry-erlang-contrib/tree/main/instrumentation/opentelemetry_tesla) automatically inject or extract context using the globally registered propagators. By default the global propagators used are the W3C [Trace Context](https://w3c.github.io/trace-context/) and [Baggage](https://www.w3.org/TR/baggage/) formats. You can configure global propagators using the OTP application environment variable `text_map_propagators`:

%% 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`](https://github.com/openzipkin/b3-propagation) and `b3multi`. ## Manual context propagation To manually inject or extract context, you can use the `otel_propagator_text_map` module:

%% 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](/docs/specs/otel/context/api-propagators/).