---
title: Cross application tracing (deprecated)
source: https://docs.newrelic.com/docs/apm/agents/python-agent/supported-features/cross-application-tracing
---

> #### ⚠️ IMPORTANT
>
> For our Python agent, [cross application tracing](https://docs.newrelic.com/docs/apm/transactions/cross-application-traces/introduction-cross-application-traces) has been deprecated since [agent version v7.0.0.166](https://docs.newrelic.com/docs/release-notes/agent-release-notes/python-release-notes/python-agent-70000166) and will be removed in a future agent release. A [distributed tracing](https://docs.newrelic.com/docs/apm/distributed-tracing/getting-started/introduction-distributed-tracing) feature is now available. Distributed tracing improves on cross application tracing and is recommended for monitoring activity in complex distributed systems.

The protocol used to communicate between applications involves attaching metadata to requests and responses. The metadata is processed by each application and the resulting data is reported by the agent.

## Requirements

Requires [New Relic Python agent version 2.92.0.78](https://docs.newrelic.com/docs/release-notes/agent-release-notes/python-release-notes/python-agent-292078) up to version 7.0.0.166. It's deprecated for versions above that.

## Custom client (HTTP)

![client_http_transport.png](https://docs.newrelic.com/images/apm_diagram_python-diagram-app-tracing.webp "client_http_transport.png")

These APIs are used for custom HTTP communication libraries that are not instrumented as part of the built in instrumentation.

HTTP cross application tracing uses HTTP headers to transport transaction metadata across applications. To generate outbound cross application tracing headers, use the `generate_request_headers` API on the `ExternalTrace` class. To process inbound cross application tracing headers, use the `process_response_headers` API on the `ExternalTrace` class.

**Outgoing requests**

````py
import newrelic.agent

transaction = newrelic.agent.current_transaction()
# returns a list of tuples in the form (header_name, header_value)
outgoing_headers = newrelic.agent.ExternalTrace.generate_request_headers(transaction)
for header_name, header_value in outgoing_headers:
  ...request.putheader(header_name, header_value) # code to be instrumented
...make_request(request) # code to be instrumented
```

````

**Incoming response**

````py
import newrelic.agent

transaction = newrelic.agent.current_transaction()
trace = newrelic.agent.ExternalTrace(transaction, 'library', 'http://example.com', 'get')
with trace:
  response = fetch_example_dot_com() # code to be instrumented
  trace.process_response_headers(...response.headers)
```

````

## Custom client (non-HTTP)

![client_custom_transport.png](https://docs.newrelic.com/images/apm_diagram_client-custom-transport.webp "client_custom_transport.png")

These APIs are used to instrument libraries that do not use the HTTP transport (and therefore may not be able to use headers as a metadata transport) and are not instrumented as part of the built in instrumentation.

### Example Instrumentation

**Outgoing requests**

````py
import newrelic.agent

transaction = newrelic.agent.current_transaction()
# returns a string value
outgoing_metadata = newrelic.agent.ExternalTrace.get_request_metadata(transaction)
...request.add_metadata(outgoing_metadata) # code to be instrumented
...make_request(request) # code to be instrumented
```

````

**Incoming response**

````py
import newrelic.agent

transaction = newrelic.agent.current_transaction()
trace = newrelic.agent.ExternalTrace(transaction, 'library', 'myproto://service', 'fetch')
with trace:
  ...response = fetch_from_service() # code to be instrumented
  # extract the metadata string sent from the service
  metadata_value = response.metadata # code to be instrumented
  trace.process_response_metadata(metadata_value)
```

````

## WSGI server [#wsgi]

> #### 💡 TIP
>
> The agent will automatically handle processing and sending responses to cross application trace metadata for all WSGI servers.

For information on instrumenting WSGI servers, see [the wsgi_application API documentation](https://docs.newrelic.com/docs/agents/python-agent/python-agent-api/wsgi_application) for details.

## Custom non-HTTP server

![server_nonhttp_transport.png](https://docs.newrelic.com/images/apm_diagram_python-nonhttp-transport.webp "server_nonhttp_transport.png")

Custom servers that are not WSGI based or do not use HTTP as a transport will have to process incoming cross application tracing metadata and generate cross application trace responses.

The following APIs allow processing of cross application tracing metadata sent on non-HTTP requests and generating response metadata to send back to the caller.

**Example instrumentation**

````py
import newrelic.agent

def handle_incoming_request(request):
  transaction = newrelic.agent.current_transaction()
  if transaction:
    transaction.process_request_metadata(...request.metadata)

  response = generate_response() # code to be instrumented

  if transaction:
    # get_response_metadata returns a string
      ...response.metadata = transaction.get_response_metadata()
  return response
```

````