on_request

The on_request lifecycle event happens on each request received by the proxy, before it even considers proxying anything anywhere.

[rules."*"."*"]
on_request = """
print(f'Hello, {request}.')
"""

rules:
  "*":
    "*":
      on_request: |
        print(f'Hello, {request}.')

The event instance passed to on_request lifecycle event scripts is a ProxyFilterEvent instance.

The same event instance will be passed to on_response, later in the lifecycle.

Patching the incoming request

You get a chance to modify the request before the proxy logic kicks in:

[rules."*"."*"]
on_request = """
request.headers['X-Hello'] = 'World'
"""

Forging a response

If a Response is set in this event, the proxy will not be involved at all.

[rules."*"."*"]
on_request = """
from harp.http import HttpResponse
response = HttpResponse('Hello, World!')
"""

It’s defined and dispatched by the proxy application, within the HttpProxyController request handler.

Using transaction markers

Transactions have a markers attribute that allows you to add metadata flags that can influence how the transaction is processed by various applications. For example, you can add markers to control storage behavior:

rules:
  "api":
    "POST /upload":
      on_request: |
        transaction.markers.add("skip-request-body-storage")

See the storage markers documentation for a complete list of available markers.

Context reference

The following variables are available in the context of the on_request lifecycle event:

  • logger: the logger instance.

  • rule: the rule name for this transaction, as defined in your configuration.

  • transaction: the Transaction instance.

  • endpoint: the endpoint name for this transaction, as defined in your configuration.

  • request: the HttpRequest instance.

  • response: an eventual HttpResponse instance, but most probably None. Set this to a HttpResponse instance to short-circuit the proxying logic.

  • stop_propagation: a function to stop the event propagation to the next event in the chain.

Warning

Don’t use stop_propagation for now, as it will stop the whole lifecycle processing (whistle#18).