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Checking requests and responses with the Common Expression Language (CEL)

There are times when you might want to include additional logic in the gateway to decide if a request has to be fulfilled or not.

The Common Expression Language (CEL) middleware enables Google’s CEL spec which implements common semantics for expression evaluation, and is a very simple and powerful option to have full control during requests and responses.

When the CEL component is enabled, any amount of expressions to check both requests and responses can be set.

Then, during runtime, when an expression returns false, KrakenD does not return the content as the condition has failed. Otherwise, if all expressions returned true the content is served.

The CEL expressions have similar syntax to expressions in C/C++/Java/JavaScript and evaluate to a boolean condition. For instance:

'::1' in req_headers['X-Forwarded-For']

This expression checks that the request header X-Forwarded-For contains the string ::1 (request comes from localhost).

CEL expressions can be used both during the request or the response of the backends and the endpoints. The flow is:

  • Request endpoint evaluation
  • Request backend evaluation (N times)
  • Response backend evaluation (N times)
  • Response endpoint evaluation (can evaluate all merged data)

Adding logic in the request

If you want to add some logic to decide whether or not to continue serving the request to an endpoint or proxy to the next backend not, use a req_* variable.

CEL variables

The following data is injected to the CEL evaluator for its inspection:


  • req_method: What is the method of this endpoint, e.g.: GET
  • req_path: The path used to access this endpoing, e.g: : /foo
  • req_params: Object with all the parameters sent with the request, e.g:
  • req_headers: Array with all the headers received, e.g: req_headers['X-Forwarded-For']
  • now: An object containing the current timestamp, e.g: timestamp(now).getDayOfWeek()


  • resp_completed: Boolean whether all the data has been successfully retrieved
  • resp_metadata_status: Returns an integer with the StatusCode
  • resp_metadata_header: Returns an array with all the headers of the response
  • resp_data: An object with all the data captured in the response
  • now: An object containing the current timestamp

JWT rejecter

CEL expressions can also be used during the JWT token validation. Use the JWT variable to access its metadata. For instance:

has(JWT.user_id) && has(JWT.enabled_days) && (timestamp(now).getDayOfWeek() in JWT.enabled_days)

This example checks that the JWT token contains the metadata user_id and enabled_days with the macro has(), and then checks that today’s weekday is within one of the allowed days to see the endpoint.

CEL Syntax

See the language definition

Unresolved issues?

The documentation is only a piece of the help you can get! Whether you are looking for Open Source or Enterprise support, see more support channels that can help you.