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Message Transformation using Interceptor APIPolicy

You can use APIPolicy CR with interceptor configurations to carry out transformations and mediation on the requests and responses. Request interceptor gets triggered before sending the request to the backend. Response interceptor gets triggered before responding to the client. Here, an interceptor is a separate microservice that handles the request, response, or both request and response transformations.

Sample Interceptor APIPolicy
kind: APIPolicy
  name: interceptor-api-policy
      - name: request-interceptor-service
      - name: response-interceptor-service
    kind: API
    name: sample-api
Sample InterceptorService Custom Resource
kind: InterceptorService
  name: request-interceptor-service
    name: interceptor-backend
    - request_headers
    - response_body
    - invocation_context

When you have attached an Interceptor APIPolicy like above example to your API, then the request flow (1, 2, 3, and 4 numbered circles) and response flow (5, 6, 7, and 8 numbered circles) can be depicted as below.

Interceptors Interceptors

If you are an API developer, you can write a custom request/response interceptor microservice in any programming language of your choice by following the Interceptor OpenAPI Definition. Then you can configure the following spec fields in the APIPolicy CR to wire your interceptor serivce with the API.

Policy Spec Field Description
requestInterceptor Interceptor service configuration for the request path
responseInterceptor Interceptor service configuration for the response path

You can define interceptors on an API level (per API) and at operation level. How to do these are explained below in detail. If a request/response interceptor is on an API level and a operation level, interceptors properties are combined using priority order defined in the following order. You can define properties under default and/or override section in API level APIPolicy and/or operation level APIPolicy. These values are combined using the priority order defined below, with priority decreasing towards the bottom of the table.

API level override
Operation level override
Operation level default
API level default

When you want the policy only to be defined in a single level, then defining the interceptor configurations in either default or override sections will work.

Configuring Interceptors

Configuring an interceptor requires the following two steps.

  1. Implement an interceptor microservice adhering to the Interceptor OpenAPI Definition

  2. For reference, a sample interceptor service for data conversion between application/json and application/xml data types with interceptor policy CRs can be found here: request-response-mediation-interceptors

  3. Create APIPolicy with interceptor configuration and attach it to your API.