Using the Response Body Transform middleware with Tyk OAS APIs

The response body transform middleware provides a way to modify the payload of API responses before they are returned to the client.

The middleware is configured in the Tyk OAS API Definition. You can do this via the Tyk Dashboard API or in the API Designer.

If you’re using the legacy Tyk Classic APIs, then check out the Tyk Classic page.

Configuring the middleware in the Tyk OAS API Definition

The design of the Tyk OAS API Definition takes advantage of the operationId defined in the OpenAPI Document that declares both the path and method for which the middleware should be added. Endpoint paths entries (and the associated operationId) can contain wildcards in the form of any string bracketed by curly braces, for example /status/{code}. These wildcards are so they are human readable and do not translate to variable names. Under the hood, a wildcard translates to the “match everything” regex of: (.*).

The response body transformation middleware (transformResponseBody) can be added to the operations section of the Tyk OAS Extension (x-tyk-api-gateway) in your Tyk OAS API Definition for the appropriate operationId (as configured in the paths section of your OpenAPI Document).

The transformResponseBody object has the following configuration:

• enabled: enable the middleware for the endpoint
• format: the format of input data the parser should expect (either xml or json)
• body: [see note] this is a base64 encoded representation of your template
• path: [see note] this is the path to the text file containing the template

For example:

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{
    "components": {},
    "info": {
        "title": "example-response-body-transform",
        "version": "1.0.0"
    },
    "openapi": "3.0.3",
    "paths": {
        "/anything": {
            "put": {
                "operationId": "anythingput",
                "responses": {
                    "200": {
                        "description": ""
                    }
                }
            }
        }
    },
    "x-tyk-api-gateway": {
        "info": {
            "name": "example-response-body-transform",
            "state": {
                "active": true
            }
        },
        "upstream": {
            "url": "http://httpbin.org/"
        },
        "server": {
            "listenPath": {
                "value": "/example-response-body-transform/",
                "strip": true
            }
        },
        "middleware": {
            "operations": {
                "anythingput": {
                    "transformResponseBody": {
                        "enabled": true,
                        "format": "json",
                        "body": "ewogICJ2YWx1ZTEiOiAie3sudmFsdWUyfX0iLAogICJ2YWx1ZTIiOiAie3sudmFsdWUxfX0iLAogICJyZXEtaGVhZGVyIjogInt7Ll90eWtfY29udGV4dC5oZWFkZXJzX1hfSGVhZGVyfX0iLAogICJyZXEtcGFyYW0iOiAie3suX3R5a19jb250ZXh0LnJlcXVlc3RfZGF0YS5wYXJhbX19Igp9"
                    }
                }
            }
        }
    }
}

In this example the response body transform middleware has been configured for requests to the PUT /anything endpoint. The body contains a base64 encoded Go template (which you can check by pasting the value into a service such as base64decode.org).

Decoded, this template is:

{
    "value1": "{{.value2}}",
    "value2": "{{.value1}}",
    "req-header": "{{._tyk_context.headers_X_Header}}",
    "req-param": "{{._tyk_context.request_data.param}}"
}

So if you make a request to PUT /anything?param=foo, configuring a header X-Header:bar and providing this payload:

{
    "value1": "world",
    "value2": "hello"
}

httpbin.org will respond with the original payload in the response and, if you do not have the response body transform middleware enabled, the response from Tyk will include:

{
    "value1": "world",
    "value2": "hello"
}

If, however, you enable the response body transform middleware, Tyk will modify the response to include this content:

{
    "req-header": "bar",
    "req-param": "[foo]",
    "value1": "hello",
    "value2": "world"
}

You can see that Tyk has swapped value1 and value2 and embedded the X-Header header and param query values from the request into the body of the response.

The configuration above is a complete and valid Tyk OAS API Definition that you can import into Tyk to try out the mock response middleware.

Configuring the middleware in the API Designer

Adding Response Body Transformation to your API endpoints is easy when using the API Designer in the Tyk Dashboard, simply follow the following steps:

Step 1: Add an endpoint

From the API Designer add an endpoint that matches the path and method to which you want to apply the middleware.

Step 2: Select the Response Body Transform middleware

Select ADD MIDDLEWARE and choose the Response Body Transform middleware from the Add Middleware screen.

Step 3: Configure the middleware

Now you can select the response body format (JSON or XML) and add either a path to the file containing the template, or directly enter the transformation template in the text box.

The Test with data control will allow you to test your body transformation function by providing an example response body and generating the output from the transform. It is not possible to configure headers, other request parameters, context or session metadata to this template test so if you are using these data sources in your transform it will not provide a complete output, for example:

Step 4: Save the API

Select SAVE API to apply the changes to your API.