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Transformations

Overview

Transformations allow you to dynamically modify requests and responses. That includes headers, query string parameters and body.

Depending on your vision and the specifics of your API products, you might use response/request transformations to:

  • restrict the data returned to customers subscribed to a free product
  • enforce a query string parameter to always have a certain value
  • remove some elements from response body
  • use Project X Products as feature toggle
  • and many more things
Example use case #1 — Truncate returned content

Say, you provide an API endpoint that returns the content of a news article. E.g.,

{
    "id": "b6355c53-44a3-464b-bc9e-759749cdb3ec",
    "title": "The title",
    "content": "Some long article body"
}

For your "Free" product, you'd like to return only the first 120 characters of article body.

Project X can do that for you, just create a product named "Free" and add a response transformation to truncate the article text.

Example use case #2 — Enforce query parameter value

If you have an endpoint that accepts a format parameter, e.g. /articles/?format=<yaml|json|xml>, you might want to restrict the format for cheaper products to always be json.

Add a request transformation to replace the query string parameter format with json — and all customers subscribed to this product will be unable to specify any value for the format parameter other than json.

This section describes product-scoped transformations. If a transformation is defined on a product level, it is only applied for the customers subscribed to this product.

Tip

Read about API-scoped transformations.

Transformations defined on an API level are always applied first.

How it works

graph TD
    input[\"GET /endpoint1"/]

    apply_api_req_tfs[Apply API-level request transformations]
    apply_product_req_tfs[Apply product-level request transformations]

    proxy_and_wait(("Proxy modified request
    and wait for response"))

    apply_api_resp_tfs[Apply API-level response transformations]
    apply_product_resp_tfs[Apply product-level response transformations]

    send_response[/Send modified response to the user\]

    input --> apply_api_req_tfs
    apply_api_req_tfs --> apply_product_req_tfs
    apply_product_req_tfs --> proxy_and_wait
    proxy_and_wait --> apply_api_resp_tfs
    apply_api_resp_tfs --> apply_product_resp_tfs
    apply_product_resp_tfs --> send_response

Transformation targets

The transformation will add/replace/remove headers in a request or response.

Header names are case-insensitive.

Add

Adds a new header, even if there is already one with the same name.

Example

Original request:

GET / HTTP/1.1
X-Some-Header: some-value

Transformation:

Add header "X-Some-Header" with value "another-value".

Transformed request:

GET / HTTP/1.1
X-Some-Header: some-value
X-Some-Header: another-value

Replace

Replaces header(s) with a new value. If there are several headers with the given name, they're removed and replaced with the new value.

Example

Original request:

GET / HTTP/1.1
X-Some-Header: first-value
X-Some-Header: second-value

Transformation:

Replace header "X-Some-Header" with value "third-value".

Transformed request:

GET / HTTP/1.1
X-Some-Header: third-value

Remove

Removes all headers with the given name.

Example

Original request:

GET / HTTP/1.1
X-Some-Header: first-value
X-Some-Header: second-value

Transformation:

Remove header "X-Some-Header".

Transformed request:

GET / HTTP/1.1

Query string parameter

The transformation will add/replace/remove query string parameters in a request.

Query string parameter names are case-sensitive.

Add

Adds a new query string parameter, even if there is already one or more with the same name.

Example

Original request:

GET /?param1=100 HTTP/1.1

Transformation:

Add query string parameter "param2" with value "test-value".

Transformed request:

GET /?param1=100&param2=test-value HTTP/1.1

Replace

Replaces query string parameter(s) with a new value. If there are several query string parameters with the given name, they're removed and replaced with the new value.

Example

Original request:

GET /?param1=100 HTTP/1.1

Transformation:

Replace query string parameter "param1" with value 200.

Transformed request:

GET /?param1=200 HTTP/1.1

Remove

Removes all query string parameters with the given name.

Example

Original request:

GET /?param1=100 HTTP/1.1

Transformation:

Remove query string parameter "param1".

Transformed request:

GET / HTTP/1.1

Body

The transformation will add/replace/remove request/response body.

Add

Adds a new body. If a request or response already has a body, it's left unchanged.

Example

Original request:

POST / HTTP/1.1

Transformation:

Add body with value {"testkey": "testvalue"}

Transformed request:

POST / HTTP/1.1

{"testkey": "testvalue"}

Replace

Replaces the body with a new value. If there is no body yet, it's added.

Example

Original request:

POST / HTTP/1.1

{"originalkey": "originalvalue"}

Transformation:

Replace body with value {"newkey": "newvalue"}

Transformed request:

POST / HTTP/1.1

{"newkey": "newvalue"}

Remove

Removes the body.

Example

Original request:

POST / HTTP/1.1

{"key": "value"}

Transformation:

Remove body.

Transformed request:

POST / HTTP/1.1

Expressions

JavaScript

You can use JavaScript expressions in your transformations to make them even more dynamic.

Expressions must evaluate to a new string value for the header/query string parameter/body.

Examples

Example — News API

Say, you provide an API endpoint that returns contents of a news article. E.g.,

{
    "id": "b6355c53-44a3-464b-bc9e-759749cdb3ec",
    "title": "The title",
    "content": "Some long article body"
}

For your "Free" product, you'd like to return only the first 120 characters of article body.

With the following transformation added to the "Free" product, you configure Project X API Gateway to do that:

Note that the expression evaluation result is used as the new value for the response body.

Note

If the original request or response body was compressed using GZip, Deflate or Brotli encoding, Project X will compress the replaced body as well.

Expression variables

There are several variables you can use in your expressions.

Path parameters

path.params.* — placeholder values specified in the endpoint URL.

Note

Placeholder names are case-sensitive.

Example

If an endpoint URL is /resource/{resourceId}

and the HTTP request URL is /resource/801d49c2-ca05-42b1-97af-baf0ddf36ba3,

then there will be a variable path.params.resourceId with value "801d49c2-ca05-42b1-97af-baf0ddf36ba3".

path.params.resourceId // "801d49c2-ca05-42b1-97af-baf0ddf36ba3"

Client IP address

request.remote_addr — Client IP address.


Request headers

request.headers['header-name'] — Request header values.

Warning

Header names are lower-case.

Example

request.headers['content-type'] == 'application/json'

Request query string parameters

request.query['query_string_parameter_name'] — Request query string parameters.

Warning

Query string parameter names are case-sensitive.

Example
request.query['page'] > 100

Request body

request.body — Request body.

Example
request.body.length > 1000

Warning

Response variables are only available in response transformations.

Response status code

response.statusCode — HTTP status code of the response from the upstream.

Example
response.statusCode == 200

Response headers

response.headers['header-name'] — Response header values.

Warning

Header names are lower-cased.

Example
response.headers['content-type'] == 'application/json'

Response body

response.body — Raw response body.

Example
response.body.length > 0

jq

You can also use jq expressions for transformations.

jq is like sed for JSON data - you can use it to slice and filter and map and transform JSON with the same ease that sed, awk, grep and friends let you play with text.

Unlike JavaScript expressions, the result of jq transformation does not necessarily have to be a string. If the expression result is not a string, it's encoded as JSON.

Examples

Example — News API

Say, you provide an API endpoint that returns contents of a news article. E.g.,

{
    "id": "b6355c53-44a3-464b-bc9e-759749cdb3ec",
    "title": "The title",
    "content": "Some long article body"
}

For your "Free" product, you'd like to return only the first 120 characters of article body.

The following jq expression will do the job:

Note that the expression evaluation result is used as the new value for the response body.

Expression input

The followind is passed as input (.) to each expression:

Input for request transformation expressions
{
    "path": {
        "params": { // path parameters, e.g. /endpoint/{path_param_1}
            "path_param_1": "path_param_value_1",
            "path_param_2": "path_param_value_2"
        }
    },
    "request": {
        "remote_addr": "127.0.0.1", // client IP address
        "headers": { // request headers
            "request_header_name_1": "header_value_1",
            "request_header_name_2": ["header_values_2", "header_values_2"]
        },

        "query": { // query string parameters
            "query_string_param_1": "query_string_param_value_1"
        },

        "body": "request body content"
    },
}
Input for response transformation expressions
{
    "path": {
        "params": { // path parameters, e.g. /endpoint/{path_param_1}
            "path_param_1": "path_param_value_1",
            "path_param_2": "path_param_value_2"
        }
    },
    "request": {
        "remote_addr": "127.0.0.1", // client IP address
        "headers": { // request headers
            "request_header_name_1": "header_value_1",
            "request_header_name_2": ["header_values_2", "header_values_2"]
        },

        "query": { // query string parameters
            "query_string_param_1": "query_string_param_value_1"
        },

        "body": "request body content"
    },
    "request_transformed": { // request object, after request transformations are applied
        // ...
    },
    "response": {
        "statusCode": 200,
        "headers": {
            "response_header_name_1": "header_value_1",
            "response_header_name_2": ["header_values_2", "header_values_2"]
        },
        "body": "response body content"
    }
}

Tips and tricks

Feature toggle

Since transformations are applied in the scope of a product, you can use them to notify your API backend, what features are enabled for the current user.

You can configure Project X to add a header (for instance, X-Features-Enabled) to each request, with different values for each product. E.g.,

Free Basic Advanced
X-Features-Enabled (empty) full_article_content full_article_content,chatgpt_suggestions

On the API backend side you can check the value of the header and enable or disable the respective features.