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Response Transformer

Transform the response sent by the upstream server on the fly on Kong, before returning the response to the client.

Note on transforming bodies: Be aware of the performance of transformations on the response body. In order to parse and modify a JSON body, the plugin needs to retain it in memory, which might cause pressure on the worker's Lua VM when dealing with large bodies (several MBs). Because of Nginx's internals, the `Content-Length` header will not be set when transforming a response body.

Terminology

  • plugin: a plugin executing actions inside Kong before or after a request has been proxied to the upstream API.
  • Service: the Kong entity representing an external upstream API or microservice.
  • Route: the Kong entity representing a way to map downstream requests to upstream services.
  • Consumer: an entity that makes requests for Kong to proxy; it represents either a user or an external service.
  • Credential: a unique string associated with a Consumer, also referred to as an API key.
  • Upstream service: this refers to your own API/service sitting behind Kong, to which client requests are forwarded.

Configuration

This plugin is compatible with requests with the following protocols:

  • http
  • https

This plugin is compatible with DB-less mode.

Enabling the plugin on a Service

With a database
Without a database

Configure this plugin on a Service by making the following request:

$ curl -X POST http://kong:8001/services/{service}/plugins \
    --data "name=response-transformer"  \
    --data "config.remove.headers=x-toremove, x-another-one" \
    --data "config.remove.json=json-key-toremove, another-json-key" \
    --data "config.add.headers=x-new-header:value,x-another-header:something" \
    --data "config.add.json=new-json-key:some_value, another-json-key:some_value" \
    --data "config.append.headers=x-existing-header:some_value, x-another-header:some_value"

Configure this plugin on a Service by adding this section to your declarative configuration file:

plugins:
- name: response-transformer
  service: {service}
  config: 
    remove.headers: x-toremove, x-another-one
    remove.json: json-key-toremove, another-json-key
    add.headers: x-new-header:value,x-another-header:something
    add.json: new-json-key:some_value, another-json-key:some_value
    append.headers: x-existing-header:some_value, x-another-header:some_value

{service} is the id or name of the Service that this plugin configuration will target.

Enabling the plugin on a Route

With a database
Without a database

Configure this plugin on a Route with:

$ curl -X POST http://kong:8001/routes/{route}/plugins \
    --data "name=response-transformer"  \
    --data "config.remove.headers=x-toremove, x-another-one" \
    --data "config.remove.json=json-key-toremove, another-json-key" \
    --data "config.add.headers=x-new-header:value,x-another-header:something" \
    --data "config.add.json=new-json-key:some_value, another-json-key:some_value" \
    --data "config.append.headers=x-existing-header:some_value, x-another-header:some_value"

Configure this plugin on a Route by adding this section to your declarative configuration file:

plugins:
- name: response-transformer
  route: {route}
  config: 
    remove.headers: x-toremove, x-another-one
    remove.json: json-key-toremove, another-json-key
    add.headers: x-new-header:value,x-another-header:something
    add.json: new-json-key:some_value, another-json-key:some_value
    append.headers: x-existing-header:some_value, x-another-header:some_value

{route} is the id or name of the Route that this plugin configuration will target.

Enabling the plugin on a Consumer

With a database
Without a database

Configure this plugin on a Consumer with:

$ curl -X POST http://kong:8001/consumers/{consumer}/plugins \
    --data "name=response-transformer"  \
    --data "config.remove.headers=x-toremove, x-another-one" \
    --data "config.remove.json=json-key-toremove, another-json-key" \
    --data "config.add.headers=x-new-header:value,x-another-header:something" \
    --data "config.add.json=new-json-key:some_value, another-json-key:some_value" \
    --data "config.append.headers=x-existing-header:some_value, x-another-header:some_value"

Configure this plugin on a Consumer by adding this section to your declarative configuration file:

plugins:
- name: response-transformer
  consumer: {consumer}
  config: 
    remove.headers: x-toremove, x-another-one
    remove.json: json-key-toremove, another-json-key
    add.headers: x-new-header:value,x-another-header:something
    add.json: new-json-key:some_value, another-json-key:some_value
    append.headers: x-existing-header:some_value, x-another-header:some_value

{consumer} is the id or username of the Consumer that this plugin configuration will target.

You can combine consumer.id and service.id in the same request, to further narrow the scope of the plugin.

Global plugins

A plugin which is not associated to any Service, Route, or Consumer (or API, if you are using an older version of Kong) is considered "global", and will be run on every request. Read the Plugin Reference and the Plugin Precedence sections for more information.

  • Using a database, all plugins can be configured using the http://kong:8001/plugins/ endpoint.
  • Without a database, all plugins can be configured via the plugins: entry on the declarative configuration file.

Parameters

Here's a list of all the parameters which can be used in this plugin's configuration:

Form ParameterDescription
nameThe name of the plugin to use, in this case response-transformer
service.idThe ID of the Service the plugin targets.
route.idThe ID of the Route the plugin targets.
consumer.idThe ID of the Consumer the plugin targets.
enabled

default value: true		Whether this plugin will be applied.
config.remove.headers
optional

List of header names. Unset the header(s) with the given name.
config.remove.json
optional

List of property names. Remove the property from the JSON body if it is present.
config.rename.headers
optional

List of original_header_name:new_header_name pairs. If the header original_headername is already set, rename it to new_headername. Ignored if the header is not already set.
config.replace.headers
optional

List of headername:value pairs. If and only if the header is already set, replace its old value with the new one. Ignored if the header is not already set.
config.replace.json
optional

List of property:value pairs. If and only if the parameter is already present, replace its old value with the new one. Ignored if the parameter is not already present.
config.add.headers
optional

List of headername:value pairs. If and only if the header is not already set, set a new header with the given value. Ignored if the header is already set.
config.add.json
optional

List of property:value pairs. If and only if the property is not present, add a new property with the given value to the JSON body. Ignored if the property is already present.
config.append.headers
optional

List of headername:value pairs. If the header is not set, set it with the given value. If it is already set, a new header with the same name and the new value will be set.
config.append.json
optional

List of property:value pairs. If the property is not present in the JSON body, add it with the given value. If it is already present, the two values (old and new) will be aggregated in an array.

Note: if the value contains a , then the comma separated format for lists cannot be used. The array notation must be used instead.

Order of execution

Plugin performs the response transformation in following order

remove –> rename –> replace –> add –> append

Examples

In these examples we have the plugin enabled on a Route. This would work similar for Services.

  • Add multiple headers by passing each header:value pair separately:

With a database

$ curl -X POST http://localhost:8001/routes/{route}/plugins \
  --data "name=response-transformer" \
  --data "config.add.headers[1]=h1:v1" \
  --data "config.add.headers[2]=h2:v1"

Without a database

plugins:
- name: response-transformer
  route: {route}
  config:
    add:
      headers: ["h1:v1", "h2:v2"]
upstream response headers proxied response headers
h1: v1
  • h1: v1
  • h2: v1
  • Add multiple headers by passing comma separated header:value pair (only possible with a database):
$ curl -X POST http://localhost:8001/routes/{route}/plugins \
  --data "name=response-transformer" \
  --data "config.add.headers=h1:v1,h2:v2"
upstream response headers proxied response headers
h1: v1
  • h1: v1
  • h2: v1
  • Add multiple headers passing config as JSON body (only possible with a database):
$ curl -X POST http://localhost:8001/routes/{route}/plugins \
  --header 'content-type: application/json' \
  --data '{"name": "response-transformer", "config": {"add": {"headers": ["h1:v2", "h2:v1"]}}}'
upstream response headers proxied response headers
h1: v1
  • h1: v1
  • h2: v1
  • Add a body property and a header:

With a database

$ curl -X POST http://localhost:8001/routes/{route}/plugins \
  --data "name=response-transformer" \
  --data "config.add.json=p1:v1,p2=v2" \
  --data "config.add.headers=h1:v1"

Without a database

plugins:
- name: response-transformer
  route: {route}
  config:
    add:
      json: ["p1:v1", "p2=v2"]
      headers: ["h1:v1"]
upstream response headers proxied response headers
h1: v2
  • h1: v2
  • h2: v1
h3: v1
  • h1: v1
  • h2: v1
  • h3: v1
upstream response JSON body proxied response body
{} {“p1” : “v1”, “p2”: “v2”}
{“p1” : “v2”} {“p1” : “v2”, “p2”: “v2”}
  • Append multiple headers and remove a body property:

With a database

$ curl -X POST http://localhost:8001/routes/{route}/plugins \
  --header 'content-type: application/json' \
  --data '{"name": "response-transformer", "config": {"append": {"headers": ["h1:v2", "h2:v1"]}, "remove": {"json": ["p1"]}}}'

Without a database

plugins:
- name: response-transformer
  route: {route}
  config:
    append:
      headers: ["h1:v2", "h2:v1"]
    remove:
      json: ["p1"]
upstream response headers proxied response headers
h1: v1
  • h1: v1
  • h1: v2
  • h2: v1
upstream response JSON body proxied response body
{“p2”: “v2”} {“p2”: “v2”}
{“p1” : “v1”, “p2” : “v1”} {“p2”: “v2”}