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API Transformer

This is a Kong Plugin that transforms requests and responses depending on your own business requirements.

Kong Plugin API Transformer

For questions, details or contributions, please reach us via the API Transformer GitHub repo.

request transformer example

local _req_uri = ngx.var.uri
local _req_method = ngx.req.get_method()
local _req_json_body = ngx.ctx.req_json_body

if _req_uri == string.match(_req_uri, ".-/folders") then
  -- modified body will pass to upstream
  if _req_method == "GET" then
    return true, '{"key": "GET /folders: body modified"}'
  elseif _req_method == "POST" then
    return true, '{"key": "POST /folders: body modified"}'
  end
else
  return false, "invalid request uri: " .. _req_uri
end

response transformer example

local _req_uri = ngx.ctx.req_uri
local _req_method = ngx.ctx.req_method
local _req_json_body = ngx.ctx.req_json_body
local _resp_json_body = ngx.ctx.resp_json_body

if _req_uri == string.match(_req_uri, ".-/folders") then
  -- modified body will return to client
  if _req_method == "GET" then
    return true, '{"key": "GET /folders: body modified"}'
  elseif _req_method == "POST" then
    return true, '{"key": "POST /folders: body modified"}'
  end
else
  return false, "invalid request uri: " .. _req_uri
end

Allowed Lua symbols in the transformer

  print
  assert
  error
  ipairs
  next
  pairs
  pcall
  select
  tonumber
  tostring
  type
  unpack
  xpcall
  string.byte
  string.char
  string.find
  string.format
  string.gmatch
  string.gsub
  string.len
  string.match
  string.rep
  string.reverse
  string.sub
  string.upper
  table.insert
  table.maxn
  table.remove
  table.sort
  table.insert
  table.concate

Available OpenResty symbols in the transformer

ngx.ctx
ngx.var
ngx.req.get_headers
ngx.req.set_header
ngx.req.get_method
ngx.req.get_body_data
ngx.req.set_body_data
ngx.req.get_uri_args
ngx.req.set_uri_args
ngx.resp.get_headers

Symbols which cached in ngx.ctx for the response transformer

This table ngx.ctx can be used to store per-request Lua context data and has a life time identical to the current request, so we use this table to store the necessary data for body_filter()

Cached Symbols Coreresponding Lua type
ngx.ctx.req_uri ngx.var.uri string
ngx.ctx.req_method ngx.req.get_method() string
ngx.ctx.req_json_body _cjson_decode_(ngx.req.get_body_data()) table
ngx.ctx.resp_json_body ngx.arg[1] talbe

Convensions in writing the transformer

In the transformer, we need to return a Lua tuple: (f_status, body_or_err), please check the detail via test case.

if f_status == true then
  body_or_err = transformed_body
else
  body_or_err = error message
end

Configuration Reference

This plugin is not compatible with DB-less mode.

Enable the plugin on a service

For example, configure this plugin on a service by making the following request:

curl -X POST http://{HOST}:8001/services/{SERVICE}/plugins \
    --data "name=api-transformer"  \
    --data "config.request_transformer=/home/foo/api_xxx/req_transformer.lua" \
    --data "config.response_transformer=/home/foo/api_xxx/resp_transformer.lua" \
    --data "config.http_200_always=true"

SERVICE is the id or name of the service that this plugin configuration will target.

First, create a KongPlugin resource:

apiVersion: configuration.konghq.com/v1
kind: KongPlugin
metadata:
  name: <api-transformer-example>
config: 
  request_transformer: /home/foo/api_xxx/req_transformer.lua
  response_transformer: /home/foo/api_xxx/resp_transformer.lua
  http_200_always: true
plugin: api-transformer

Next, apply the KongPlugin resource to a Service by annotating the Service as follows:

apiVersion: v1
kind: Service
metadata:
  name: {SERVICE}
  labels:
    app: {SERVICE}
  annotations:
    konghq.com/plugins: <api-transformer-example>
spec:
  ports:
  - port: 80
    targetPort: 80
    protocol: TCP
    name: {SERVICE}
  selector:
    app: {SERVICE}

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

Note: The KongPlugin resource only needs to be defined once and can be applied to any service, consumer, or route in the namespace. If you want the plugin to be available cluster-wide, create the resource as a KongClusterPlugin instead of KongPlugin.

For example, configure this plugin on a service by adding this section to your declarative configuration file:

plugins:
- name: api-transformer
  service: {SERVICE}
  config: 
    request_transformer: /home/foo/api_xxx/req_transformer.lua
    response_transformer: /home/foo/api_xxx/resp_transformer.lua
    http_200_always: true

SERVICE is the id or name of the service that this plugin configuration will target.

Enable the plugin on a route

For example, configure this plugin on a route with:

$ curl -X POST http://{HOST}:8001/routes/{ROUTE}/plugins \
    --data "name=api-transformer"  \
    --data "config.request_transformer=/home/foo/api_xxx/req_transformer.lua" \
    --data "config.response_transformer=/home/foo/api_xxx/resp_transformer.lua" \
    --data "config.http_200_always=true"

ROUTE is the id or name of the route that this plugin configuration will target.

First, create a KongPlugin resource:

apiVersion: configuration.konghq.com/v1
kind: KongPlugin
metadata:
  name: <api-transformer-example>
config: 
  request_transformer: /home/foo/api_xxx/req_transformer.lua
  response_transformer: /home/foo/api_xxx/resp_transformer.lua
  http_200_always: true
plugin: api-transformer

Then, apply it to an ingress (Route or Routes) by annotating the ingress as follows:

apiVersion: networking/v1beta1
kind: Ingress
metadata:
  name: {ROUTE}
  annotations:
    kubernetes.io/ingress.class: kong
    konghq.com/plugins: <api-transformer-example>
spec:
  rules:
  - host: examplehostname.com
    http:
      paths:
      - path: /bar
        backend:
          serviceName: echo
          servicePort: 80

ROUTE is the id or name of the route that this plugin configuration will target.

Note: The KongPlugin resource only needs to be defined once and can be applied to any service, consumer, or route in the namespace. If you want the plugin to be available cluster-wide, create the resource as a KongClusterPlugin instead of KongPlugin.

For example, configure this plugin on a route by adding this section to your declarative configuration file:

plugins:
- name: api-transformer
  route: <route>
  config: 
    request_transformer: /home/foo/api_xxx/req_transformer.lua
    response_transformer: /home/foo/api_xxx/resp_transformer.lua
    http_200_always: true

ROUTE is the id or name of the route that this plugin configuration will target.

Enable the plugin globally

A plugin which is not associated to any service, route, or consumer is considered global, and will be run on every request. Read the Plugin Reference and the Plugin Precedence sections for more information.

For example, configure this plugin globally with:

$ curl -X POST http://{HOST}:8001/plugins/ \
    --data "name=api-transformer"  \
    --data "config.request_transformer=/home/foo/api_xxx/req_transformer.lua" \
    --data "config.response_transformer=/home/foo/api_xxx/resp_transformer.lua" \
    --data "config.http_200_always=true"

Create a KongClusterPlugin resource and label it as global:

apiVersion: configuration.konghq.com/v1
kind: KongClusterPlugin
metadata:
  name: <global-api-transformer>
  annotations:
    kubernetes.io/ingress.class: kong
  labels:
    global: \"true\"
config: 
  request_transformer: /home/foo/api_xxx/req_transformer.lua
  response_transformer: /home/foo/api_xxx/resp_transformer.lua
  http_200_always: true
plugin: api-transformer

For example, configure this plugin using the plugins: entry in the declarative configuration file:

plugins:
- name: api-transformer
  config: 
    request_transformer: /home/foo/api_xxx/req_transformer.lua
    response_transformer: /home/foo/api_xxx/resp_transformer.lua
    http_200_always: true

Parameters

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

Form Parameter Description
name
required

Type: string		 The name of the plugin, in this case api-transformer.
service.id

Type: string		 The ID of the Service the plugin targets.
route.id

Type: string		 The ID of the Route the plugin targets.
enabled
required

Type: boolean

Default value: true		 Whether this plugin will be applied.
config.request_transformer

The .lua script to be used for the transformation. Available OpenResty variables and utils: Check README
config.response_transformer

The .lua script to be used for the transformation. Available OpenResty variables and utils: Check README
config.http_200_always

Default value: true

We may need to use HTTP 200 approach in API error handling in some business cases.