API Transformer

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

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.
• 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

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

$ curl -X POST http://kong: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"

Without a database

Configure this plugin on a Service by adding this section do 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

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

Enabling the plugin on a Route

With a database

Configure this plugin on a Route with:

$ curl -X POST http://kong: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"

Without a database

Configure this plugin on a Route by adding this section do 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

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

Global plugins

• 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.

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.

Parameters

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

Kong Plugin API Transformer

For questions, details or contributions, please reach us at https://github.com/qnap-dev/kong-plugin-api-transformer

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()

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