How can I manipulate the JSON body of a POST request using Nginx and Lua?

Question

I am doing a proof of concept to demonstrate how we might implement 3scale in our stack. In one example I want to do some POST request body manipulation to create an API façade that maps what might be a legacy API format to a new internal one. Eg. change something like

{ "foo" : "bar" , "deprecated" : true }

into

{ "FOO" : "bar" }

The Lua module docs for content_by_lua, which seems like the appropriate method say

Do not use this directive and other content handler directives in the same location. For example, this directive and the proxy_pass directive should not be used in the same location.

My understanding is that the content_by_lua is a content handler like proxy_pass, only one of which can be used per location.

I don't think there's any way to remove proxy_pass as that's the basis of how the proxying works, so is it possible capture the request in a separate location, use content_by_lua, then pass to the location implementing proxy_pass or is there a different method like rewrite_by_lua which is more appropriate?

---

If it helps anyone else, I added the following function (my first bit of Lua) which removes the user_key parameter which 3scale requires for authorization but is invalid for our API if forwarded on:

function remove_user_key()
  ngx.req.read_body()
  -- log the original body so we can compare to the new one later
  local oldbody = ngx.req.get_body_data()
  log(oldbody)
  -- grab the POST parameters as a table
  local params = ngx.req.get_post_args()

  -- build up the new JSON string
  local newbody = "{"

   for k,v in pairs(params) do
     -- add all the params we want to keep
     if k ~= "user_key" then
        log("adding"..k.." as "..v)
        newbody = newbody..'"'..k..'":"'..v..'",'
     else 
        log("not adding user_key")
     end
   end
  --remove the last trailing comma before closing this off
  newbody = string.sub(newbody, 0, #newbody-1)
  newbody = newbody.."}"

  ngx.req.set_body_data(newbody)
  log(newbody)
end

if ngx.req.get_method() == "POST" then
  remove_user_key()
end

Answer 1

I will suggest you to use access_by_lua
in nginx.conf

location / {
                #host and port to fastcgi server
                default_type text/html;
                set $URL "http://$http_host$request_uri";
                access_by_lua_file /home/lua/cache.lua;
                proxy_pass http://$target;
                -------
                ---------

in cache.lua file you can do something like :

if ngx.req.get_method() == "POST" then
    -- check if request method is POST 
    -- implement your logic 
    return
end

Answer 2

Adding to what Prashant already mentioned: when you download your Nginx configuration files from 3scale you will notice that there is a Lua file included. This file is already being called from access_by_lua_file.

That file would be in my opinion the best place to add your body manipulation code. It will be executed for every request before the proxy_pass to your API server is sent.

Also, this is a really good in-depth blog article about how to accomplish different transformation to your requests using Lua inside Nginx:

Answer 3

Recently I had to manipulate upstream based on JSON value in post request and I found this useful: NGINX LUA and JSON

It is basic configuration but gives an idea how to do it.