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Nginx RApi

22 Apr 2015

I put Lua Rclient Library for Nginx and complete setup at GitHub repository for your convenience

I want to share a very simple and light way to setup custom api to R functions using Nginx server

There are of course more complete solutions available, for example using:

You will need Nginx for Windows from http://nginx-win.ecsds.eu/ This is a compiled version of Nginx server that includes support for Lua I.e. OpenResty

I used the Nginx image processing server with OpenResty and Lua as the guide to make this setup.

I want to design following API:

  • http://localhost:8080/rapi/calc?1+2+3 will return 6=1+2+3
  • http://localhost:8080/rapi/D?x*x will return derivative of x*x = x + x

First, let’s create a Nginx configuration in conf\nginx.conf file:

# These three directives should be tweaked for production
error_log stderr notice;
daemon off;
events { }

http {
	include       mime.types; 

	# rclient
	lua_package_path 'lualibs/?.lua;'; 

	server {
		listen 8080;

		# examples
		# http://localhost:8080/rapi/calc/23+92
		# http://localhost:8080/rapi/deriv/x*x    
		location ~ ^/rapi/(?<program>[^/]+)/(?<param>.*)$ {
			content_by_lua_file lualibs/serve_rapi.lua;
		}
	}

	server {
		listen       80;

		location / {
			root   html;
			index  index.html index.htm;		
		}
	}
}

Next we need to process http://localhost:8080/rapi R API requests in the lualibs/serve_rapi.lua file:

local program, param =
ngx.var.program, ngx.var.param

-- helper function to indicate not valid request

local function return_not_found(msg)
  ngx.status = ngx.HTTP_NOT_FOUND
  ngx.header["Access-Control-Allow-Origin"] = "*"
  ngx.header["Content-type"] = "text/html"
  ngx.say(msg or "not found")
  ngx.exit(0)
end

if not(program == "calc" or program == "deriv") then
  return_not_found("invalid program. only calc and deriv are supported")
end

local R = require "rclient"  
local r = R.connect()
r.text1 = param

local status, exception
-- catch errors

status, exception = pcall(function() 
  if program == "calc" then
  	r "text1 <- eval(parse(text=text1))"
  else
  	r "text1 <- deparse(D(parse(text=text1),'x'))"
  end
end)


local res
if not status then
	res  = exception
else
	res  = unpack(r.text1)
end

status = r.disconnect

--[How do I add Access-Control-Allow-Origin in NGINX?](http://serverfault.com/questions/162429/how-do-i-add-access-control-allow-origin-in-nginx)

ngx.header["Access-Control-Allow-Origin"] = "*"
ngx.header["Content-Type"] = "text/plain"
ngx.say("param: " .. param)
ngx.say("result: " .. res)
ngx.exit(0)

Now it is time to run a first test.

Please start Rserve, following the example from Lua Rclient Library

library("Rserve")
Rserve(args="--no-save")

I usually start Rserve with a batch file: Rterm.exe -e "library('Rserve');Rserve(args='--no-save')"

Next, start Nginx server and try following URL in your browser:

  • http://localhost:8080/rapi/calc?1+2+3 you should return 6
  • http://localhost:8080/rapi/D?x*x you should return x + x

Now we can setup a simple html page to access above RAPI:

<html>
<head>
<meta content="text/html;charset=utf-8" http-equiv="Content-Type">
<script type="text/javascript" src="jquery-1.11.2.min.js"></script>
</head>
<body>
<h3>Sample Nginx RApi</h3>
<div id="input">
    <select id="operation">
        <option value="calc">Calculator</option>
        <option value="deriv">Derivative</option>
    </select> 
    <br><br>
    <span>Please enter your expression and click Value</span>
    <br><br>
    <input id="expression" type="text" value="1+2" onkeydown="if (event.keyCode == 13) doValue()" />
    <button id="valueBtn" onclick="doValue()">Value</button>
</div>
<br><br>
<div id="output"></div>
  
<script type="text/javascript">
function doValue() {
    var url = "http://localhost:8080/rapi/" + $("#operation").val() + "/" + $("#expression").val();
    $.get(url, function (data) {
        //[How do I replace all line breaks in a string with <br /> tags?](http://stackoverflow.com/questions/784539/how-do-i-replace-all-line-breaks-in-a-string-with-br-tags)

        $("#output").html(data.replace(/(?:\r\n|\r|\n)/g, '<br />'));
    });
}
</script> 
<br>
<br>
</body>
</html>

This page can be accessed by going to http://localhost/

I put Lua Rclient Library for Nginx and complete setup at GitHub repository for your convenience

It adds support for native OpenResty sockets (i.e. cosocket api) instead of using LuaSocket following steps done at pgmoon is a PostgreSQL client library

Aside:

Lua Rclient Library at GitHub:

  • https://github.com/stepelu/lua-rclient
  • https://github.com/jucor/rclient/

To Debug Lua script, i highly recommend getting ZeroBrane Studio Following are sample tutorials:

For example you can try following script

local R = require "rclient"
  
local r = R.connect()
  
-- Lua --> R:

r.myvec  = { 1,2,3 } -- Array.

r.text1 = '1+2'
r.text2 = 'x*x'
  
-- Execute R commands and evaluate expression as in R interpreter:

r "text1 <- eval(parse(text=text1))"
r "text2 <- deparse(D(parse(text=text2),'x'))"
r "text1" --> [1] 3

r "text2" --> [1] x + x


local status = r.disconnect

You might also want to try LuaJIT:

Interesting Lua project castl

I also recommend installing Gow - The lightweight alternative to Cygwin to have easy access to curl and other useful utilities.

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