Creating a node.js GeometryService : part 1

Ever since I encountered the geoservices-rest-specification (pdf) I've been thinking about creating my own implementation. I also wanted wanted to try out node.js so I combined both ideas and started implementing a GeometryService in node.js. If you've never heard of node.js, this is the short introduction from the homepage of node.js:
Node.js is a platform built on Chrome's JavaScript runtime for easily building fast, scalable network applications. Node.js uses an event-driven, non-blocking I/O model that makes it lightweight and efficient, perfect for data-intensive real-time applications that run across distributed devices. The GeometryService is a web service that contains GIS related utility methods like project, intersect, buffer,... A sample server can be found here http://sampleserver1.arcgisonline.com/ArcGIS/rest/services/Geometry/GeometryServer.

I first downloaded and installed the windows installer of node.js version 0.65. After a reboot all locations where added to my path so that I can now start a new cmd window and type node. Note that when you install packages globally with the node package manager called npm (npm install <package> -g) npm (the node package manager) then the packages are installed in Windows 7 under %AppData%\npm (C:\Users\<username>\AppData\Roaming\npm). The full list of packages is located here search.npmjs.org.

To get started learning node I first read the node.js tutorial at nodebeginner.com. By following this tutorial I got a great insight in how to create your own webserver. At this moment I'm continuing to build my webservice based on this code but I plan to use some framework like express or journey in a later stage. This is not very urgent because I'll first focus on the JSON output of the GeometryService and thus won't need to output any html.

For my first version of the GeometryService I decided to use PostGIS as my geometry processor. I know this introduces an extra overhead. But I think this is the easiest way to get something up and running in a short period of time. If you don't want to bother with installing PostgreSQL and PostGIS I suggest you to download the Community edition of the Open Geo Suite. To be able to connect to the database I installed node-postgres with following command line: npm install pg -g. If you're on windows and get build errors you might try to add a file called true.cmd to the directory where node.exe resides (e.g. C:\Program Files (x86)\nodejs) with as content exit 0. Below is a short snippet on how I connect to the database. Make sure to replace all placeholders in the connection string.

var pg = require("pg");

function executeSQL(sql, parameters, resultCallback){
  var connectionString = "pg://username:password@host:port/databasename";
  pg.connect(connectionString, function(err, client) {
    client.query(sql,parameters, function(err, result) {
      // TODO add error handling
      console.log(result);
      resultCallback(result);
    });
  });
}

That was it for today. In the next part of the series I'll write about debugging node.js and about my progress on the GeometryService.

As I announced in this post, I open sourced the code of this project. It and can be found in this bitbucket repository.

1 comment:

  1. It is so good and informative! Thanks for your good sharing!

    ReplyDelete