This demo Java command-level web application refers to an embedded Neo4j graph database containing publicly available railroad locations. A graph database consists of nodes connected by links. Nodes and links can have name / value properties. The DBMS provides an API to navigate the resulting graph. A Lucene index enables an application to find a node-of-interest.
In our case, each node is a integer-numbered "station" from a subset of public 1996 census data of the USA railroad network. A link (track) connects two stations. Each link has a node-to-node distance property. Neo4j has a built-in A* shortest path algorithm that returns an iterator across the shortest path through the graph of "stations".
Our mobile / desktop web application uses a "from" and "to" slider to select rail station route endpoint numbers (1..133752) to compute a shortest path route between the two stations. A server-side controller calls Neo4j to obtain an iterator used to emit Google KML for the shortest path between the two stations. The controller is independently addressable via a URL. A browser-side button click handler reads the station slider values. Then it uses a Google API to insert a KML layer into a Google Map of the USA. The insertion API takes our controller URL as a parameter, thus obtaining the KML for the route layer.
We deploy to a Heroku application, myjavane4j. The application is the lowest-scale dyno, meaning that it "spins down" after a period of disuse. The dyno may take 30 seconds to restart, and another 30 seconds for our application to insert its Neo4j database to Heroku ephemeral storage. After that, each request is fairly instantaneous -- until an hour or so of disuse. That's a low price to play in a sandbox. Try the application:
http://myjavaneo4j.herokuapp.com/
Clone this repository to your file system. We used Eclipse 3.7 Juno. The directory tree is suitable to import into Eclipse as a Maven project. You may need environmental adjustments if your computer is not a Mac on Mountain Lion. We created a web application server runtime for Apache Tomcat 7.29 in Eclipse.
To publish a derivative of this application on your own site, you would need your own Google API key to programmatically supply a KML layer to Google Maps. It's located in the index.html file, the only html file. The map API requires that your KML be publicly available on the web. Our development order was:
- Test the shortest path logic with JUnit
- Write a controller that used that logic to return a stream of KML. Test the controller by dropping its URL into Google Maps' search field in a browser.
- Publish a web application containing the controller to a publicly visible site (we first opened an HTTP port on our router for this)
- Only then did we flesh out the view logic.
Note that if you test locally, your logic must always refer to a publicly available KML source. We eventually deployed our WAR to a public Heroku application, as we refined the browser-side "logic." Our single HTML page always referred to our Heroku version of the application developed in steps 1 - 3.
See github project https://github.com/mauget/railroutes_cmd for creating the database and for an command-invoked version of this application that displays in Google Earth.
You'll need to change two values.
- The Google API key in the
index.htmlfile. - The
APP.urlStemvalue of your public deployed controller URL in thescripts/app.jsfile.
Changing these two dependency values to integrate via a Mustache or Handlebars template would be a good TODO.
- A route displays as a blue line -- Google Earth zooms to accommodate.
- An example from an iPHone 4 follows.
