WhatTheBus gets its move on: live maps arrive

The “livemap” tag for WhatTheBus completes the work that I seeded in the last commit.

In the last update, we had finished a page that used jQuery AJAX updates to get the latest bus location from the cache.  Using the simulator to provide updates, the AJAX updates shows that we could get location updates as latitude and longitudes.  By watching the dev web server, I also saw that these requests were super light (0ms DB, 0ms view).

In this update, I took the same basic code and added Google maps (v3) interaction.  Using Google and jQuery together maps is refreshingly easy.

The first step was to render the map when the page loads.  This required adding the Google javascript library and pointing the page’s onload event  to initMap.  The initMap function creates a new map object centered on the bus’ location, creates a bus marker at the center, and then replaces the page’s live_map div with the map.

<script type="text/javascript" src="http://maps.google.com/maps/api/js?sensor=true&amp;key=<%= MAP_KEY || "not_set_in_#{RAILS_ENV}_config" %>"> </script>
<script type="text/javascript">
var map;
var busMarker;
var xref = '<%= @bus.xref %>';
var name = '<%= @bus.name %>';
var tstamp = new Date();
function initMap() {
  var centerCoord = new google.maps.LatLng(<%= @pos %>);
  var mapOptions = {
    zoom: 16,
    center: centerCoord,
    mapTypeId: google.maps.MapTypeId.ROADMAP
  };
  map = new google.maps.Map(document.getElementById("live_map"), mapOptions);
  busMarker = new google.maps.Marker({
    position: centerCoord,
    map: map,
    title: name,
    icon: "/images/bus.png"
  });
  window.setInterval('updateMap();', <%= MAP_UPDATE_SECS %>000);
}

The second step was to add the AJAX call on a timer.  After adding the update function timer registration, the updates simply extended the existing AJAX request.  This existing request already had the bus’ position so the work centered on interacting with the map.

To keep things friendly, we turn the last marker into a bullet point and change the bus name to the last time.  Then we create a new marker based on that latest position and center the map on that position too.  To prevent reduce server load for non-reporting buses, we move off the map page if there is no position data.

function updateMap() {
  busMarker.setIcon("/images/track.png");
  busMarker.setTitle(tstamp.getHours() + ":" + tstamp.getMinutes() + ":" + tstamp.getSeconds());
  tstamp = new Date();
  // get the data for the map
  jQuery.getJSON("/bus/index/"+ xref +".json?cache", {}, function(data){
    if (data.length==0) window.location.href("/bus/index/<%= @bus.xref %>");
      var newCoord = new google.maps.LatLng(parseFloat(data.buses[xref].lat), parseFloat(data.buses[xref].lng));
      busMarker = new google.maps.Marker({
        position: newCoord,
        map: map,
        title: name,
        icon: "/images/bus.png"
      });
      map.setCenter(newCoord);
  });
}

After a few tweaks to add navigation between the existing pages, the first use-case of WhatTheBus is in the bag!

The next step is to setup the code online and get a district to send updates!

WhatTheBus Map seeds planted, looks like Cucumbers

Today’s WhatTheBus progress (see tag “MapSeeds”) created the foundation for live maps. These maps will only show a single bus at a time because our initial use case is that a parent wants to monitor their child’s bus.

To accomplish this use case, we need to provide a list of all the buses in the system (grouped by district) and then allow users to select a bus and see the map.

This foundation started with a series of Cucumber tests that verified the navigation structure and the base map page.  These pages used the existing page tests (yawn).

In fact, most of this work is pretty dull Rails web navigation stuff.  The interesting parts were:

  1. DRY the bus location from cache by moving it into the model
  2. Add jRails so that we can use jQuery goodness
  3. Optimization: added index for Bus xref
  4. Optimization: added ?cache flag for the bus json requests to bypass database calls on location only requests

In our next pass, we’ll add support for Google Maps (v3).  Until then, we just have some simple script that pulls the current bus position using our existing bus JSON request.  I am using the simulator (“rake sim:move”) to verify this; unfortunately, Cucumber does have native AJAX support.

Here’s the <script>

<script type="text/javascript"> var xref = '<%= @bus.xref %>'; var name = '<%= @bus.name %>'; function initMap() { window.setInterval('updateMap();', <%= MAP_UPDATE_SECS %>000); updateMap(); } function updateMap() { jQuery("#ll").text("?,?"); // get the data for the map jQuery.getJSON("/bus/index/"+ xref +".json?cache", {}, function(data){ jQuery("#ll").text(data.buses[xref].lat + ',' + data.buses[xref].lng); }); } </script>

WhatTheBus fun with Cucumber and MemCacheD

Sometimes a problem has to kick you upside the head so you can learn an important lesson.  Tonight’s head slapper was an interaction between Cucumber and MemCacheD.

If you are using CUCUMBER AND MEMCACHE read this post carefully so you don’t get burned.  If you’re using MemCache and not writing tests then return to Jail, do not collect $200.

It’s important to note that Cucumber has the handy side effect of running each scenario in a transaction.  The impact is that the data from each scenario does not impact the next scenario.  (note: you can pre-load data into cucumber using fixtures).

However, Cucumber does not do any rollback for Cache keys added into MemCache.  In fact, your MemCache entries will happily persist between your development and test systems.

WhatTheBus has a simple check to reduce database writes – it only writes to the database if there is no cache hit for the bus.  My thinking is that we only need to add a new bus if there is no key as shown in this partial snippet:

  cache = Rails.cache.read params[:id]
  if cache.nil?
     bus = Bus.find_or_create_by_xref :name => params[:name], :xref => params[:id]
  end

This works great for live testing, but fails in technicolor for Cucumber because tests with the same ID will not make it to the find_or_create.

To solve the problem, I had to add a pre-condition (‘given’ in Cucumber speak) to each scenario to make sure the cache was cleared.  It looks like this in the scenario feature:

  Given no cache for "1234"

And that’s translated as code in the steps like so:

  Given /^no cache for "([^\"]*)"$/ do |id|
   Rails.cache.delete id
  end

WhatTheDB? Adding mySQL into WhatTheBus

Today’s WhatTheBus update added data persistence to the application. Ultimately, I am planning to use CouchDB for persistence; however, I wanted to show a SQL to document migration as part of this process. My objective is to allow dual modes for this application.

In the latest updates, I continued to show Test Driven Development (TDD) process using Cucumber. Before starting work, I ran the test suite and found a bug – spectacular failure if MemCacheD is not running. So my first check-in adds recovery and logging around that event. Next I wrote a series of tests for database persistence. These tests included checking a web page that did not exist at this time. I ran the tests – as expected, all failed.

The persistence was very simple: models for bus and district. These minimal models are created dynamically when a bus location is updated. The data contract is that the first location update should include the bus name and distract in the url. After the first update, only ID and location (lat, lng) are expected. In addition to the model and migrations, I also updated the database.yml to use mySQL.

Creating a web page for the bus (bus/index/[xref id]) required the addition of a little infrastructure for the application. Specifically, I had to add an application layout and style sheet. Just because I have a styles sheet, does not mean there is any style (I’ve got style, brother. I’ve got million dollar charm, sister. I’ve got headaches and toothaches and bad times too).

To preserve simplicity, I am not storing the location information in the database. Location is so time sensitive that I don’t want to create any storage burden and I’m using cache expiration to ensure that we don’t keep stale locations around.

Up next…. I’m going to add a simulator (in rake) to make it easier to work on the application.

WhatTheBus, Day1: MemCacheD roundtrip

Today I got the very basic bus data collection working using Cucumber TDD.  That means that I wrote the basic test I wanted to prove BEFORE I wrote the code that operates the test.

The Cucumber feature test looks like this:

Feature: Mobile Access
In order to ensure that location updates are captured
School Bus Location providers
want to have data they send stored on the site

Scenario: Update Location
When bus named “lion” in the “eanes” district with a id of “1234″ goes to “32,-97″
When I go to the bus “1234″ page
Then json has an object called “buses”
And json has a record “1234″ in “buses” with “lat” value “32″
And json has a record “1234″ in “buses” with “lng” value “-97″

There’s is some code behind this feature that calls the web page and gets the JSON response back.  The code that actually does the work in the bus controller is even simpler:

The at routine takes location updates just parses the parameters and stuffs it into our cache.  For now, we’ll ignore names and district data.

def at

Rails.cache.write params[:id], “#{params[:lat]},#{params[:lng]},#{params[:name]},#{params[:district]}”, :raw=>:true, :unless_exist => false, :expires_in => 5.minutes
render :nothing => true

end

The code that returns the location (index) pulls the string out of the cache and returns the value as simple JSON.

def index

data = Rails.cache.read(params[:id], :raw => true).split(‘,’)
if data.nil?
render :nothing => true
else
render :json => {:buses => { params[:id].to_sym => { :lat => data[0], :lng => data[1] } } }
end

end

Not much to it!  It’s handy that Rails has memcache support baked right in!  I just had to add a line to the environment.rb file and start my memcached server.