In the rankers app that I’ve recently finished for a client, I cached the users data offline, but needed a way to quickly and easy to see if the user had the freshed version of the data.

Downloading database updates

To speed up loading time, and enable offline access, I download the list of places (about 350kb) by an ajax call, and then cache the json and the etag of the latest revision in localStorage. Then when the app launches next time, I send a request for the newest version of the places list, and attach the etag to the request, so that rails can send a not-modified response, and save the users some data.

Here’s the rails pseudocode code I used:

@places = do 

response.etag = @etag = [
  Place.find(:first, :order => 'updated_at desc').updated_at.to_i,

if request.fresh?(response)
   head :not_modified
  render :action => 'index', :mime_type => 'application/json'

And in the view:

<% cache(@etag) do %><%= %><% end %>

This cached the json (since json generation can be pretty slow in ruby), and sends the appropriate header. To handle this on the client side, I used something like this, to cache the places data, and the etag.

$.ajax {
  url : ""
  dataType : 'json'
  headers : { 'If-None-Match' : localStorage.getItem('etag') }
  success : (data, textStatus, xhr) =>
    if textStatus == "success"
      localStorage.setItem('etag', xhr.getResponseHeader('Etag'))
      localStorage.setItem('placesData', JSON.stringify(data))
    else if textStatus == "notmodified"
      data = JSON.parse(localStorage.getItem('placesData'))
      throw "your toys"


I was going to implement my own version of etags and if-none-match, since I thought I might get heisenbugs with different implementations of xmlhttprequest on different mobile webkits, but in the end, I decided not to reinvent the wheel and use what already exists. So far this has worked well.