Photos Calendar; an extension for Google Calendar using Java App Engine and Picasa API

Posted by jimblackler on Nov 6, 2011

A few years ago there was a lot of talk about web ‘mash ups’; using web APIs to combine web applications.

Photos Calendar is an old-school mash up that brings together Google+, Picasa and Google Calendar, all using a Java web app running on Google App Engine.

To try it out click here. Or read on for the inspiration for the app, and how it was built.


Now that Android enables instant upload of photos to the cloud, I’ve been sharing lots of pictures taken with my phone onto Google+. Also, as I’ve used Picasa Web Albums for some time, the cloud now has quite a bit of my photo history.

Another web app I’ve been using for years is Google Calendar. It occurred to me that it would be interesting to see links to my cloud-based photos appearing on Google Calendar, in the form of events placed at the time the pictures were taken. In some cases these would appear next to the original appointment from the event.

As well as making the task of finding particular pictures easier, it also may help you to recall what you were doing on a particular day by checking your calendar. But above all it would simply add an interesting new way of browsing your existing data.


I believed the idea was practical. The Picasa service is the storage engine for Google+ picture posts, and it has a public API. Also, Google Calendar has an API and other ways that calendar data can be imported to the app.

In fact, developing the app wasn’t particularly hard. The toughest bits were handling the Picasa API’s authentication mechanism via App Engine, fixing ambiguities with the time zone on photo time stamps, and creating the pages to explain to users what to expect (since the app has no user interface of its own).

App Engine

Ideally I wanted the app to run on Google App Engine, because this service provides a scalable and robust way of serving simple apps.

I considered the best way to get the events to appear on Google Calendar. I could have used Calendar’s gData API to import events to users’ existing calendars. However this approach would have modified the main event calendar rather than adding an extra layer, and would have created the need to synchronize data on a schedule.

It’s much easier to serve an iCal calendar on demand and have users add this to their calendars as an extra layer. iCal is the name of the built-in calendar app on Macs, and it’s format, of the same name, is the the main format used by Google Calendar to import calendars.

Fortunately App Engine allows virtually any content to be served by implementing the HttpServlet class. If an App Engine app can serve iCal format calendar data, extra calendar data can simply be imported into Google Calendar by adding a URL. This means that for most users the app can be set up with just a few clicks. Users can also toggle display of the photos calendar on and off as they see fit.


I discovered that the excellent iCal4j library could work on App Engine. Not all Java libraries will work on App Engine ‘out of the box’ because of restrictions imposed by the platform, such as a lack of threading support. iCal4j allows Java apps to create data in the iCal format. From the developer’s perspective, adding events to an iCal4j calendar is pretty simple.

If the calendar service is also a client to the Picasa API, the two services can be bridged. The servlet queries the API, iterates the results, and spits out a calendar with one event per photo.


So how to enable the servlet to make API queries? The problem is, to serve a user’s private data Picasa requires an authentication token to be sent with every query. This token is obtained as part of an authentication process where the user is sent to the Google website in order to agree to give the application access to his or her data.

If the iCal service is to make Picasa queries then it will need this token. But as the service is to serve many users, how is it to know which token to use to serve requests? The answer is to have the calendar URI carry the token, meaning that the service need stores no information about each user. The information can be carried in the URL encoded in a ‘data’ parameter. The URL is prepared by the servlet that receives the data following the authentication request, for each user, embedded with all information required to create the calendar layer for each user.

However it is necessary for security reasons to encrypt the authentication token. Otherwise, anyone who can access the calendar URL could also reveal the Picasa authentication token which could allow them to make any Picasa API calls with full permissions granted by the user. If it is encrypted with a key private to the app this attack vector is heavily reduced. There is no way to avoid discovery of the calendar URL allowing access to the photos calendar layer to anyone with that URI, but there are no way that these URLs will be revealed to outsiders in normal use of the app.

Time Stamps

It’s crucial for this app that the pictures events are created on the correct date and times, since that’s all the app does! It’s easy to get the timestamp the pictures were created, Picasa serves this information from the camera EXIF (picture metadata) data accompanying the picture. The problem is that these time stamps are local to the time they were taken, but no time zone is given. This is a big problem when combined with the fact that iCal calendars imported into Google Calendar have to be given an explicit time zone. If I don’t get the combination right, pictures could appear up to 24 hours removed from the actual time the picture was taken.

Using location data from the EXIF might offer one solution, but location data is usually removed from the pictures at upload time for security reasons. I have no option but to assume that all pictures are taken in the time zone of the user.

It’s actually quite difficult to get the time zone of the current user, even though the app has their Picasa credentials. The information does not appear to be available through the general Google Account, but there is a roundabout way of getting it through the Calendar API (by looking at the timezone on the default calendar). However I didn’t want the time overhead of using this API, or to ask for another permission that users may be dubious about, and more tokens that would need to be carried on the URL.

My fallback was to use a technique I’d employed on my Events Clock project. It is possible to use JavaScript to make an educated guess as to the user’s current time zone based on the offset applied to the Date object.

I used an excellent snippet from the Redfin Developer’s Blog to detect the time zone. This, being JavaScript, happens on the client side, but the data is stored in a cookie for later retrieval on the server side following successful authentication of the Picasa API. It’s an approximation, and it relies on the assumption that the user’s browser timezone is the same as their camera timezone, but it got good results in my tests.

You can see the app for yourself here:

The app is free software under a GPL license. The source can be found here