Monday, 6 January 2014

Transactional websites and navigation

There's lot's of things that make me happy. In my professional life, it's getting stuff done, helping other people or learning something new. Recently I learnt something which was probably widely known but I'd managed to miss all these years. I'm so inspired that I'm going to share it with you all.

A lot of transactional websites crumble when you dare to do something as reckless as use the back button or open more than one window on the site. The underlying reason is that the developer is storing data relating to the transaction – i.e. specific to a navigation event in a single window - in the session – which is common to all the windows. A very poor way to mitigate the problem is to break the browser functionality by disabling the back button or interaction via a second window. I must admit to having used this in the past to mitigate the effects of problems elsewhere in the code (alright, if you must know – I bastardized Brooke Bryan's back button detector, and as for the new window....well the history length is zero)

But how should I be solving the problem?

The obvious solution is to embed all the half-baked ingredients of a transaction in each html page sent to the browser and send the updated data model back to the server on navigation. This can work surprisingly well as long as the data on the browser is cleared down between sessions. But with increasingly complex datasets, this becomes rather innefficient, particularly on slow connections. Further there are times when we want the transaction state to reflect the session state: consider a shopping basket – if a user fills a shoppng basket then ends their session we might want to retain the data about what they put in their shopping basket – but we might also want to release any stock reserved by the act of adding it to the basket. Often the situation arises where we end up with (what should be) the same data held in more than one place (browser and server). At some point the representations of the truth will diverge – and at that point it all goes rather pear shaped.

A while back I created a wee bit of code for point and click form building – PfP Studio. Key to the utility of this was the ability to treat a collection of widgets (a form) as a widget itself. And the easiest way to achieve that was to support multiple windows. When I first wrote this, I decided that the best way to handle the problem was to add partitioned areas to the session – one for each window. This depended on the goodwill of the user to open a new window via the functionality in the app rather than the browser chrome: each window had to carry a identifier (the “workspace”) across navigation events. Effectively I was rolling my own session handling with transids

This has a number of issues – the PHP sites warns about leaking authentication tokens but there's also a lot of overhead when you start having to deal with javascript triggerred navigation, and PRGs.

Then the other day I discovered that the browser property was writeable in all major browsers! Hallelujah! Yes, it still means that you need to do some work to populate links and forms, but it's a lot simpler than my previous efforts – particularly with a javascript heavy site.

Any new window (unless it has been explicitly given a name in an href link or via has an empty string as the name – hence you simply set a random value if it's empty – and the value persists even if you press the back button.

While I think I've explained what I'm trying to say, a real example never goes amiss:

if ('' {
   var t=new Date();;