Gojko Adzic

HTTP Sessions allow us to develop web applications as if they were running on a desktop machine, making the web so much more useful. Although HTTP is a stateless protocol and there is a lot of work involved in providing this abstraction, web servers make it very easy to use — perhaps too easy. Taking a quote from Spiderman, with great power comes great responsibility. That is why misusing HTTP sessions is probably the number one obstacle to building scalable web sites today. Here are some tips how to consume HTTP sessions responsibly. Continue Reading »

It is fairly obvious that web site performance can be increased by making the code run faster and optimising the response time. But that only scales up to a point. To really take our web sites to the next level, we need to look at the performance problem from a different angle.

How much can you handle?

Although an average web server is able to process a few thousand requests per second, the number of requests it can actually handle at the same time is severely limited. Here are some simple figures: Continue Reading »

One of the most important architectural decisions that must be done early on in a scalable web site project is splitting the data flow into two streams: one that is user specific and one that is generic. If this is done properly, the system will be able to grow easily. On the other hand, if the data streams are not separated from the start, then the growth options will be severely limited. Trying to make such a web site scale will be just painting the corpse, and this change will cost a whole lot more when you need to introduce it later (and it is “when” in this case, not “if”). Continue Reading »

The print-on-demand service that I’ve chosen for my book has a fairly nice web user interface with lots of Ajax code. Generally, that web application is one of the best aspects of doing business with them. That’s why I was surprised to find a rather amateurish flaw in their shopping cart. Here is the screenshot:

Notice the “do not click Place Order more than once” message. Someone has obviously identified that customers have been clicking more times than needed, but offering a warning message is not the way to solve the issue. In fact, such a message is a huge warning signal that something is wrong.

I’m pointing out this case in particular because it is one of the most frequent problems I’ve seen in Ajax sites, and one so easy to avoid. The proper solution is incredibly simple: always disable the form as the first step of asynchronous processing. It is very easy to block the “Place Order” button before an Ajax call goes out to the server, and doing so would technically prevent anyone using a proper browser to place the same order twice. For high volume sites, like this one, I always recommend implementing some sort of double-order analytics on the server just in case, because exceptions do happen, but just a simple JavaScript command to disable that button would save everyone a lot of trouble.

Effective content caching is one of the key features of scalable web sites. Although there are several out-of-the-box options for caching with modern web technologies, a custom built cache still provides the best performance. Continue Reading »