The RC for GWT 1.4 provides native support for image concatenation in a feature called ImageBundle. If you’ll recall, I discussed image concatenation in a round up last month, and it’s a key embodiment of our “First Principal of Performance Optimization” — reduce network requests as much as possible.
Given that this is Google, they had to take things to that next level. And by that, I of course mean that their implementation has a few twists and refinements so brilliant and elegant that they seem obvious in retrospect and make the rest of us feel unworthy. Because we are.
The aspect of ImageBundle that jumps out at me is the infinite cacheability of the images: the file name includes an MD5 of the image contents, so there is no need for even periodic HTTP round trips for if-modified-since or if-none-match checks. You can set the expiration date to the expected heat death of the universe, or beyond. Thus, network requests are only made when they are absolutely necessary. Only one network round trip is made at that time. Everything is as simple as possible, but not one bit simpler.
And to those naysayers who may suggest a pathological case where minor modifications to one or two contained images cause frequent downloads of the larger, concatenated image: it just doesn’t matter. As this post at edgeblog explores, it’s still the latency, stupid. The real cost of object downloads is the round tripping. Establishing a connection, sending a request, and waiting for the first byte dominate network times. The added marginal cost of additional contents is always a fraction of those setup costs as the bytes of the reply are in flight by the time you begin receiving the reply. And as bandwidth increases while latency remains relatively flat, the dominance of latency will only increase.
