ArcGIS Explorer Build 480: Threading!

ArcGIS Explorer (AGX to its friends, apparently) has a new release out, with a great number of promised new features. I'd watched previous builds of AGX through an HTTP proxy to see what all it got up to while fetching images from the web. Investigation showed that it was brutally single-threaded and not terribly bright about what order to retrieve tiles in.

Build 480 changes those things a great deal for the better. Even when looking at a single layer, such as the default satellite/aerial imagery provided, you can see AGX running 2 to 4 simultaneous connections to fetch tiles. Their documentation promises multi-threaded downloads only on a dual core machine, which I've got, so your mileage may vary. I would have thought that even a single core machine would benefit from having several HTTP requests in flight, as they involve so much waiting around, but one suspects ESRI did some performance testing.

When connecting to a very slow service, such as most of the WMS servers out there, I was able to see AGX with as many as 10 connections in flight at once. This is good! There are still some starvation issues (the crappy WMS service kept the fairly snappy ESRI service from showing up for a long time), but this is a great improvement.

The order in which tiles are retrieved still seems suspect to me, with the world view first fetching Asia, the north pole, and Antarctica before the western hemisphere view which is the default. Grabbing multiple images at once compensates for the imperfect tile fetching strategy. And like before, shutting down AGX while it has several open HTTP connections is not pretty: it waits until they are timed out to shut down completely (even continuing to fetch new tiles while it tries). These flaws mean that you still need to be careful using any service which is slow or broken -- other imagery will get stuck behind it.

More news as investigations continue. So far, so encouraging!