Sunday, October 4, 2009

Thoughts on making the ESRI REST API a little more scalable

Spørg om jeg har haft problemer med at fremstille, flytte og håndtere cached tiles ...

Have I had problems producing, managing and moving cached tiles ...


[...] One of the concerns I (and others) have with the current architecture of the REST server in AGS 9.3 is with the way how the cached map services are served out by the server. Even though, all the tiles have already been cooked and saved on disk. The default way the tiles are served out by the server is through the AGS REST server. This means that the tiles/images on the disk are not directly served out by IIS itself but the AGS REST handlers (implemented as IHTTPHandler in the library) need to get involved. Every tile request coming into the handler is served out really fast but IIS does have to call into the ASP.NET module that spins up a new thread for this request and this thread is blocked until the image request has been completed. Again, let me re-iterate. This image request-response by the REST handler is very fast. But, why do we need to be satisfied with fast when we can be FASTER. If the tile requests are made directly to just the tile images on a web server (doesn’t need to have AGS) by default, the request doesn’t even need to get to the AGS (which now doesn’t need to be externally accessible and can be behind your firewall if need be) and this tile request can be satisfied by just the web server without having AGS involved. This way cached layers can be served to the light-weight clients by just the web server which could in the cloud or the images can be stored and served directly from Amazon storage services. These things are currently possible with the light-weight clients but ESRI can definitely do their part to make that process much easier. All that’s fine but why is the performance going to be any better? Well, if the web server (IIS) is just serving out tile images from the disk, we can enable kernel-mode caching in IIS to server out these tiles which is magnitudes faster than serving it through the handler. When kernel-mode caching is enabled in IIS, the request is satisfied purely by IIS without the request having to enter the ASP.NET and be served by the IIS worker process. So, yeah it is MUCH MUCH faster with kernel-mode caching. [...]

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