Vishful thinking…

Thoughts on making the ESRI REST API a little more scalable

Posted in ESRI by viswaug on September 22, 2009

These are just a couple of my thoughts on how the ESRI REST API can be made a little more scalable. The ESRI REST API has been a boon to me and probably many other GIS developers out there who had to get knee deep in the ADF and build usable apps with it. The ADF has gotten a lot better in 9.3 and the ADF guys at ESRI have done a good job of improving the framework by leaps and bounds. But still, I stay away from the ADF partly because it is still not easy to use and debug. Definitely not as easy to use as the REST API with JS/Flex/Silverlight clients. The REST API inherently leverages the power of ‘HTTP goodness’ and makes the scalability of the apps that are built with the light-weight clients much more scalable. The scalability is achieved because of the fact that the data is cached as necessary both the REST server and the client browser.

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.

Also, in my snooping around the REST API, I found that the handlers in the REST API are implemented as IHTTPHandlers. The scalability of the API can be improved a little if these handled are written as IHTTPAsyncHandlers. When using IHTTPAsyncHandlers the threads serving these requests are not blocked until the request is served. Since, the serialization to JSON etc currently happen on the REST web server in AGS 9.3, the time to serve the requests might be non-trivial. So, this makes a higher number of threads available in the thread pool for the IIS worker process to use. This will definitely increase the scalability of the ESRI REST API and your web application. Given, there are things that we need to pay attention to when using this async programming model since, the number of threads could explode when serving very large number of requests and the process might just spend a lot time setting up the threads and cleaning up after them.

Those are just two thoughts I had on the subject today and I managed to put in words. I am also all ears to other ideas/criticisms on the subject.

I am also hoping that in the near future, ESRI’s SOAP and REST API have the exact same service contracts that are just exposed at different endpoints. Currently, the SOAP API seems to more feature-filled and the REST API seems to be playing catch up. Aren’t SOAP and REST just different ways of accessing the same exact GIS feature set in ArcGIS Server? Alright, I will end the rant right here. 🙂