In this article I’m going to describe a novel solution for making cached versions of dynamic content available, while attempting to strike a balance between flexibility, performance and the origin of dynamic content. This solution may not be suited for very dynamic content (where the updates are better triggered by rewriting the cached version when the content changes), but in those situations where the dynamic content may be built from a very large dataset on request from the users. I have two use cases detailing applications I’ve been involved in building where I have applied this strategy. This could also be implemented with a caching service in front of the main service, but will require the installation of a custom service and hardware etc. for that service.
The WMS Cache
WMS (Web Map Service) is an OGC (Open Geospatial Consortium) specification which details a common set of parameters for how to query a web service which returns a raster map image (a regular png/jpg/bmp file) for an area. The parameters include the bounding box (left,bottom,right,upper) and the layers (roads,rivers,etc) and the size of the resulting image. The usual approach is to add a caching layer in the WMS itself, so any generated image is simply stored to disk, and then checked if the disk exists before retrieve the data and rendering the image (and if it exists, just return the image data from disk instead). This will increase the rate of requests the WMS can answer and will take load off the server for the most common requests. We are still left with the overhead of parsing the request, checking for the cached file and most notably, loading our dynamic language of choice and responding to the request. An example of such a small and naive PHP application is included:
The next request which arrives with the identical set of GET-parameters, will be served with the overhead of loading PHP, parsing the PHP-script (which is less if you have APC or a similar cache installed), sorting the GET-parameters (so that bbox=..&x=.. is the same as x=..&bbox=..), serializing the response, checking that the file exists on disk (you could simplify this to just doing a read and checking if the read succeeded), copying the data from disk to memory and then outputting the data to the client (you could also use fpassthru() and friends which may be more optimized for simple reading and output of data, but that's not the main point here).
To relate this to our use case of the WMS, we need to take a closer look at how map services are used today. Before Google showed the world what a good map solution could look like with modern web technology, a map application presented an image to the user, allowed the user to click or drag the image to zoom or move, and then reloaded the entire page to generate the new image. If it took 0.5s to generate the image, that were not really a problem, as the data set is not updated very often and it is very easy to do these operations in parallel across a cluster. When Google introduced Google Maps, they loaded 9 visible images (tiles) in the first image, and then started loading other tiles in the background (so that when you scroll the map, it looks like the images are already in place). If you run an interface similar to Google Maps against a regular WMS, most WMS servers would explode and take the whole 42U rack with them. Not a very desirable situation. The easy solution if you have an unlimited set of resources, disk space and money is to simply generate all the available tiles up front, in the same way as Google has done it. This will require disk space for all the tiles, and will not allow your users to choose which layers then want included in the map (this will change as map services are starting to build each layer as a separate tile and then superimposing them in the user interface).
The problem is that most of us (actually, n - 1) are not Google, but most of us do not build map services either. For those of us who do, we needed a way of living somewhere in between of having to render our complete dataset to image tiles up front or running everything through the WMS. While working with Gunnar Misund at Østfold University College, I designed a simple scheme to allow compatible clients to fetch cached tiles automagically, while those tiles which did not exist yet, were generated on the fly from the background WMS. The idea was to let Apache httpd handle the delivery of already generated and cached content, while our WMS could serve those areas which were viewed for the very first time (or where the layer selection were new). It would not be as fast as Google Maps for non-cached content, but it wouldn't require us to run through our complete service to generate images either.
(This is just an example, and does only contain the center point of the image). This is decomposed into:
This is all good as long as image.jpg exists in the local path provided, so that Apache can just serve the image as it is from the location. Apache httpd (or lighttpd and other "serve files fast!"-httpds) are able to serve these static files in large numbers (it's what they were written for, you know..) with a minimum overhead. The problem is what to do when the file actually does not exist, which will happen each time a resource is requested for the first time, and we do not have a cache yet. The solution lies in assigning a PHP-file as the handler for any 404 error (file not found). This is a well known trick used all over the field (such as handling www.php.net/functionname direct lookup). In PHP you can use $_SERVER['REQUEST_URI'] to get the complete path of the request that ended in the 404.
The .htaccess file of the application is as simple as cake:
ErrorDocument 404 /wms/handler.php
I've enclosed a simple specification which were written as a description of the implementation when the project was done in 2005.
Generating thumbnails can also be transformed into the same problem set. In the case where you need several different sizes of thumbnails (and different rescales are needed for different applications), you can apply the same strategy. Instead of handing all the information to a resize script with the file name etc. as the argument, simply have the xsize and the ysize as part of the URL. If the file exists in the path, it's served directly with no overhead, otherwise the 404 handler is invoked as in the previous example. The thumbnail can then generated, saved in the proper location and the world can continue to rotate at it's regular pace.
This application can then be extended by adding new parameters in the url, such as the resize method, if the image should be stretched, zoomed and other options.
This is a very simple scheme that does not require any custom hardware or server software installed, and places itself neatly in between having a caching front end server between the client and the application and the hassle of generating the same file each and every time. It allows you to remove the overhead of invoking the script (PHP in this case) for each request, which means that you can serve files at a much greater rate and let your hardware do other, more interesting things instead.