Building a cache of tiled map images can be a great way to speed up your map services, especially detailed basemaps that give your application a geographic context. If you’ve attempted to build a map cache before, you’re probably aware that it can take a significant investment of time and disk space to create a large cache.
Server administrators naturally want to understand how long it will take to build a map cache and how much space on disk the cache will require. A couple of new features in ArcGIS 10.1 make it easier to estimate the size of your cache and track its progress as tiles are being built.
Estimating cache size
A big difference with caching in 10.1 is that you can define the parameters of the cache before you even publish the service. ArcGIS can take this information and provide an idea of the disk space required by the cache.
A good cache size estimate obviously helps you understand if your hardware will be adequate, but it can also lead you to make a better decision about whether to build all the tiles immediately at publish time (another 10.1 feature) or build the tiles in stages by running the caching tools manually.
As soon as you adjust the scale levels and image format of your cache, you’ll see a rough estimate of the cache size is provided for you as the Estimated Cache Size. Although this estimate is better than nothing, it is determined only by mathematics and won’t reflect the characteristics of your map. You should go ahead and click the new Calculate Cache Size button to get a more refined guess at the size of your cache.
The Calculate Cache Size utility actually generates sample tiles of your cache and then extrapolates the estimate based on the sample tile size. The better the quality of the estimate you request, the more sample tiles will be created. In many cases it’s worth the few extra minutes to get the most precise estimate.
If you’ve defined an area of interest feature class for your cache, you should understand that the Calculate Cache Size utility returns an estimate for the entire extent rectangle of the feature class. Your resulting cache may be smaller than estimated.
Getting progress reports as the cache is built
When you install ArcGIS Server 10.1, you might notice that a new preconfigured service called ReportingTools begins running on your server. This service allows you to request a progress report on any map cache, whether tiles are currently being built or not. To view the report, click the tiles icon next to any cached service in ArcGIS Server Manager. (Or, in ArcMap, right-click any cached map service in the Catalog tree and click View Cache Status.)
The initial report window is small and just shows the percentage of cache completed. Click Show Details to get an expanded window with a full level-by-level report, including information about whether tiles are currently being generated and which scale levels are being worked on. The number of tiles built for each scale level is compared with the expected number of tiles to return a completion percentage. Immediately, you’ll begin to see how the larger scales of your cache require the most time and resources.
The percentage of cache completion in the report is based on the full rectangular extent of your map. Therefore, if you submitted an irregularly shaped area of interest feature class when you ran the caching job, your percentage may not reach 100%.
Contributed by Sterling Quinn of the ArcGIS for Server development team