Map cache usage by clients
This topic contains three parts. The first section outlines how ArcGIS applications access and use the tiles in a map cache. The second part explains how these applications store the tiles for later use. The final section provides some instructions for overlaying map caches in ArcGIS applications.
How applications access and use the cache
When you finish generating a map cache, the map service is automatically restarted and immediately begins using the cache. Any ArcGIS application that can display a map service uses the cache, although how the tiles are retrieved and used varies between applications.
When using the Web Application Developer Framework (ADF), you add cached map services to your applications the same way that you add any other map service. The Map control recognizes whether the service is cached and works with the service to get the cache tiles if they are available. If you are overlaying caches and the Web ADF cannot get any tiles for one of the services, it treats that service as dynamic.
After an initial connection to the GIS server, the Web ADF retrieves tiles directly from the virtual cache directory. This is the fastest way for the application to get the tiles. Tiles can only be retrieved this way for caches stored in the exploded format.
If the virtual cache directory is unavailable or the cache is stored in compact format, the Web ADF application attempts to get the tiles from the cache directory on the file system. See the Map Control discussion to learn the ways that a Web ADF application tries to retrieve a tile that is not immediately available through the virtual cache directory.
If you are viewing a single cached service in any Web application and pan to an area where tiles do not exist, the application does not display a dynamic image; instead, you don't see anything. One way to ensure that you see a map when you pan to an uncached area is to enable on-demand caching.
If the map appears more slowly than expected, examine the URLs of the map images to verify that the application is retrieving tiles. One way to do this is to open the application in Mozilla Firefox and click Tools > Page Info > Media.
- If you see URLs like this, your application is successfully getting tiles from REST requests:
- If you see URLs like this, your application is successfully getting tiles from the virtual cache directory:
- If you see URLs like this, your application cannot get tiles out of the virtual cache directory, but it is successfully getting the tiles from the server's file system:
- If you see some other URL format for your map images, your application is retrieving the tile in a less efficient way or the cache is not being used.
Multilayer caches are never recommended for use in Web applications. If you need to turn layers on and off, you can overlay two or more fused caches to achieve the same visual effect as a multilayer cache. To do this, use the following workflow:
- Place each group of layers that would have participated in the multilayer cache in its own map document. Make sure the full extent of the map documents is the same.
- Publish each map document as a map service.
- Create a fused cache for each of the map services using the same tiling scheme (matching image formats are not required).
- Add the services to your map with the desired layer order and transparencies. Users of your map will be able to turn the services on and off.
You add cached map services to ArcMap using the Add Data button the same way you add any other map service. There are two ways you can view a cache in ArcMap:
- Access the cache through a map service. To view a cache this way, browse to the GIS server and the map service that was used to create the cache. In this scenario, ArcMap makes an initial connection to the GIS server to determine if the service has a cache. Then ArcMap retrieves tiles from the cache directory on the server's file system.
- Access the cache as a raster dataset. To view a cache this way, browse to the directory containing the cache tiles and choose to add the dataset to ArcMap. The cache is represented with the same icon used to add all other rasters using the Add Data button. A cache accessed as a raster is for viewing only and cannot be queried. The advantage with this type of cache is that it is not tied to a map service and can be viewed when disconnected from the server, as long as you can still access the cache directory.
When a request is made for a tile at a scale that exactly matches a scale level in the cache, the map service returns the tile directly. Most often, requests do not exactly match the scale levels in the cache. In this situation, ArcMap generally requests the tile from the next closest scale level and resamples it to match the requested scale. This resampling is still much quicker than generating a tile dynamically; however, it does result in an image that looks different than the original tile. Map labels that have been cached may be difficult to read at certain scales in ArcMap due to this resampling. For best results, view the map at or near the scales from which the cache was created.
ArcMap is the most practical client for consuming multilayer caches. This is because ArcMap has the ability to blend and label multilayer cache tiles on the client side (unlike Web ADF applications).
ArcGlobe and ArcGIS Explorer
ArcGlobe and ArcGIS Explorer can read 2D caches and drape them over the surface of the globe. You'll get the best performance if you build the cache with one of the built-in tiling schemes that are used by ArcGIS Online, Google Maps, and Bing Maps. If you cannot use one of these tiling schemes, your best performance will come from using ArcGIS Server globe services.
One scenario where you may want to add a map service to ArcGlobe is when you've created a 2D map cache and want to create an identical 3D globe cache from the 2D cache. In this scenario, you would only be concerned with generating the cache and would not be navigating the globe.
Local caching of layers on the client
ArcMap, ArcGIS Explorer, and ArcGlobe all maintain local caches of the tiles you have visited. When you zoom and pan back to these areas, the application uses the local cache and does not have to retrieve the tiles from the server again.
You can choose whether you want to save the local cache on disk for use in future sessions or use the local cache for the current session only.
You can also choose not to cache tiles locally. Although this causes you to lose the performance benefit of local caching, you can always be sure that you are getting the latest tile from the server.
It's possible that the server administrator has disabled local caching of the layer, which also means that you always get the most up-to-date tile. If this is the case, the other options for saving the cache locally are unavailable.
If the server administrator updates an area of the server cache that you have already viewed, you must clear your local cache to see the changes. ArcMap, ArcGIS Explorer, and ArcGlobe cannot detect if a cache has been updated. If you are a server administrator, you need to alert your users when updated data is available so that they know to clear their cache.
When designing map caches that will be overlaid with other map caches, there are some important cache design factors to consider depending on which application will be used to view the caches.
Overlaying map caches in Web applications
The easiest way to do this is to match tiling schemes for both caches, then only create tiles at the scales that make sense for each cache. This way, you can be sure that you've matched coordinate system and tile size and that the software recognizes that the two caches have scales in common.
When designing map caches that will overlay other map caches in a Web ADF application, there are certain rules you need to follow to ensure that the caches overlay successfully. See Overlaying map caches in the .NET Web ADF to learn more about these conditions.
Overlaying map caches in ArcMap
You can overlay any number of map caches in ArcMap. The caches do not need to have the same tiling schemes or projections; however, because of the resampling and reprojecting that ArcMap performs, using equivalent tiling schemes and projections may improve the performance and aesthetic quality of your map.