What is map caching?
What is map caching? |
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| Release 9.3 |
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Map caching is a very effective way to make your ArcGIS Server maps run faster. When you create a map cache, the server draws the entire map at several different scales, and stores copies of the map images. The server can then distribute these images whenever someone asks for a map. It's much quicker for ArcGIS Server to hand out a cached image than to draw the map each time someone requests it. Another benefit of caching is that the amount of detail in the image doesn't noticeably affect how quickly the server can distribute the copy.
With caching, the only time you pay the performance penalty for drawing the map is when you create the cache. For this reason, creating the cache might take a long time. However, the benefits of caching make up for the time it takes to create the cache. These are some of the main benefits of caching:
Performance: When you deploy cached maps, your users can get more done because they spend less time waiting for the map to draw.
Quality: With caching, performance isn't affected by how much detail is in the map. It doesn't matter whether or not you used shaded relief, transparency, or the Maplex labeling engine to create your map. A cache is just a collection of images, and the server can return one cached image in more or less the same amount of time it can return any other cached image.
Industry standard: Have you ever gone to a popular Internet mapping site and watched as the little square tiles fill in as you pan and zoom? Often, it takes just a second for all the tiles to appear. That's because they're using caching. With ArcGIS Server map caching, you can achieve similar performance for your maps.
Caching does not happen automatically. To make a cache, you first need to design the map and publish a service from it. Then, you'll set some cache properties and begin creating tiles. You can choose to create all the tiles at once or allow some of the tiles to be created on demand, that is, when someone first visits them.
ArcToolbox contains the Server Tools toolbox which is used for caching. The tool you'll use most often is Manage Map Server Cache Tiles. You can access this tool through the Caching tab of the Service Properties dialog box in ArcCatalog.
When you cache a map, you draw it at more than one scale so that users can zoom in and out of the map. When choosing properties for your cache, you'll need to determine what scales you want to use for caching. If you're just trying out the caching tools for the first time, you can let the computer pick some scales for you. However, you should usually pick your scales earlier so that you can design your map to look good at those scales. Write down the scales and use them as you do your cartographic design in ArcMap. When it's time to create the cache, you can enter the scales into the cache generation tool.
There are other properties that are important to understand when you create the cache. You can find out more by reading Choosing cache properties.
The cache is stored in your server cache directory. If you installed all the components of ArcGIS Server on one machine, the installation process created a server cache directory for you. If you installed ArcGIS Server across multiple machines, you need to create the server cache directory yourself. See Creating a server directory if you need help with this.
The scales that you pick and the properties you set for the cache are the tiling scheme. Each cache has a tiling scheme file that you can import when you create new caches so that all your caches use the same tile sizes and scales. This can help the performance of your ArcGIS Server Web applications that contain more than one cached service. Optionally, you can choose to use the well-known tiling schemes of ArcGIS Online, Google Maps, and Microsoft Virtual Earth so that you can easily overlay your caches with data from these online mapping services.
A map cache represents a snapshot of your map at one point in time. Because of this, caches work best with maps that do not change frequently. These include street maps, imagery, and terrain maps.
If your data tends to change, you may still be able to use the caching tools to periodically update the cache. You can even schedule these updates to occur automatically. To understand whether your frequently changing map can still be cached, it helps to ask these questions:
How up-to-date does my map need to be?
If the data you see on the map needs to be live, with no time delay acceptable, then caching is not appropriate. However, if a short delay is acceptable and the cache updates can be performed within that time window, you can still use caching.
How big is my cache and how widespread are the changes to my data?
These two questions go together. A large cache takes more time to create. It may only be practical to update your large cache if you can isolate the changed areas and update those only. If the cache is small, you may be able to quickly rebuild the entire cache.
If the update cannot keep up with the changes in an acceptable amount of time, the map is not appropriate for caching.
After considering the questions above, use caching whenever it's appropriate. The performance benefit that you gain is probably worth the investment it takes to create and update the cache.
To learn more about updating your cache, see Updating a map cache.
Although map caches represent pictures of the data, you can still allow others to perform identify, search, and query operations on your map service. These tools can get the geographic locations of features from the server and return the results. The application will draw the results in its native graphics layer format on top of the cached image.