Manual Caching and Automatic Caching for Programs

Part 2 in our series on Working Offline in Windows 2000: the pros and cons of automatic and manual caching for programs.

 By Brien M. Posey
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In Part 1 of this series, I discussed the idea that Windows 2000 allows you to cache network-based files and folders so that mobile users can use them on the go, just as if they were attached to the network. In that article, I explained that there are three distinct types of caching for files and folders: automatic caching, manual caching, and automatic caching for programs. In this article, I'll continue the discussion by explaining the pros and cons of manual caching and automatic caching for programs.

Manual Caching

As I mentioned in Part 1, manual caching works best for users who need access to a large number of files or directories. The only significant downside to manual caching is that it requires user intervention. The biggest issue that mobile users need to keep in mind about manual caching is that there are no preset cache size limits. Therefore, it's possible for a user to run low on hard disk space by caching too much data. According to Microsoft, the amount of data that can be manually cached is limited to 2 GB. However, if the hard disk on the mobile computer has less than 2 GB of free space, nothing will stop the mobile user from running the machine out of space with cached files.

Automatic Caching for Programs

The Automatic Caching for Programs option allows mobile users to access network-based programs while working offline. For example, suppose your users run Microsoft Word by double-clicking on an icon that points to a copy of Microsoft Office that's stored on a network share. In this case, under normal circumstances, mobile users won't be able to use Microsoft Word while on the go. However, automatic caching for programs can make a program such as Microsoft Word available to mobile users even when they are working offline. Unfortunately, there are some very serious issues that you'll encounter when using this feature.

To understand why automatic caching for programs can be problematic, consider the way that automatic caching works in general. In the basic implementation of automatic caching, files are cached only when accessed. The same thing happens in automatic caching for programs. To see how this affects mobile users, let's return to my earlier example of a user who wants to use Microsoft Word while offline.

If a mobile user attempts to use a network-based copy of Microsoft Word (or any other program) while offline, what they will be able to do depends on several factors. Remember that automatic caching only caches files as they are accessed and replaces cached files with newer files as the cache fills up. This means that if a mobile user has never accessed Microsoft Word while online, then the program won't be available to them offline. Likewise, if the mobile user has previously used Microsoft Word while online, but has accessed other automatically cached files since then, Microsoft Word may or may not be available, depending on whether the cache had to remove files to make room for new files.

It would seem that when mobile users need to use a program offline, they need to open that program just before they disconnect, to make sure it will be available when working offline. Actually, this isn't entirely true. Remember that only the program files the user has accessed will be cached. Therefore, if a user doesn't use all of the program's features, there's a good chance that not all of the program's files will be cached. For example, suppose a mobile user opens Microsoft Word just before disconnecting. Later, when the user tries to use Microsoft Word while offline, he will be able to access the programbut because not all the files have been cached to the local hard disk, some features (such as the spell check) won't be available. If, however, the user had opened Microsoft Word while online and used the spell check at that time, then the spell check files would have been cached (disk space permitting) and the spell check feature would now be available offline.

This article was originally published on Dec 19, 2000
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