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Tony Leininger
Microsoft Corporation
June 22, 1998
Contents
Introduction
Tips for Scheduling
Tips for Content Updates
Tips for Optimizing Your Subscriber's Experience
Some Issues that Are Fixed in Internet Explorer 5
Some Unresolved Issues
Summary
For More Information
Microsoft® Internet Explorer 4.0 brought you Active Channel™ technology and the Channel Definition Format (CDF) as tools to organize and deliver information to users in a timely manner. After almost a year with this new technology, our channel partners and many users have provided some tips on how to make the subscription updates work better for everyone. Along the way, we gathered a few troubleshooting ideas and came across one or two bugs, which we'd like to provide information on as well.
This article shares tips on scheduling, content updates, and optimizing your subscriber's experience, and provides links to other parts of MSDN Online where you'll find technical information and detail on these subjects. You'll learn how to optimize updates, choose the best schedule, balance server loads, and find out what issues have been resolved in Internet Explorer 5.
<SCHEDULE> tags are at the heart of server-load management and your ability to deliver the right content to your audience. Here are a few essential details to keep in mind when building your CDF file.
For more details, see the Web Men Talking column and read the CDF Specification.
Obviously, you'll want to keep your content fresh, but remember that although scheduling frequency can be important for offline viewing, research shows that most users check for updated content only once a day. So there isn't much benefit in having updates occur more often than that. When you consider your content and your production cycle, you may even find that you can set the CDF to update less often -- say, every two or three days. For specifics, see the Web Men's Good Schedule Scenario #1.
Although you'll obviously want to avoid scheduling updates during the peak load times for your servers, keep in mind the time of day your target audience is most likely to have their machines on and be looking for your updates. Some content is targeted to business users and some is targeted to an audience at leisure. Which audience you are targeting will influence whether you schedule updates for daytime or evening hours. For example, our content partners in the entertainment industry have found that their subscribers visit their channels to find the latest movie reviews just before leaving work. When scheduling your update to occur, distribute the server load over the longest manageable period (see Update Tip #3 for other factors). For a summary of scheduling issues, see the Web Men's Rules of CDF Scheduling. The CDF Specification also has more scheduling information.
Internet Explorer will update your channel at a random time within the INTERVALTIME that you specify, so scheduling the update to download during the widest possible timeframe will help reduce your server load. Creating a 23-hour INTERVALTIME is one method you might use. In this scenario, Internet Explorer will randomly choose a time to download your new content. However, this may not be practical, depending on your situation. Schedule the widest update window that works for you. See the CDF Specification for more information on the scheduling tags.
Definitely keep your updates smaller than 500K, and around 250K is the ideal maximum size. Downloading 100K over a 28.8K modem takes 28 seconds, so a rule of thumb to remember is that it takes half a minute to download each 100K of content. Many channel updates have been significantly over the 500K mark and well above the 250K ideal size. When updating, take a moment to consider which files you need to download to your user's machine, and the total size of those files.
If a client machine isn't turned on when your channel is scheduled to update, or if the update didn't get a chance to download during the scheduled interval, Internet Explorer will queue up to update the next time the user goes online. Sounds simple enough. However, we uncovered a few details that will interest you and are possibly issues:
Client-side issues:
Server-side issues:
See the CDF Specification for more information on server behavior.
Internet Explorer 4.0 tags provide flexibility for controlling the actual files that download during an update, but some of these tags are tiresome to implement all of the time. For more efficient downloads, include the LASTMOD attribute and the crawler will download only the changes you've made. If your Web pages don't change often, using LASTMOD won't be a problem. Of course, if your content changes frequently, a dynamically generated CDF is in order.
A built-in feature of Internet Explorer 4.0 is the "gleam," a red asterisk that appears over your channel icon to notify users when there is new content. You can also choose for users to receive e-mail notification of new content by specifying an HTML page as e-mail within the CDF. For users to receive either the gleam or e-mail message, you must post a changed CDF file when you post your new content. If you only change other channel pages, such as .htm or .asp files, users will not receive either type of notice. Changing the CDF file can be as simple as adding a blank space somewhere and saving the file on the server. For more details, see the Web Men Talking column and CDF 201: Beyond the Basics.
When you use personalization, chances improve that you will give people exactly what they want from your channel. Letting users select the specific content they want to have downloaded also increases your channel's usefulness. Users have told us that they are looking for unique and useful content when subscribing, and customizable downloads are a big draw. For some pointers on channel personalization, see The Internet Start Site: Now It's Personal, which looks at how the Internet Start Site 
was personalized.
Active Server Pages (ASP) lets you run scripts on the server. If you can write HTML, you can create an ASP file. ASP helps you give channel users a uniform and simple interface, while making use of business logic. ASP also gives users a safe environment for Web applications that involve financial transactions. While ASP files can be listed among the linked files in your channel, you will have to include a MIME type of "application/x-cdf" in your ASP file, which generates a CDF file. Include the following line at the very top of the ASP file:
<% Response.ContentType = "application/x-cdf" %>.
If you author just a straight CDF file, you don't need this line of code. You also don't need it if you're using Internet Information Server (IIS) 4.0 and an extension of .cdx.
Also, if you use a third-party tool rather than writing the ASP pages by hand, you may not be able to dynamically generate the CDF file. See the CDF Specification for more information on dynamically generating your CDF file.
After downloading a channel, users may occasionally notice that some of the files are missing when they return to it. This is a side effect of the Internet Explorer 4.0 "scavenger," which removes old content from the user's machine. All downloaded Internet files go into the user's cache, and the scavenger removes excess data to keep the cache manageable. It prevents the cache from growing until it fills the user's hard drive.
While the scavenger operates in the background and is invisible to users, sometimes they can see this unintended effect on your channel. The scavenger should not delete channel content, especially if you use the LASTMOD attribute as described in Update Tip #3. Still, the scavenger prefers large items that are relatively old, so it may pull icons and images from the channel bar and replace them with default icons. In Internet Explorer 4.0, once the scavenger has deleted images, it can take a very long time for them to show up again, even if the crawler downloads those images back into the cache. These problems have been corrected in Internet Explorer 5.
To simplify the updating process, Internet Explorer 4.0 has a built-in auto dialer for machines to connect to the Internet to download the updates. This is very useful for Desktop items that need to get the latest information for stock quotes, sports scores, headlines, and the like. It's also useful for users who want to automate channel updates.
We have seen two behaviors that can affect the user experience. Sometimes the auto dialer stays connected, which means an unattended machine may not log off the Internet connection. A second issue is that the auto dialer may not trigger another update during the same Windows® session, but will reset if the user restarts their computer. Both of these behaviors have been corrected for Internet Explorer 5.
It would be helpful to predict crawl cycles so you would know when your server might experience peaks in demand. With this information, you could plan for and balance the expected server load more effectively. There currently isn't a way to do this. The best suggestion we've heard from publishers is to track the number of times your CDF files have downloaded in a single update cycle, and multiply that by the number of files included in the update. This will at least give you an estimate of the number of hits to expect the following day. If you have more ideas, let us know!
One of the best features of Desktop items is their ability to deliver extremely timely information directly to the desktop. Stock quotes, sports scores, and the latest headlines are examples (see Optimizing Tip #3: Use Active Server Pages). However, to work optimally, this requires a continuous connection to the Internet. When the connection is interrupted, or if a user restarts their computer, the Desktop item may go blank or return a "navigation canceled" message. Ideally, there would be a redirect to a cached file as a backup. At this time, there is no workaround for this.
By using these tips, you can change your CDF files so that update schedules are defined correctly for the best performance, schedule updates at the best times for you and your users, and distribute your server load over the longest manageable time period. Also, by keeping your updates small and using the LASTMOD attribute diligently, you will increase the speed of download times.
When you consistently change the CDF, the gleam or e-mail notification will alert end users that your channel has new content. In addition, using personalization and Active Server Pages will help you satisfy your end user's desires for the best and most usable information you have to offer.
Use these tips and let us know if you have others to share. Check back often for additional information about working with Active Channels, and let us know what other issues you'd like to see covered. Most of all, have fun with Active Channels!
Did you find this material useful? Gripes? Compliments? Suggestions for other articles? Write us!
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