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Server-Side Caching Options

Server-Side Caching Options

by John Peterson

Why Cache?

Back in the dark ages (a year or so ago) most ASP developers were happy when they got their code to actually run. Then they either went home for some much needed rest or went out to celebrate the fact that their code actually worked and the project was finished.

Well that was then and this is now. Now your code not only has to work, it has to be scalable enough to handle thousands of users. On top of that, it also needs to be fast for all these users (because somewhere along the line the average user went from a slow analog modem to a cable modem or DSL line that's almost as fast as your server). Unfortunately, this means that you can't immediately blame the graphic designers anymore and you've got to make sure your code runs as quickly as possible (if not quicker). Throwing more hardware at the problem will sometimes help, but if you can't afford it (or can't get it approved by your boss) you're not left with too many choices except trying to increase the speed of your code. This is where caching can come into play.

In general when you mention caching most ASP developers immediately get scared. This is understandable because caching and dynamic content generally don't work well together, but used correctly, caching can help solve many of your performance and scalability problems and turn a sluggish site into a real speed demon.

Types of Server-Side Caching

First let me emphasize that I'm not talking about client-side or proxy server caching. While these are great technologies, they're not going to do us much good from the point of view of dynamic content. In addition, they are usually outside of our control except for the setting of some expiration headers that we can't even ensure that they will pay attention to.

What I am talking about is moving your data as close as possible to the state in which it'll be delivered to the client. I'll illustrate the point using a very common example, which I'm sure everyone has seen: an html <select> element which contains a list of states from which you're supposed to choose your home state.

<select name="state">
  <option value="AL">ALABAMA</option>
  <option value="AK">ALASKA</option>
  <option value="AS">AMERICAN SAMOA</option>
  ... 
  <option value="WV">WEST VIRGINIA</option>
  <option value="WI">WISCONSIN</option>
  <option value="WY">WYOMING</option>
</select>

To start with, let's assume you have a table in a database with the names and abbreviations of each state (if you don't you will once you download the code at the end of this article). The goal is to build the dropdown box using the database so that as you add states or need to make changes, those changes are reflected in your web pages. That being said, how often do we actually add a state and need to change this information in our forms? Well not being a big history buff, I'm not sure, but what I am sure of is that it won't be happening in the next couple hours in which time hundreds of people could possibly be hitting your web page. So why should you hit the database to dynamically build the dropdown each time? The answer is you shouldn't be!

I'm going to illustrate 4 different methods of building the state selection box. They are:

No Caching Hits the data source for each request of the page
Data Caching Stores the data in a temporary high-speed location and then builds the output using it
Element Caching Takes the data and transforms it into the appropriate output and then caches the output
Page Caching Caches the whole page and saves it as an file

No Caching

This is the easiest method and doesn't really need much discussion. I only mention it to provide a baseline for the others. This is the only option for truly dynamic data. Most people wouldn't appreciate it if when they went to see their stock quotes or bank statement they weren't up to date. Similarly, shopping carts and any other real-time data generally shouldn't be cached.

Data Caching

What I mean by Data Caching is taking a copy of the data from the database or other slow data source (like external web pages) and storing it in a faster location. Where you store it is really dependent on how it'll be used and how quickly it needs to be accessed. In general, I tend to use Application level variables to store a disconnected / custom recordset or an array. Text or XML files can also be very useful for larger amounts of data that you're not willing to devote the memory to by storing it in an Application variable. The main issue with using files is that it slows things down. So, unless you're caching data from a very slow link, you might actually be slowing things down. That being said, I've used it quite often for caching http requests from other servers. Eventually this won't be needed, but with all the progress we've made, the web still isn't 100% reliable and it certainly isn't always fast. This way if I don't get an answer back or if it takes too long I can go to the cached version and at least give the user something besides an error!

While this method can substantially increase performance it still leaves us with the data in a state that we can manipulate it. In our example, we can not only build a select box of states, we could also display them in a table of the new quarters that have been released by the US Mint, or sort them into reverse order to really confuse people. In short, the data is still flexible and we can manipulate it easily in order to use it for a variety of purposes.

Element Caching

This takes Data Caching and goes one step further. If you know you're always going to be building a select box out of the state data then why are you caching the data itself and not the select box? The concept here is to speed things up even more by only doing the processing to build the box once and then caching the output. Then when you want to display the box again it's as quick and easy as simply doing a Response.Write.

While this method is faster and just as easy as Data Caching, the one drawback is that you do lose some of the flexability. Trying to build a table of states from a string containing a select box of the states is a lot of work and at that point it's probably easier to go back to the database to get the data again!

Page Caching

This is the ultimate in server-side caching. In this scenario we do all the processing ahead of time and build static .htm files. The benefit here is that there's no processing involved at all when requesting the cached files. The ASP interpreter never gets involved once the page is built. It's just as fast as if you had hand written the page and hard coded the data in... except for the fact that you didn't have to do it by hand! There are a couple different ways to build the files. In the example I just use the FileSystemObject to write the text I want in the file because it's simple to illustrate and you won't need to install a component. Many people find using a HTTP component to retrieve the dynamic file from the web server and then writing it out to a static file to be easier... especially if you're attempting to cache multiple pages.

The main drawback here is that determining when to refresh the cache can be a little difficult since no processing is happening. You can't do it in the page itself since it doesn't process. You also can't use global.asa unless you've got other asp files in the web being requested so it'll run (a decent option if you're just caching a few high traffic pages). Otherwise, you're pretty much left to refreshing the cache on your own or setting up a scheduler to do it for you.

Because this is a little more complex and I know there's a product that does this well, I feel I'd be remiss if I didn't mention XBuilder and XCache. Both products address page level caching, but they do it in slightly different ways. For more information see the link at the end of this article.

Refreshing the Cache

The most difficult part of any caching system is knowing when the content should not be cached anymore. If you don't cache it long enough, it can defeat the whole point of doing it at all. On the other hand, if you don't refresh often enough you'll be dealing with old data and that's not good either.

Unfortunately this is an issue you'll need to decide for yourself. It's really dependent on the volatility of the data. In our example, I was joking around about how rarely it changes, but in all seriousness that's why I choose it. It's content like this, that's really in the database just for management and is used a lot but rarely changes that is the perfect candidate for some form of caching.

The Code

Ok so enough talk already... here's what you're really after... the code. I've included the database of the states and a quick example of each of the 4 methods mentioned. I use Application variables to cache for the Data and Element Caching samples and the file system (naturally) for Page Caching. This means you'll need to have an Application setup and you'll need permission to write to the file system to run the Page Caching sample, but besides that they should all be pretty easy to get to run.

Getting Started

So now that you've read about the options and played with the code how do you actually go about getting started? I usually look for pages on which to implement caching based on two criteria: traffic to the page and the amount of rarely changing data that the page uses. If a page gets a lot of traffic then you want it to be as fast as possible and use as few resources as possible. Even a small performance increase on a busy page can make a big difference in your site's apparent speed and processor utilzation on your server. Along the same lines, if a page uses a lot of slow data sources then the page is a natural candidate for caching even if it doesn't get a lot of traffic just so that it loads faster when it is requested.

Additional Information:
MSDN Web Workshop - Got Any Cache?
SQL Server Solutions - Caching SQL data in the IIS Application object
ASPWatch - Boosting performance by generating 'static' ASP pages
"Cache No More" by Phil Paxton - Discouraging proxy and client-side caching

Related Products:
Post Point Software - Maker of XBuilder and XCache commercial caching software
WebGecko - Maker of Active Page Generator (APGen) commercial caching software


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