Caching happens at all levels, on the hard drive, in RAM, and within the
operating systems. It is a performance fundamental of computer science.
In 1999, a wave of caching products swept the Internet to serve graphics
faster. These products drove the stock of companies like Akamai
Technologies, Inc., and Inktomi Corp. into the sky. However, they were
focused on caching graphics and streaming media.
Now a new wave of caching products are emerging that cache dynamic Web
pages. Dynamic Web pages are those that are created between the request
for the page and the response back to the browser. Usually they are
written in a scripting language like perl, CGI, VBScript, or Cold Fusion.
In the past, these pages were used to provide personalization and
e-commerce transactions. However, as the Web evolves, dynamic pages are
being used to generate content-like promotions for the front page, changing
indexes and mainstream content. Because these types of pages are being
used for Web-site maintenance and not user interactivity, they can be
cached for short periods of time.
Dynamic caching's time has come, Post Point Software launched a
released of XCache in May, Microsoft plans to release ASP+ and Acceleration
Server next year, Cold Fusion released SPECTRE at the end of 1999, and
Vignette's Story Server lists caching as one of their main features.
However, if you are writing your Web site in Active Server Pages (ASPs), it
was not until XCache that you could implement partial page caching (the
ability to cache just a portion of the page, leaving the rest dynamic).
This ability allows you to incorporate personalization without losing
performance.
This article will cover three ways to cache your dynamic Web site written
in ASPs, including Microsoft Acceleration Server, Microsoft's ASP+,
and Post Point Software's XCache, and offer the pros and cons of
using each product. If you are looking for speed and scalability for the
Web site, read on.
Microsoft Security and Internet Acceleration Server (ISA)
Microsoft's Security and Internet Acceleration Server, a.k.a. ISA
(<a href="http://www.microsoft.com/isaserver/">
http://www.microsoft.com/isaserver</a> ), is the third version
of their reverse proxy server (still in beta). The ISA server is typical
of reverse proxy servers and is somewhat advanced in its handling of URLs
with question marks (dynamic pages). While most reverse proxy servers
don't recognize query strings as unique identifiers to the pages, ISA
does cache each page with unique query strings as separate pages.
What is a Reverse Proxy Server?
The proxy server sits between the Internet and the Web site and handles all
traffic before it can reach the Web server. A reverse proxy server
intercepts requests to the Web server and instead responds to the request
out of a store of cached pages. This method improves the performance by
reducing the amount of pages actually created "fresh" by the Web server.
Reverse proxy servers cache pages based on HTTP header tags that are
returned from the dynamic pages. The four most important are:
Last-Modified Tells the proxy when to change the page.
Expires Tells the proxy when to drop the page.
Cache-Control Tells the proxy if the page should be cached.
Pragma. Tells the proxy if the page should be cached.
For example, by default all Active Server Pages return "Cache-control:
private." Which means that none of your Active Server Pages will be cached
with Microsoft's Acceleration Server.
Advantages
The ISA server runs on a separate machine that distributes the load from
the Web server to the Acceleration Server. The ISA server also serves
everything from RAM, making it extremely quick.
Disadvantages
With the ISA server you need to recode your Active Server Pages in order to
cache them -- Active Server Pages default HTTP response headers will tell
the ISA server not to cache its pages. Because the ISA server sits on a
separate machine, none of the cached requests make it to the Internet
Information Server's log files. This means that even though those
pages are being requested, there is no record of it on the Web server, so
your traffic numbers are not accurate when running ISA.
Example
When you want to change your cache in ISA, you will need to change to code
that controls the ASP page. In order to turn CacheControl off, you will
need to reprogram your ASP pages by adding:
<%
Response.AddHeader "CacheControl","public"
%>
You also need to tell the proxy when the page expires so that it can
refresh the cache appropriately. Add this line to set the cache period to
1 hour:
<%
Response.Expires = 60
%>
Microsoft's Security and Internet Acceleration Server can be used for
caching Web pages if you don't mind recoding your site, however other
caching systems handle dynamic pages better, such as Microsoft's ASP+
and Post Point's XCache.
Active Server Pages Plus
ASP+ is an extension of Microsoft's Active Server Pages. This
completely new approach to Active Server Pages is scheduled to be release
in early 2001. Basically, ASP+ is a set of tools that extends the
Microsoft Internet Information Services (IIS) server. However, unlike
other ASP upgrade paths, you need to recode your current ASP pages to take
advantage of Microsoft's new "managed code" scripting languages and
ASP+ features. If you do recode your Web site, you can add a directive to
each page to cache it.
ASP+ provides two types of page caching: one called output caching that
caches a whole page for a period of time, and another called partial page
caching that caches just a portion of the page or several portions. Both
require you to recode each page you want to cache adding the correct
directive.
The concept of partial page caching is a very powerful tool, since pieces
of the page might stay the same no matter what user is viewing the page and
other pieces might be different on a per-user basis. Partial page caching
allows you to put your most expensive routines in a long-term cache and
leave your banner rotation and personalization pieces of the Web site
completely dynamic.
Advantages
ASP+ provides both output page caching and partial page caching for free.
It allows you to divide your cache by the browser's language and
browser type. This gives you a bigger cache, however if you are creating
unique pages by language or browser type, this is a big benefit.
Disadvantages
ASP+ requires that you recode your ASP pages in order to take advantage of
the new ASP+ features. Microsoft also requires that you add directives to
the top of the page in order to cache. ASP+ is in beta and will launched
in early 2001.
Example
The following output caching directive is applied to the top of the page:
<%@ OutputCache Duration="60" %>
Because ASP+ is still in beta, Microsoft has not yet released details on
the syntax for the partial page cache directive.
XCache
XCache is a third-party tool from Post Point Software ( http://www.postpointsoft.com) that
allows you to cache Active Server Pages in IIS 4.0 or IIS 5.0. Unlike
ASP+, it is currently available. It was released in May 2000. Like ASP+,
XCache has the ability to perform output caching and partial page caching
-- called Active Caching.
XCache installs on your Web server, handling requests before they reach
the Web server. In this regard it is much like a reverse proxy server.
However, unlike a reverse proxy server, XCache makes sure all requests that
are served from the cache are reflected in your log files. This can be a
problem with other caching solutions that are not on the box -- when
requests are served from the cache, hits are not logged. This makes
analyzing your Web site traffic impossible.
To start using XCache, install it on your Web server and start it from the
MMC Snapin that is provided. You can pick and choose those pages you want
to cache, leaving pages that change on a per-user basis uncacheable.
Advantages
XCache doesn't require recoding to do output caching and provides
partial page caching. Requests severed from the cache are logged to the
Web server log files. XCache is currently available.
Disadvantages
XCache uses a disk cache to hold files instead of a RAM cache like the ISA
server.
Example
If you want to implement Active Caching (partial page caching), just turn
it on from the Web site properties page. Once you have it enabled, you can
choose which section of the page you don't want to cache by changing
the delimiters from <% %> to <$ $>. XCache will then
cache everything in <% %> and leave everything in <$ $>
to execute when the user requests the page.
XCache also allows you to include pages when the file is cached by using
the standard #include or include pages when the page is requested using
#dynamic. For example, changing:
<!-- #include virtual="/dir/include.asp"-->
to
<!-- #dynamic virtual="/dir/include.asp"-->
will make all the code in include.asp execute when the page is requested.
Includes left in
<!-- #include virtual="/dir/include.asp"-->
will execute when the page is cached.
Performance Gains
You can see significant performance gains using ISA, ASP+, or XCache,
depending on the scenario. Imagine you have hundreds of graphics on your
Web server that are taking the majority of the servers processing time and
bandwidth. You could cache these with the ISA server, reducing the load on
the Web server and allowing it to process more ASP pages. At the same time
the ISA server would load the graphics into memory, serving them faster
than the Web server could from the hard drive.
Now image you have a product catalog in your SQL Server database of 5,000
products. Typically you would have a category page that lists products in
that category and a detail page that displays all the information about the
product. These pages would be driven directly from the database, allowing
you to change and add products easily. Now imagine that certain products
are not changed for months. This means that their detail page has not
changed. However, every time that page is requested, code has to be
executed and SQL Server calls have to be made to see exactly the same page
that was displayed to the last user..
In this scenario, caching is the solution. You would cache your detail
pages, and all requests would come out of cache, instead of executing.
This would reduce the calls to the SQL Server and reduce the CPU load on
the Web server. Pages would return faster and the Web server would be able
to serve more pages since it has less work to do.
Suppose on the same Web site, you have a shopping basket that allows each
user to purchase those products in their shopping cart. In this scenario
each user would see a different version of the page since they would be
purchasing different products. You would not want to cache these pages
since they change for every user and the pages need to interact with the
database.
However, on the site's home page you have specialty promotions that
are based on information you have tracked about each user. Along with
these promotions there is a list of all the categories. This page changes
on a per-user basis, however most of the information is the same for all
users. This is a perfect scenario for partial page caching. You could
cache the category list, indexes, and navigation, essentially everything
but the promotions -- and this would leave the promotions dynamic.
Summary
There is a new wave of caching products hitting the market that allow
interactive pages to benefit from Web page caching. Three of the
front-runners are Post Point Software's XCache, Microsoft's ASP+,
and Microsoft's Security and Acceleration Server. Take a close look
at your Web site and determine if you could benefit from dynamic caching.
About the Author
Wayne Berry is the
president and CEO of Post Point Software, as a former
Microsoft design engineer and one of the top Active Server Page developers
in the country, Berry's expertise includes software design, development,
marketing and online business. Berry served as a Software Development
Engineer at Microsoft and as Director of Development at Freeshop prior to
Post Point Software.
A popular speaker, Berry has been invited to speak to international ASP
Developers conferences, BackOffice Conference and Internet World. He has
authored several technical books, including ActiveX Unleashed Programming,
Windows Registry Guide, and Special Edition Using Microsoft Internet
Information Server 4.0. , as well as many articles in print and online trade
publications.
Berry holds a B.S. in Computer Science from Western Washington University.
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