gregoryyoung is rapidly becoming one of our favorite Page Editors. The original version of this article, including all of the code samples, is located at http://www.cityofangels.com/Experts/proxy.pdf.

A few years ago I was working on a team developing fairly large system. We had purchased a library that provided a lot of useful functionality for our application. We integrated this library very easily, abstracting our object creation using a factory [GoF]. All was wonderful with this component until we came upon scalability issues with it. The data that it returned us was momentarily static; in other words it would rarely change and when it did change, we didn't really care if it took ten minutes for us to get the update, as it was non-critical data. Unfortunately this component would issue a remote call every time it was called, which would take on average between 500ms and 750ms. Low and behold, the vendor had not provided us with a caching mechanism!

The problem was compounded by the fact that we did not have the source code for the program. After a bit of research I happened upon the proxy pattern and in less than 15 minutes solved our problem.

Originally defined in [GoF] the proxy (surrogate) pattern is one of the most under-utilized yet extremely useful patterns available. The proxy pattern is also one of the simplest you will ever use. My personal introduction to the proxy pattern was a typical case of where you will find a proxy in action.

The secret to the proxy pattern's success is the fact that the proxy and the subject meet an identical interface; it is a level of indirection. Due to this they can be used interchangably from the client's perspective. The proxy is returned to the client as if it were the original object and the client need not be concerned on whether it has been returned the original object or the proxy. Due to this, the behavior of a proxy is said to be transparent. The transparency of a proxy is limited to its external contract, though it can significantly alter the behaviors of an object and can cause some limitations.

The proxy pattern is very similar to and often times confused with the adapter pattern by developers new to patterns. The key difference between the adapter pattern and the proxy pattern is that the adapater pattern takes an object and provides a different external contract for it whereas the proxy pattern uses the same contract.

As you may have realized by now, there are some definite design concerns when working with proxies as there are some objects which are not very conducive to being proxied. Proxies are most conducive to objects which can be presented in their entirety as an abstract contract. Objects which have public variables or have sealed methods are not compatible as the proxy would not be transparent -- the behavior of the subject object would change once the proxy was used. Since objects with purely abstract contracts are the most conducive to the proxy pattern, many people use the proxy pattern with interfaces as opposed to classes.

The process for creating a proxy for an interface is identical to the process for creating a proxy for a class. The nice thing is that since interfaces are by their very definition purely abstract contracts, they are guarenteed to be compatible with the proxy pattern. In fact if you have a class that has a contract which cannot be defined abstractly then you will likely need an interface (or an adapter and an interface if you do not have access to source) in order to make the object compatible with a proxy.

This class could become at least partially proxy compatible if it were to implement the IDoSomething interface, though the proxy would only represent the contract of IDoSomething. Although I would truly love the ability to add exceptions to my object contracts, this is one area where .NET's lack of a strong exception contracting system has actually added to its flexibility. If .NET were to support such a system, the proxy pattern would be affected due to the fact that the proxy could not throw an exception which was not defined by the original object.

Greg Young

Merete, who has a Guru certificate in Windows XP, received some email and forwarded it to us. It is reprinted, courtesy of the author.

Dear Merete,

I know it has only been 24 hours since my last email but this news is just surfacing on the Internet and could be a timely warning for you.

It seems an "unofficial" Windows XP Service Pack has been made available on the Internet and anyone who installs it on their computers could be in trouble!

Here is one quote from Microsoft I have read:

"Anyone who installs this thinking they are getting SP3 (even as a preview) is being grossly mislead and is posing a significant potentially non-recoverable risk to their PC and data," wrote Mike Brannigan, an enterprise strategy and senior consultant at Microsoft.

SO BE WARNED!

...if you come across a link with a suggestion you download the software then AVOID it.

Microsoft has said that its priority is Windows Vista, the next major release of its operating system.

While the company does have plans to release XP SP3, it has not provided details on what the service pack will contain. Microsoft has issued a statement saying that the "content on the unofficial website(s) does not represent Microsoft's plans for Windows XP SP3."

Got a friend who uses Windows XP? Then why not send them a copy of this email...

Kind Regards

Marc Liron
Microsoft MVP
Windows Digital Media
http://www.updatexp.com
http://www.marcliron.com
http://www.paywhatyoucanafford.com

Remember last issue when I mentioned that my new Compaq computer, which came preloaded with Windows XP (and therefore Internet Explorer) has Netscape's home page as the default? I'm glad it is, because there's been some pretty interesting stuff there.

One article that caught my eye was one that talked about how banks are finally upgrading security for the people who use on line banking. My bank offers it, and though my husband swears by it -- I think he's just too laz ... er... busy to balance his checkbook -- I've never tried it. I guess it comes from reading and writing about phishing attacks and such.

Anyway, the article says that while most overseas banks have already incorporated so-called "two-factor authentication", American banks are trying to tweak their web systems, because it's a lot less expensive than handing out SmartCards to all of their customers. It's not that the banks are losing THAT much money; for them, it's just a trade-off in operating expenses. Just wait until the CEO of some bank gets his identity ripped off.

HP and WalMart were in the news last week when a website that watches retail sales posted an item saying that WalMart will start selling HP desktops and laptops for $398 beginning the day after Thanksgiving -- the traditional beginning of the Christmas shopping season. HP sold a $499 laptop (after rebates) earlier this year at the beginning of the back-to-school season, and other companies have sold inexpensive laptops as well, but those usually didn't come with Windows, and they didn't have AMD processors.

Finally, I got a newsletter the other day that says that a "recent poll" -- am I the only one who wonders who does these polls and why doesn't anyone ever say who they are -- showed that 88 per cent of email users don't care if they lose some legitimate email as long as the junk mail gets filtered out. The question the newsletter asked was, "What good is e-mail going to be if you can no longer rely on it?"

I see that as a Hobson's choice. We need email at this point; it can be an efficient and inexpensive communications tool. But the technology brings with it the 100 or so pieces of garbage I wind up deleting every day -- and even though my ISP is kind enough to tag it as junk, a lot of it still gets past their filtering system untagged.

I know the arguments. It's not the ISPs fault, because all they do is send my mail to me OR they're just providing a service to their customers. But I wish I could grab one of those spammers and shove a printed copy of each of their emails... you get the idea.