Scott on Writing

# re: URL Rewriting Redux 9/8/2004 12:57 PM Adam

You can still have a race condition in your code here. Two threads (two separate requests) could get a null reference to the config and both attempt to insert. You really should be locking to be thread safe.

private static object ConfigLockObject = new object();

RewriterConfiguration config = null;
if( HttpContext.Current.Cache["RewriterConfig"] == null ) {
lock( ConfigLockObject ) {
// you have to check a second time since multiple threads
if( HttpContext.Current.Cache["RewriterConfig"] == null ) {
config = ConfigurationSettings.GetConfig("RewriterConfig");
HttpContext.Current.Cache.Insert("RewriterConfig", config);
}
}
}
else {
config = (RewriterConfiguration) HttpContext.Current.Cache["RewriterConfig"];
}

# re: URL Rewriting Redux 9/8/2004 12:59 PM Adam

My code above missed an else statement:

private static object ConfigLockObject = new object();

RewriterConfiguration config = null;
if( HttpContext.Current.Cache["RewriterConfig"] == null ) {
lock( ConfigLockObject ) {
// you have to check a second time since multiple threads
if( HttpContext.Current.Cache["RewriterConfig"] == null ) {
config = ConfigurationSettings.GetConfig("RewriterConfig");
HttpContext.Current.Cache.Insert("RewriterConfig", config);
}
else {
config = (RewriterConfiguration) HttpContext.Current.Cache["RewriterConfig"];
}
}
}
else {
config = (RewriterConfiguration) HttpContext.Current.Cache["RewriterConfig"];
}

# re: URL Rewriting Redux 9/8/2004 1:06 PM Scott Mitchell

Adam, I don't disagree with you w.r.t. the race condition, but I fail to see how this is a "big deal." Can you explain what can go awry in the above situation? Why would it matter if two threads insert the same value to the cache?

# re: URL Rewriting Redux 9/8/2004 3:01 PM Adam

I didn't say it was a big deal, I just said it fixes the race condition that the supplied fix doesn't actually fix (since it wasn't synchronizing access to a critical section of code).

# re: URL Rewriting Redux 9/23/2004 11:15 AM Scott Allen

Scott:

I think it's a great article.

Something I learned from working with the CSK is the action attribute in the form tag can be set properly by calling RewritePath a second time during the request inside the page's Load event - it's a one liner you could stick in a base class. See http://odetocode.com/Blogs/scott/archive/2004/09/22/509.aspx/

# re: URL Rewriting Redux 1/19/2006 11:50 PM François

Anyway, caching the configuration settings in your context is useless as far as I know. The configuration system is cached. You can try it yourself by making a custom IConfigurationSectionHandler, and calling ConfigurationSettings.GetConfig several times, even in separate threads, you will always get the same object. Which again raises a situation dangerous for thread safety : for example, the NameValueSectionHandler returns a NameValueCollection, what happens if in a thread jokes around by adding or updating a value in the collection while somebody else is reading the collection ? Of course, it's not a good practice to update configuration settings on your code, but I'm somewhat astonished that the configuration system doesn't return a copy of the object given by your configuration handler, making the implementation ICloneable mandatory.

By the way, configuration settings are like any static variables, they are the same in the same application domain. And when the web.config file is updated, ASP.NET reloads the application domain, including flushing the configuration cache. But I guess it wouldn't cause a problem in your code since your HttpContext will be flushed too :) If you have any wuestion, you can reach me at sossalemaire at yahoo.fr...

# UrlRewriter Bug Fix 2/12/2006 5:15 PM Steven Smith

Some of you may know that I'm pretty interested in caching as a means to improving performance. ...

# Upcoming Talk at the SoCal .NET Technical Summit 8/24/2006 7:28 AM Scott on Writing

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On Saturday, September 23rd, I'll be speaking at the SoCal .NET Technical Summit in Irvine, CA at the

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