IIS7 Deployments calls for ETag Usage Review if using


Entity Tags (ETags) are commonly used in Web applications to effectively leverage the power of using web farms, which is a non-fancy term for HTTP/S load balancing.  In web farms, a common practice is to set what is called ETags as it helps enhance performance in web farm scenarios.  ETags is controlled in IIS by a metabase property (ETAG_CHANGENUMBER) and this value is sent back to clients via response headers as instructed by the HTTP RFC 2616

If the ETag value is the same across a number of servers, then clients are not required to re-download content that already exists in the clients cache.  Without this value set to the same, clients might communicate on subsequent requests to another server in the web farm and receive an unnecessary response of data from the IIS server.

For purposes of this blog, I will focus on modifications rather than further definition of ETags.  However, you can read more in the IIS 6.0 documentation on ETags to get more familiar with them.

...Web Farms, IIS 6.0 & RTM

A change that has caused a few of our customers issues when upgrading to Windows Server 2003 & IIS 6.0 is a change to our ETag computation.  At RTM, we changed our behavior from IIS 5.0 hence causing IIS customers large problems as increase in traffic began to occur once migrating to IIS 6.0.  The cause of this issue was a known bug, 823544, and was fixed prior to the release of Service Pack 1.  Beyond that, we found an additional bug, 900245, that caused the ETag value to change when the IIS 6.0 metabase configuration was changed.  This would be an expensive change in web farms because this would cause sync scripts to be sure to validate this new ETag value across all servers during their deployment.

As with any major update, it is in the best interest for customer's to avoid these bugs by installing Service Pack 1 which integrates these two fixes for delivery in a major milestone.

...Web Farms, IIS 6.0 & Service Pack 1 (slipstream)

In slipstream installs, you do not need to do anything other than set the ETag value to match across your web farm servers.  The problems with ETag mentioned above have been fixed and should work as designed.

...IIS 6.0 & IIS 7.0 - Making it work

In comes IIS7, the next generation platform for web applications for Microsoft.  The headaches caused by these changes in IIS 6.0 were heard loud and clear as we have changed this behavior.  In IIS7, the ETAG_CHANGENUMBER value is now set to "0" as to not interfere with deployments to web farms.

The key take away is that as IIS7 servers start to integrate into your environment with other IIS 6.0 servers is that you will need to change the ETAG value property to "0" to match the IIS7 servers.  If you do not, then you will see performance go down as client's caches are invalidated.

To do so, do the following:

  1. Locate adsutil.vbs (typically in %systemroot%\inetpub\adminscripts)
  2. In that directory, issue the following command:

cscript adsutil.vbs set w3svc/etag_changenumber 0


I bring this up as it recently came up internally as we have some internal deployments that had this value set in the metabase on their IIS 6.0 installations.  Upon deploying IIS7, they came upon this problem and had to modify the behavior as they integrated IIS7 & IIS 6.0 servers together in their web farms. 

As mentioned above, if you dedicate your IIS7 servers to their own web farm as to avoid this problem.  I hope this might help some of you avoid this issue in your future deployments.

Additional Resources 


  • I am trying to fix just this problem for our web servers. We have a number of web servers and each server is returning a different etag value for a given static file. I have attempted to fix this on machines running both windows server 2003 Std. edition SP1 and SP2 and I keep getting the same error message when running the above command:

    ErrNumber: -2147463164 (0x80005004)
    Error Trying To GET the Schema of the property: IIS://localhost/Schema/etag_changenumber

    If I look in the MBSchema.xml file, I can see an entry fro the value. I have also tried to use the create command with adsutil with no luck. I'm not terribly well versed with using adsutil to update the metabase so pardon my ignorance.

  • Sorry I neglected to clarify that this actually a problem for us with IIS6 not 7.

  • Are Etags on by default in IIS? I'm trying to find documentation on how to turn them on.

  • What you posted, cscript adsutil.vbs set w3svc/etag_changenumber 0, doesn't seem to work in IIS 6. You can add the property manually with the metabase explorer as http://support.microsoft.com/kb/922703/ describes, but it doesn't seem to be exposed in some way that adsutil can get to it. This is on a WIndows 2003 SP2 Server.

  • Is there a way to turn etags off in IIS7, I just want the browsers to use the Content Expiration date.

  • I'd also like to know how to turn off Etags in IIS6 & IIS7. (Steve Souders' High Performance Web Sites recommends turning them off, if not required, to save on payload).

  • Remove ETags from the Http Response by setting a blank ETag header. In IIS Manager, right click Web Site (or any folder), click Properties, select HttpHeaders tab, add Custom Http Header called ETag but leave the value blank.

  • You cannot leave the value field blank, you must enter something. The OK button is disabled until something is entered. You would think there was an easy way to do this in IIS7!

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