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Pure.Krome
Asked: 2009-07-27 20:14:38 +0800 CST

What is the downside to sticky sessions with load balancers?

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We have a web farm of IIS7 machines which work great. In front of them is an F5 Big-IP hardware load balancer, also working fine :)

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(source: www.f5.com)

Currently we're using an ASP.NET State Service to handle our OutProc state. This is required when you have a web farm to maintain any type of session information.

I was wondering if we could have sticky sessions on the F5 Big-IP and therefore change from OutProc back to InProc? If so, what is the downside of this? I know the downside of InProc vs OutProc, so don't worry about explaining that. I'm more interested in the pros/cons of sticky sessions with out F5 Big-IP.

Can anyone shed some light and/or experience?

load-balancing iis-7 f5-big-ip sticky-sessions

3 Answers

  1. Best Answer

    There are two main downsides:

    1. Your load isn't evenly distributed. Sticky sessions will stick, hence the name. While initial requests will be distributed evenly, you might end up with a significant number of users spending more time than others. If all of these are initially set to a single server, that server will have much more load. Typically, this isn't really going to have a huge impact, and can be mitigated by having more servers in your cluster.

    2. Proxies conglomerate users into single IP's, all of which would get sent to a single server. While that typically does no harm, again other than increasing individual server loads, proxies can also operate in a cluster. A request into your F5 from such a system would not necessarily be sent back to the same server if the request comes out of a different proxy server in their proxy cluster.

    AOL was at one point using proxy clusters, and really screwed with load balancers and sticky sessions. Most load balancers will now offer sticky sessions based off of C-Class net ranges, or with the case of F5, cookie based sticky sessions which store the end node in a web request cookie.

    While cookie based sessions should works, I've had some problems with them, and typically choose IP based sessions. BIG HOWEVER: I'm mostly working on internal apps - DMZ milage might vary.

    All that being stated, we've had some great success with sites running behing F5 with sticky sessions and In-Proc sessions.

    You also might want to take a look at one of the in memory distributed caching systems like Memcached or Velocity for an alternative to session being stored in SQL or the out of proc memory service. You get close to the speed of in-proc memory with the ability to run it across several servers.

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  2. In addition to the excellent answer from Christopher, sticky sessions mean that you've lost a couple of the huge benefits of redundant servers -- the ability to take one or more down for maintenance, and transparency in the face of system failure.

    I consider sticky sessions a strong indicator of poor application architecture and/or poor programming. "Avoid at all costs" is my motto.

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  3. I recently read a great article in TechNet regarding "Providing Scalability for ASP.NET Applications". It went into the pros and cons of each possible solution. Take a read:

    TechNet June 2009 - Providing Scalability for ASP.NET Applications

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