Stateful Lift + dynamic load balancing: how does Lift scale down?

Byron Gibson

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May 20, 2013, 4:54:40 PM5/20/13

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Hi all, when you run a stateful Lift app on, say, AWS using Elastic Load Balancer, and it spawns additional Lift instances in response to spikes, and then despawns them when the spike ends, what's SoP for handling the user state in the despawned instances?

Do you just assume the users on those instances were part of a temporary spike and won't be back anyway, so don't need their state preserved? Or something else?

I haven't had to cross this bridge yet, but it recently occurred to me I might, and I don't see anything in the Cookbook or Assembla wiki specifically addressing this.

Thanks!

Byron Gibson

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May 20, 2013, 5:02:06 PM5/20/13

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Edit: Oh, found it in a blog post referenced from the ELB docs:

http://shlomoswidler.com/2010/04/elastic-load-balancing-with-sticky-sessions.html

"When scaling up (adding more application servers) ELB maintains stickiness of existing sessions. Only new connections will be forwarded to the newly-added application servers.

When scaling down (removing application servers), you should expect some of your clients to lose their sessions and require logins again. This is because some of the stored sessions were on the application server that is no longer servicing requests."

So unless there's a trick with Lift I don't know about, if you're using stateful Lift then you'll lose the state of the new connections made during the spike. I can live with that.

David Pollak

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May 20, 2013, 5:08:23 PM5/20/13

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If you use ProtoExtendedSession, your users stay logged in. The current page they are viewing will be refreshed when the session is lost.

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