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I'm running an AWS Load Balancer with Web Servers at different Data Centres.

I need to start using PHP Sessions on Elasticache to Centralize Session Management.

I can create an a 2 node Elasticache setup with one node in each data centre but I'm concerned that there will be an impact to speed if cache data needs to be access on a cache server that is in the opposite data center to the requesting web server.

Is this the case? Or is there no or very minimal impact to speed?

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Having two ElasticCache nodes replicating across regions isn't going to work very well. The network latency caused by being separated by 2,500 miles will negate most/all of the performance gain.

You shouldn't need to be sharing session data across regions though. Setup latency-based DNS routing in Route53, which will route each user to the closest datacenter automatically. Their sessions will live within their region.

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When you say 'Data Centres' do you mean Regions (us-west-1, us-east-1) or Availability zones (us-west-1a, us-west-1b, etc).

If you're just using two availability zones in the same region, the latency is negligible and you can treat availability zones as if they were the same data center (even though they're isolated, they're connected by highspeed fiber).

If you're talking two Regions (us-west-1 and us-east-1 for example) then the latency will be a lot higher (plus ELB doesn't balance between regions, only availability zones).

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