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I'm relaunching a site (~5mm+ visits per day) on EC2, and am confused about how to deploy nodes in different data centers. My most basic setup is two nodes behind a Varnish server.

Should I have two Varnish instances in different availability zones, each with WWW nodes that talk to a shared RDS database? Each Varnish instance can be load balanced w/ Amazon's load balancer.

Something like:

1 load balancer talking to:

  • Varnish in Virginia, which talks to its own us-east-x nodes
  • Varnish in California, which talks to its own us-west-x nodes
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We use amazon EC2 extensively to do load balancing and fault tolerance. While we still don't extensively use the LoadBalancers provided by Amazon we have our own load balancers(running outside Amazon). Amazon promises that the LoadBalancers will never go down, they are internally fault tolerant, but I havent tested that well enough.

In general we host two instances per availability zone. One acting as a mirroring server to the real server. If in case one of the servers go down we send the customers to the other one. But lately Amazon has shown a pattern that a single availability zone goes down quite often.

So the wise technique I presume is to set up servers across availability zones like you mentioned. We use postgres so, we can replicate content in the database across the instances. With 9.0 there is Binary replication that works great for two way replication. This way both the servers can take the load when up but when a availability zone does go down all the users are sent to one server. Since a common database is available it does not matter where the users go to. Just that they will experience a slight slowness, if they go to the wrong server.

With this approach you can do tandem updating of the web sites. Update one ensure that it is running fine and then update the next. So even if the server failed to upgrade the whole website is always up.

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how do you send customers to the other servers when one availability zone goes down?
Our custom load balancers will do load checks from servers every minute. We use subdomain url's to direct customers. So if a cluster is down we send them a different subdomain url. The amazon load balancer acts as a abstraction layer and internally routes a http request to the server. It checks for availability every 10 seconds by requesting for a web page from your server. You can just put a blank.html. If the instance is down then amazon will not send any requests to this server.

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