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I was looking at RightScale's architecture diagrams and came across their Basic 4-Server Setup (with EBS): http://support.rightscale.com/12-Guides/EC2_Best_Practices/EC2_Site_Architecture_Diagrams#Basic_4-Server_Setup_with_EBS

I'm assuming that in front the top 2 IP's are meant to be EC2 static IPs (where you publish multiple IPs for your domain).

The two things I didn't really understand are

  1. Why isn't it using Amazon's Elastic Load Balancing (ELB)?
  2. Why isn't it using Amazon's Relational Database System (RDS)?

Is this just so that things are more portable to another cloud offereing?

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Why isn't it using Amazon's Elastic Load Balancing (ELB)? There are at least 3 reasons reasons why they might not use ELBs:

  1. Additional cost - you can run your own micro instance for less than the cost of an ELB, that should have more than enough power to do load-balancing.
  2. ELBs are (or at least were) layer 4 load balancers - they didn't support the layer 7 logic that HAProxy supports. Additionally, until recently they did not support 'sticky sessions'.
  3. Redundancy - you would typically run a single ELB - whereas here you have a backup. While ELBs should be extremely reliable, not having a single point of failure is good. Of course, portability is a factor as well.

Why isn't it using Amazon's Relational Database System (RDS)?

  1. Again, cost is a factor - if you have a '4 server' model - adding an RDS really increases that. The prices of RDS instances are 30% higher than the equivalent EC2 instance. The advantage lies primarily in that 'they will take care of everything for you'. If you can set up something, you will likely have more control over it.
  2. Control: no access to my.cnf (although some API equivalents); no binlog; no read replication; no shutdown/super privileges; maintenance is scheduled weekly (and you may not always be certain of what that entails); if something goes wrong with RDS (e.g. problems due to an upgrade), you have to rely on them to fix it; they run a specific version of MySQL - if you happen to need an older version, you can't use it.
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I don't suppose you've considered asking RightScale these questions, given that it's their diagram?

I'd say that the IPs at the top are supposed to be your application IPs (although why they think more than one is a good solution I'm not sure). It's possible that the diagrams were drawn before ELB and RDS were available, or otherwise I'd say it's likely they're trying to stay portable.

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