When building a solution in AWS you have to understand the difference between affinity and availability, and the terminology that Amazon uses. We will define affinity as the physical location of resources within a data center and availability as the isolation of resources for disaster recovery scenarios.
The AIS solution I’ll reference throughout this post uses a Virtual Private Cloud (VPC) to enable our engineers to develop a multi-tiered application within a private sub-netted network split across two availability zones. The VPC as defined by Amazon allows for “… you to create a virtual network topology—including subnets and route tables—for your Amazon Elastic Compute Cloud (Amazon EC2) resources.” The use of a VPC ensures you have high affinity for your EC2 resources. Availability zones refer to the geographically separated data centers Amazon uses for hosting EC2 resources. Therefore, your disaster recovery architecture must account for placement of EC2 resources in different Availability Zones.
High Availability vs. Disaster Recovery in AWS
To muddy the terminology waters further, we have the concept of high availability, where the effects of hardware or software failure are essentially masked and downtime for end users reduced, such as in Microsoft’s SQL Server. Our solution had to address the issue of high availability and allow for a swift recovery in the event of a disaster or failure of EC2 resources in one availability zone. A high-level picture of what we needed to achieve is shown below.