Availability Zones (AZs)¶
Each region contains multiple distinct locations called Availability Zones, or AZs. Each Availability Zone is engineered to be isolated from failures in other Availability Zones, and to provide inexpensive, low-latency network connectivity to other AZs in the same region.
An Availability Zone (AZ) is one or more data centers in close geographic proximity connected together over low-latency/high-speed links.
By launching instances in separate Availability Zones, you can protect your applications from the failure of a single location. Note: Each AWS region provides a minimum of two AZs.
Rackspace Availability Zone Recommendations¶
Rackspace typically recommends a two AZ deployment, which provides availability and redundancy while reducing complexity, operational overhead, and cost.
There are situations where a third AZ may be required to address specific application-centric requirements:
Example 1: MongoDB's Election and Quorum constraints require three AZs to survive a single AZ failure that contains the primary and a secondary in a three-node cluster.
Example 2: Applications that have strict load and availability requirements that cannot be met by relying on Auto Scaling Groups require over-provisioning. Adding a third AZ could be considered to reduce costs by lowering needed the over-provisioning.