Availability Zones
- What it is
- One or more discrete data centers within a Region, each with redundant power, networking, and cooling.
- Every Region has at least three AZs (typically three to six).
- The two properties that matter
- Isolated — AZs are physically separated by a meaningful distance so a single flood, fire, or power failure cannot take out two. They do not share single points of failure.
- Connected — linked by high-bandwidth, low-latency (single-digit millisecond) private fiber, so synchronous replication between AZs is practical.
- Why deploy across AZs
- The standard way to achieve High Availability within a Region.
- Examples: Amazon RDS Multi-AZ, load balancers across subnets in multiple AZs, AWS Auto Scaling groups spanning AZs.
- Naming nuance
us-east-1amaps to a different physical AZ in each AWS account, so AWS spreads load evenly. Use AZ IDs (use1-az1) when coordinating across accounts.
- Scope reminder
- Amazon Elastic Block Store (EBS) volumes and subnets are AZ-scoped. Amazon S3 and Amazon Elastic File System (EFS) are Regional.