
Understanding AWS Availability Zones: Boosting SaaS Resilience and Uptime
Learn about AWS availability zones, what they are, why they are essential for SaaS Resilience and uptime

AWS resources are hosted in multiple locations world-wide. These locations are composed of AWS Regions, Availability Zones, and Local Zones. Each AWS Region is a separate geographic area. Each AWS Region has multiple, isolated locations known as Availability Zones.
- Each region has a different number of AZs, but they all have at least 3.
- All AZs in a region are interconnected via high-bandwidth, low-latency, fully redundant metro fiber.
- Traffic between AZs is encrypted and supports synchronous replication.
- Partitioning applications across AZs improves protection against issues like power outages and natural disasters.
- AZs are physically separated by a meaningful distance, typically within 100 km (60 miles) of each other.
- Your service might not utilise ALL the AZs.

Aurora stores copies of the data in a DB cluster across multiple Availability Zones in a single AWS Region. When data is written to the primary DB instance, Aurora synchronously replicates the data across Availability Zones to six storage nodes associated with your cluster volume — AWS docs
AWS maps the physical Availability Zones randomly to the Availability Zone names for each AWS account. This approach helps to distribute resources across the Availability Zones in an AWS Region, instead of resources likely being concentrated in Availability Zone “a” for each Region. As a result, the Availability Zone us-east-1a for your AWS account might not represent the same physical location as us-east-1a for a different AWS account.

- Two are better than one. Deploy to a minimum of two AZs. For consistency across the organization, see the code example below for how this can be achieved. Make sure all the organizations use the same AZs (1 and 2, for instance). See the ‘AZ Selection via IaC’ section below to see how to do it via IaC.
- Critical SaaS services that are willing to pay extra for improved SLI and performance during partial AZ malfunction are encouraged to deploy to three or more AZs.
- Before adding an AZ, it’s crucial to calculate the cost of both the DATA TRANSFER and the extra infrastructure. This step ensures that the deployment remains cost-effective and aligns with the organization’s budget.
- For critical SaaS services that deploy to regions with more than 3 AZs, evaluate the cost to determine whether the added AZs are worth it. As a reference, such regions have failed before, so even a region with 6 AZs can be done.
We recommend that you distribute the primary instance and reader instances in your DB cluster over multiple Availability Zones to improve the availability of your DB cluster. — AWS docs
