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CLF-004
What is the main purpose of using multiple Availability Zones?
Choices
Explanation and answer
Read the explanation and correct answer (spoiler — try answering first)
Correct answer: D. To reduce single points of failure and improve availability.
Explanation
By placing resources across multiple Availability Zones, services can continue to operate even when a single AZ experiences a failure.
Per-choice explanations
A. To increase the number of IAM users.
Incorrect. This has nothing to do with the number of IAM users.
B. To make all AWS usage free.
Incorrect. A multi-AZ configuration can increase cost in some cases.
C. To disable CloudWatch Logs.
Incorrect. This has nothing to do with enabling or disabling CloudWatch Logs.
D. To reduce single points of failure and improve availability.
Correct. A multi-AZ configuration is the basis of higher availability.
Practical Use
A multi-AZ configuration that uses multiple Availability Zones (AZs) is a basic design that prevents the entire service from stopping when a single physical data center failure occurs, such as a power outage, network equipment failure, or natural disaster. In practice, standard configurations include a web tier in which EC2 instances are placed in multiple AZs via an Auto Scaling Group with an ALB in front to distribute traffic, a database tier in which RDS is configured for Multi-AZ placement with a standby DB in a different AZ, and a file storage layer placed on services that are already redundant within a Region, such as EFS. A multi-AZ configuration also involves trade-offs. Data transfer across AZs incurs charges, and the latency between AZs is higher than within the same AZ. In addition, RDS Multi-AZ launches an additional standby DB instance, so it incurs roughly double the compute cost compared with a single-AZ configuration. Because higher availability also raises cost, you must decide in stages whether a single AZ is sufficient, whether Multi-AZ is needed, or whether you should expand all the way to multi-Region based on the SLA required by the workload (for example, 99.9% or 99.99%). On CLF-C02, the purpose of multi-AZ placement is tested in the context of eliminating single points of failure (SPOFs) and achieving high availability. Typical distractors apply topics that are unrelated to availability, such as reducing cost, increasing the number of IAM users, or making monitoring unnecessary. You should also be careful about confusing multi-AZ with multi-Region. Multi-AZ is a measure for availability within the same Region, while multi-Region is positioned as a measure for geographic disaster recovery or latency optimization.