DOP-C02 practice question: Load balancers, target health and cross-zone
Resilient Cloud Solutions A free question from our original DOP-C02 practice exam. Try it, then open the answer to see why each option is right or wrong.
An airline runs its booking API on instances in three Availability Zones, behind an Application Load Balancer. The target group's health check runs against a /health path every 30 seconds.
During a recent incident, one instance in one Availability Zone kept failing its checks, but the load balancer continued to send it requests for several minutes. Customers saw timeouts on about one request in ten during that period.
The team wants unhealthy targets to stop receiving requests within a short, predictable time, and wants requests that are already in progress on a deregistered target to finish.
Which change meets these requirements?
Choose one answer.
AA. Turn off the target group's stickiness and increase the number of instances in each Availability Zone, so that a single failing instance receives a smaller share of the requests.
BB. Increase the health check interval to 60 seconds and the healthy threshold to five, so that the load balancer only marks an instance healthy after it has been stable for several minutes.
CC. Replace the Application Load Balancer with a Network Load Balancer, which removes unhealthy targets from service immediately without any health check configuration.
DD. Set the health check interval to 10 seconds, the unhealthy threshold to two, and a deregistration delay so that in-flight requests can finish on a deregistered target.
Show the answer and explanation
Answer: D
A. Wrong: Smaller shares still send some requests to a failing instance, and stickiness does not affect health checks. More instances add cost without ending the timeouts.
C. Wrong: Network Load Balancers still rely on health checks to remove targets, and changing the load balancer type is a large change for a timing problem. The design would still need its health check tuned. Learn more: How Elastic Load Balancing works (opens in a new tab)
Application Load Balancer health checks determine how quickly a failing target stops receiving requests, based on the interval, timeout and unhealthy threshold. The deregistration delay lets in-flight requests finish when a target leaves the group. Stickiness, longer intervals and a different load balancer type do not fix the problem.
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