Load balancers, target health and cross-zone: DOP-C02 study notes and practice questions
What to know about Load balancers, target health and cross-zone for the DOP-C02 exam, from the Resilient Cloud Solutions domain (15% of the exam guide), then 1 free question to test it. Part of the DOP-C02 study guide.
These questions ask why users hit failing targets, how fast unhealthy targets are removed, or which load balancer fits.
Types
Load balancer
Layer
Fits
Application (ALB)
7 (HTTP/HTTPS, gRPC)
Path and host routing, Lambda targets, authentication with Cognito or OIDC, weighted target groups for blue/green
Network (NLB)
4 (TCP, UDP, TLS)
Very high throughput, static IPs, PrivateLink, preserving client IP
Gateway (GWLB)
3
Inserting firewalls and inspection appliances
Health checks
Interval, timeout, healthy threshold and unhealthy threshold decide how fast a failing target stops getting requests.
Point the check at a path that reflects real health (such as /health that checks dependencies lightly).
Auto Scaling groups should use ELB health checks so instances failing the app check are replaced, not just ones failing EC2 status checks.
Graceful changes
Deregistration delay (connection draining) lets in-flight requests finish before a target is removed.
Slow start ramps traffic to new targets.
Lifecycle hooks plus deregistration let apps finish work before termination.
Distribution
Cross-zone load balancing: on by default for ALB; off by default for NLB and GWLB. Turning it on spreads traffic evenly across all targets whatever their AZ.
Stickiness (cookies) keeps users on one target; it can unbalance load.
Zonal shift and ARC can move traffic away from an impaired AZ.
Monitoring
HTTPCode_Target_5XX_Count, UnHealthyHostCount, TargetResponseTime and access logs in S3 help diagnose problems.
Common traps
EC2 status checks only in the Auto Scaling group, so broken apps stay in service.
A health check on / that passes while the app is broken.
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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