ALB Least Outstanding Requests Routing Algorithm

SAA-C03 By islever Editorial Last updated

Design scalable and loosely coupled architectures.

Short answer

B Use the least outstanding requests algorithm based on the RequestCountPerTarget and ActiveConnectionCount CloudWatch metrics.

Question

A company runs a highly available web application on Amazon EC2 instances behind an Application Load Balancer. The company uses Amazon CloudWatch metrics. As the traffic to the web application increases, some EC2 instances become overloaded with many outstanding requests. The CloudWatch metrics show that the number of requests processed and the time to receive the responses from some EC2 instances are both higher compared to other EC2 instances. The company does not want new requests to be forwarded to the EC2 instances that are already overloaded. Which solution will meet these requirements?
  1. Use the round robin routing algorithm based on the RequestCountPerTarget and ActiveConnectionCount CloudWatch metrics.
  2. Use the least outstanding requests algorithm based on the RequestCountPerTarget and ActiveConnectionCount CloudWatch metrics. Correct Answer
  3. Use the round robin routing algorithm based on the RequestCount and TargetResponseTime CloudWatch metrics.
  4. Use the least outstanding requests algorithm based on the RequestCount and TargetResponseTime CloudWatch metrics.

Key Takeaways

The question tests the understanding of ALB routing algorithms and the difference between aggregate vs. per-target CloudWatch metrics, specifically avoiding the trap of using RequestCount instead of RequestCountPerTarget.

This page explains how to configure Amazon Application Load Balancer (ALB) to distribute traffic efficiently using the least outstanding requests routing algorithm and specific CloudWatch metrics.

Common mistake: Many learners choose D because they recognize 'least outstanding requests' but incorrectly select 'RequestCount', which is an aggregate metric for the entire load balancer, not individual targets.

Expert Analysis

Why the Answer Is Correct

The correct answer is B. The scenario describes a need to prevent overloaded instances from receiving new requests by analyzing performance differences between instances. The 'Least Outstanding Requests' routing algorithm is designed exactly for this purpose; it routes traffic to the target with the fewest in-flight requests. To make this decision effectively, the ALB needs metrics that reflect the state of each individual instance. 'RequestCountPerTarget' provides the number of requests processed by each target, allowing the ALB to identify which instances are currently busier than others.

Why the Other Options Are Wrong

Options A and C use the 'Round Robin' algorithm, which distributes requests evenly regardless of the current load on the instances. This contradicts the requirement to avoid sending traffic to already overloaded instances. Option D uses the correct algorithm ('Least Outstanding Requests') but pairs it with the wrong metric ('RequestCount'). As noted in community comments like "RequestCount applies to the ALB rather than individual targets" (FlyingHawk), RequestCount is an aggregate metric and cannot help the ALB distinguish between overloaded and underloaded instances.

Community Comment Notes

Community consensus strongly favors B. User h0ng97_spare_002 correctly identified that RequestCountPerTarget is necessary to identify the amount of requests for each EC2 instance. Another commenter, FlyingHawk, highlighted that Option D is incorrect because RequestCount is unsuitable for identifying overloaded instances as it applies to the whole ALB. While some users initially debated D, the distinction between aggregate and per-target metrics was the deciding factor for most.

Official Reference

These links open the official vendor documentation in a new tab.

Exam Strategy

Always check if the required metric is available at the target level or only at the load balancer level. If you need to make decisions based on individual instance health or load, ensure the metric name includes 'PerTarget'.

Frequently Asked Questions

Why is RequestCount not suitable for this scenario?

RequestCount is an aggregate metric for the entire Load Balancer, not individual targets. It cannot tell you which specific EC2 instance is overloaded.

When should I use Least Outstanding Requests over Round Robin?

Use Least Outstanding Requests when request processing times vary significantly or when instances have different capacities, ensuring balanced load distribution.

More SAA-C03 FAQ →

Community Votes

B
83%
D
17%

83% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 15 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 15

  1. h0ng97_spare_002 👍 11 Answered B
    Option B is correct because can use "RequestCountPerTarget" to identify the amount of requests for each EC2 instance. Then use "least outstanding requests algorithm" to route to targets with the lowest number of in progress requests. Option D is wrong because "RequestCount" cannot identify the amount of requests for each EC2 instance. "RequestCount" is for the whole ALB.
  2. Andy_09 👍 8
    Option B would be the correct choice
  3. Kezuko 👍 4 Answered B
    https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-cloudwatch-metrics.html To understand the types
  4. osmk 👍 3 Answered D
    The least outstanding requests routing algorithm routes requests to the targets with the lowest number of in progress requests > https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-target-groups.html
  5. haci 👍 3 Answered B
    The question is not asking for better performance in response time. It is just simply asking to distribute the number of requests among instances. So B seems more logical.
Show 10 more comments
  1. dkw2342 👍 2
    IMO the correct answer is option D: This is from an earlier version of the AWS documentation on ALB target groups - for some reason they removed this information in the current revision: "Consider using least outstanding requests when the requests for your application vary in complexity or your targets vary in processing capability. Round robin is a good choice when the requests and targets are similar, or if you need to distribute requests equally among targets. You can compare the effect of round robin versus least outstanding requests using the following CloudWatch metrics: RequestCount, TargetConnectionErrorCount, and TargetResponseTime." https://web.archive.org/web/20200426172626/https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-target-groups.html#modify-routing-algorithm
  2. alawada 👍 2 Answered B
    distribute the number of requests among instances
  3. FlyingHawk 👍 1 Answered B
    D will be correct if change ReqeustCount to RequestCountPerTarget. RequestCount applies to the ALB rather than individual targets, making it unsuitable for identifying overloaded instances. ActiveConnectionCount: This metric indicates the current number of active TCP connections. It complements RequestCountPerTarget by helping identify instances that might be overwhelmed with connections, even if their request count seems moderate.
  4. osmk 👍 1
    D>>> The least outstanding requests routing algorithm routes requests to the targets with the lowest number of in progress requests > https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-target-groups.html
  5. xBUGx 👍 1 Answered D
    I think TargetResponseTime is the best indicator for telling is a server is overloaded or not
  6. sk1974 👍 1 Answered D
    I will go with D since the question asks for the target response.
  7. sk1974 👍 1 Answered B
    Actually ,I now vote for B and not D
  8. TruthWS 👍 3
    Option B
  9. Moon239 👍 2
    Why not D?
  10. Scheldon 👍 2 Answered B
    AnswerB

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