Health checks configuration in AWS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of a health check in AWS?

A health check monitors the status of resources like servers or load balancers to ensure they are working properly. If a resource fails the health check, AWS can stop sending traffic to it.

beginner

Name two types of health checks used in AWS Elastic Load Balancing.

1. HTTP/HTTPS health checks: They send a request to a specific URL path and expect a response.
2. TCP health checks: They check if a TCP connection can be established on a port.

intermediate

What does the 'Healthy threshold' setting control in a health check?

It defines how many consecutive successful health checks are needed before considering a resource healthy again.

intermediate

Why is the 'Interval' setting important in health checks?

It sets how often AWS performs the health check. A shorter interval means faster detection of failures but more network traffic.

beginner

What happens if a resource fails the health check 'Unhealthy threshold' times?

AWS marks the resource as unhealthy and stops sending traffic to it until it passes health checks again.

Which AWS service commonly uses health checks to route traffic?

A IAM

B S3 Bucket

C CloudWatch Logs

D Elastic Load Balancer

Elastic Load Balancers use health checks to decide which instances should receive traffic.

What does a TCP health check verify?

A If a TCP connection can be established on a port

B If the DNS resolves correctly

C If the server CPU is under 80%

D If a web page returns status 200

TCP health checks test if a TCP connection can be made to the resource on a specific port.

If the 'Unhealthy threshold' is set to 3, how many failed checks mark a resource unhealthy?

A 1

B 3

C 2

D 4

The resource is marked unhealthy after 3 consecutive failed health checks.

What is a benefit of setting a shorter health check interval?

A Faster failure detection

B Less network traffic

C Longer resource downtime

D Reduced CPU usage

Shorter intervals detect failures faster but increase network traffic.

Which protocol is NOT typically used for AWS health checks?

A HTTP

B TCP

C FTP

D HTTPS

FTP is not used for AWS health checks; common protocols are HTTP, HTTPS, and TCP.

Explain how AWS uses health checks to maintain application availability.

Think about how AWS decides where to send user requests.

Describe the key settings you configure when setting up a health check in AWS.

Consider what AWS needs to know to check if a resource is working.

Practice

1. What is the main purpose of a health check in AWS services like Elastic Load Balancer?

easy

A. To verify if a server or service is responding correctly

B. To increase the server's CPU capacity automatically

C. To backup data from the server regularly

D. To encrypt data sent between servers

Solution

1. Step 1: Understand the role of health checks

   Health checks monitor if a server or service is working properly by sending requests and checking responses.

2. Step 2: Identify the main goal

   The main goal is to detect unhealthy servers so they can be removed from traffic, ensuring reliability.

3. Final Answer:

   To verify if a server or service is responding correctly -> Option A

4. Quick Check:

   Health check purpose = verify server response [OK]

Hint: Health checks confirm server responsiveness, not capacity or backups [OK]

Common Mistakes:

• Confusing health checks with scaling or backup tasks
• Thinking health checks encrypt data
• Assuming health checks improve server hardware

2. Which of the following is the correct way to configure a health check path for an HTTP service in AWS Elastic Load Balancer?

easy

A. HealthCheckInterval: 60

B. HealthCheckPath: "/status"

C. HealthCheckProtocol: TCP

D. HealthCheckPort: 8080

Solution

1. Step 1: Identify the correct parameter for HTTP path

   For HTTP health checks, the path to check is set using HealthCheckPath with a URL path string.

2. Step 2: Differentiate from other parameters

   HealthCheckPort sets the port, HealthCheckProtocol sets the protocol, and HealthCheckInterval sets the time between checks, not the path.

3. Final Answer:

   HealthCheckPath: "/status" -> Option B

4. Quick Check:

   HTTP health check path = HealthCheckPath [OK]

Hint: Path is set with HealthCheckPath, not port or interval [OK]

Common Mistakes:

• Using HealthCheckPort to set the path
• Confusing protocol with path
• Setting interval instead of path

3. Given this health check configuration for a load balancer:
IntervalSeconds: 30
TimeoutSeconds: 5
HealthyThresholdCount: 3
UnhealthyThresholdCount: 2
How many consecutive failed health checks will cause the instance to be marked unhealthy?

medium

A. 30

B. 3

C. 5

D. 2

Solution

1. Step 1: Understand UnhealthyThresholdCount meaning

   This value defines how many failed health checks in a row mark the instance unhealthy.

2. Step 2: Identify the value from configuration

   UnhealthyThresholdCount is set to 2, so after 2 failed checks, the instance is unhealthy.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   Failed checks to unhealthy = UnhealthyThresholdCount = 2 [OK]

Hint: UnhealthyThresholdCount = failed checks before unhealthy [OK]

Common Mistakes:

• Confusing HealthyThresholdCount with unhealthy count
• Using IntervalSeconds as count
• Assuming timeout affects count

4. You configured a health check with TimeoutSeconds set to 10 seconds, but your service responds in 3 seconds. The health check still fails. What is the most likely cause?

medium

A. HealthyThresholdCount is set too low

B. TimeoutSeconds is too high causing failure

C. The health check path is incorrect or unreachable

D. IntervalSeconds is set to 1 second

Solution

1. Step 1: Analyze TimeoutSeconds vs response time

   TimeoutSeconds is 10, service responds in 3 seconds, so timeout is not the issue.

2. Step 2: Consider other causes for failure

   If health check fails despite quick response, likely the path is wrong or the service is unreachable.

3. Final Answer:

   The health check path is incorrect or unreachable -> Option C

4. Quick Check:

   Fast response but failure = wrong path/unreachable [OK]

Hint: Fast response but failure? Check health check path [OK]

Common Mistakes:

• Assuming timeout too high causes failure
• Confusing threshold counts with timeout
• Ignoring path correctness

5. You want to configure a health check for a web service behind an AWS Application Load Balancer. The service sometimes takes up to 8 seconds to respond under load. Which configuration best ensures accurate health status without false failures?

hard

A. Set TimeoutSeconds to 10, IntervalSeconds to 30, HealthyThresholdCount to 3, UnhealthyThresholdCount to 2

B. Set TimeoutSeconds to 5, IntervalSeconds to 10, HealthyThresholdCount to 2, UnhealthyThresholdCount to 2

C. Set TimeoutSeconds to 3, IntervalSeconds to 5, HealthyThresholdCount to 5, UnhealthyThresholdCount to 5

D. Set TimeoutSeconds to 1, IntervalSeconds to 15, HealthyThresholdCount to 1, UnhealthyThresholdCount to 1

Solution

1. Step 1: Match TimeoutSeconds to max response time

   The service can take up to 8 seconds, so TimeoutSeconds should be at least 8 to avoid false failures. Set TimeoutSeconds to 10, IntervalSeconds to 30, HealthyThresholdCount to 3, UnhealthyThresholdCount to 2 sets 10 seconds, which is suitable.

2. Step 2: Choose thresholds and intervals for stability

   Longer IntervalSeconds (30) and thresholds (3 healthy, 2 unhealthy) help avoid flapping due to temporary slow responses.

3. Step 3: Evaluate other options

   Options A, C, and D have TimeoutSeconds too low or thresholds too sensitive, risking false unhealthy marks.

4. Final Answer:

   Set TimeoutSeconds to 10, IntervalSeconds to 30, HealthyThresholdCount to 3, UnhealthyThresholdCount to 2 -> Option A

5. Quick Check:

   Timeout ≥ max response + stable thresholds = Set TimeoutSeconds to 10, IntervalSeconds to 30, HealthyThresholdCount to 3, UnhealthyThresholdCount to 2 [OK]

Hint: Timeout must exceed max response time to avoid false failures [OK]

Common Mistakes:

• Setting TimeoutSeconds less than max response time
• Using too low thresholds causing flapping
• Ignoring service response variability