Health checks configuration in AWS - Step-by-Step Execution

Process Flow - Health checks configuration

Start: Configure Health Check

↓

Set Protocol, Port, Path

↓

Define Interval & Timeout

↓

Set Healthy & Unhealthy Thresholds

↓

Load Balancer Sends Health Requests

↓

Check Response: Success or Fail

↓

Update Instance Health Status

↓

Use Health Status for Traffic Routing

↓

End

This flow shows how health checks are configured and used to monitor instance health and route traffic accordingly.

Execution Sample

AWS

HealthCheck:
  Protocol: HTTP
  Port: 80
  Path: /health
  IntervalSeconds: 30
  TimeoutSeconds: 5
  HealthyThresholdCount: 3
  UnhealthyThresholdCount: 2

Defines a health check that sends HTTP requests every 30 seconds to /health on port 80, marking instances healthy or unhealthy based on response.

Process Table

Step	Action	Health Check Request Sent	Response Received	Health Status Update	Traffic Routing
1	Send health check to instance	HTTP GET /health on port 80	200 OK	Instance marked healthy count 1	Instance does not receive traffic
2	Send health check to instance	HTTP GET /health on port 80	200 OK	Instance marked healthy count 2	Instance does not receive traffic
3	Send health check to instance	HTTP GET /health on port 80	200 OK	Instance marked healthy count 3 (healthy threshold met)	Instance confirmed healthy, receives traffic
4	Send health check to instance	HTTP GET /health on port 80	500 Error	Instance marked unhealthy count 1	Instance still receives traffic
5	Send health check to instance	HTTP GET /health on port 80	Timeout	Instance marked unhealthy count 2 (unhealthy threshold met)	Instance marked unhealthy, traffic stopped
6	Send health check to instance	HTTP GET /health on port 80	200 OK	Instance marked healthy count 1	Instance still does not receive traffic
7	Send health check to instance	HTTP GET /health on port 80	200 OK	Instance marked healthy count 2	Instance still does not receive traffic
8	Send health check to instance	HTTP GET /health on port 80	200 OK	Instance marked healthy count 3 (healthy threshold met)	Instance marked healthy, traffic resumes
9	End monitoring	No request	No response	Health check monitoring continues	Traffic routed based on latest health status

💡 Health check monitoring runs continuously; traffic routing updates based on health status thresholds.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8	Final
HealthyCount	0	1	2	3	0	0	1	2	3	3
UnhealthyCount	0	0	0	0	1	2	0	0	0	0
InstanceHealth	Unknown	Healthy (pending)	Healthy (pending)	Healthy	Unhealthy (pending)	Unhealthy	Unhealthy (pending)	Unhealthy (pending)	Healthy (pending)	Healthy
TrafficRouting	No traffic	No traffic	No traffic	Receives traffic	Receives traffic	No traffic	No traffic	No traffic	Receives traffic	Receives traffic

Key Moments - 3 Insights

Why does the instance not immediately become healthy after one successful health check?

Because the HealthyThresholdCount is 3, the instance must pass 3 consecutive successful checks before being marked healthy, as shown in steps 1-3 in the execution_table.

Why does traffic stop only after two failed health checks?

The UnhealthyThresholdCount is 2, so the instance must fail 2 consecutive checks before being marked unhealthy and traffic is stopped, as seen in steps 4-5.

What happens if the instance recovers after being unhealthy?

It must pass the healthy threshold count again (3 successful checks) before traffic resumes, as shown in steps 6-8.

Visual Quiz - 3 Questions

Look at the execution_table, what is the HealthyCount after step 3?

A 2

B 3

C 1

D 0

💡 Hint

Check the 'HealthyCount' variable in variable_tracker after step 3.

At which step does the instance stop receiving traffic due to health check failure?

A Step 4

B Step 3

C Step 5

D Step 6

💡 Hint

Look at the 'TrafficRouting' column in execution_table where traffic changes to stopped.

If the UnhealthyThresholdCount was set to 1, how would the traffic routing change after step 4?

A Traffic would stop immediately after step 4

B Traffic would continue as normal

C Traffic would stop after step 5

D Traffic would never stop

💡 Hint

Refer to how UnhealthyThresholdCount affects traffic routing in execution_table steps 4 and 5.

Concept Snapshot

Health checks monitor instance health by sending requests at intervals.
Set protocol, port, and path for checks.
Define thresholds for healthy/unhealthy counts.
Instance marked healthy after consecutive successes.
Instance marked unhealthy after consecutive failures.
Traffic routed only to healthy instances.

Full Transcript

Health checks configuration involves setting up periodic requests from a load balancer to instances to verify their health. The configuration includes protocol, port, path, interval, timeout, and thresholds for marking instances healthy or unhealthy. The load balancer sends requests at the defined interval and waits for responses within the timeout. If an instance responds successfully enough times consecutively, it is marked healthy and receives traffic. If it fails enough times consecutively, it is marked unhealthy and traffic is stopped. This process repeats continuously to ensure traffic is only sent to healthy instances.

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