Target groups concept in AWS - Step-by-Step Execution

Process Flow - Target groups concept

Client sends request

↓

Load Balancer receives request

↓

Load Balancer checks Target Group

↓

Select healthy target from Target Group

↓

Forward request to selected target

↓

Target processes request and responds

↓

Load Balancer sends response back to client

The load balancer receives client requests, checks the target group for healthy targets, forwards the request to one, and returns the response.

Execution Sample

AWS

Client -> Load Balancer -> Target Group -> Target Instance
Target Instance -> Load Balancer -> Client

Shows the flow of a request from client through load balancer to a target in the target group and back.

Process Table

Step	Action	Load Balancer Decision	Target Group State	Result
1	Client sends request	Receives request	Targets: [Instance1(healthy), Instance2(healthy)]	Request accepted
2	Load Balancer checks target group	Selects healthy target	Targets: [Instance1(healthy), Instance2(healthy)]	Instance1 selected
3	Forward request to Instance1	Forwarding	Targets unchanged	Request sent to Instance1
4	Instance1 processes request	No change	Targets unchanged	Response generated
5	Load Balancer sends response to client	No change	Targets unchanged	Client receives response
6	Health check detects Instance2 unhealthy	No request	Targets: [Instance1(healthy), Instance2(unhealthy)]	Instance2 marked unhealthy
7	Next request arrives	Selects healthy target	Targets: [Instance1(healthy), Instance2(unhealthy)]	Instance1 selected
8	Forward request to Instance1	Forwarding	Targets unchanged	Request sent to Instance1
9	Instance1 processes request	No change	Targets unchanged	Response generated
10	Load Balancer sends response to client	No change	Targets unchanged	Client receives response

💡 Execution stops after client receives response and target group updates health status.

Status Tracker

Variable	Start	After Step 2	After Step 6	After Step 9	Final
Targets Health	[Instance1: healthy, Instance2: healthy]	[Instance1: healthy, Instance2: healthy]	[Instance1: healthy, Instance2: unhealthy]	[Instance1: healthy, Instance2: unhealthy]	[Instance1: healthy, Instance2: unhealthy]
Selected Target	None	Instance1	Instance1	Instance1	Instance1
Request Status	None	Received	Received	Processed	Processed

Key Moments - 3 Insights

Why does the load balancer skip Instance2 after it becomes unhealthy?

Because the load balancer only forwards requests to healthy targets in the target group, as shown in steps 6 and 7 where Instance2 is marked unhealthy and skipped.

What happens if all targets in the target group are unhealthy?

The load balancer cannot forward requests to any target, so requests will fail or be dropped. This is implied by the load balancer selecting only healthy targets in the execution table.

How does the load balancer know which target to select?

It uses health checks to determine which targets are healthy and then selects one based on the load balancing algorithm, as shown in step 2 where it selects Instance1 because both are healthy.

Visual Quiz - 3 Questions

Look at the execution table, which instance is selected at step 7?

A Instance1

B Instance2

C No instance selected

D Both instances

💡 Hint

Check the 'Load Balancer Decision' column at step 7 in the execution table.

At which step does Instance2 become unhealthy?

A Step 8

B Step 4

C Step 6

D Step 10

💡 Hint

Look at the 'Target Group State' column for changes in health status.

If Instance1 also became unhealthy, what would happen to the load balancer's selection?

A It would select Instance1 anyway

B It would have no healthy targets to select

C It would select Instance2 anyway

D It would forward requests to both instances

💡 Hint

Refer to the key moment about what happens if all targets are unhealthy.

Concept Snapshot

Target groups hold registered targets (like servers).
Load balancers send client requests only to healthy targets.
Health checks update target health status.
Unhealthy targets are skipped until healthy again.
This ensures reliable request routing and availability.

Full Transcript

A target group is a set of servers or instances that a load balancer uses to distribute incoming client requests. When a client sends a request, the load balancer receives it and checks the target group for healthy targets. It selects one healthy target and forwards the request. The target processes the request and sends a response back through the load balancer to the client. Health checks run regularly to mark targets as healthy or unhealthy. Unhealthy targets are skipped by the load balancer until they become healthy again. This process ensures requests are only sent to working servers, improving reliability and availability.

Practice

1. What is the main purpose of a target group in AWS Elastic Load Balancing?

easy

A. To create virtual private clouds

B. To store user data securely

C. To manage DNS records

D. To group servers for load balancing and health checks

Solution

1. Step 1: Understand target group role

   A target group groups servers (targets) to distribute incoming traffic.

2. Step 2: Identify key functions

   It also performs health checks to ensure traffic goes only to healthy servers.

3. Final Answer:

   To group servers for load balancing and health checks -> Option D

4. Quick Check:

   Target groups = server grouping + health checks [OK]

Hint: Target groups organize servers for load balancing [OK]

Common Mistakes:

• Confusing target groups with security groups
• Thinking target groups store data
• Mixing target groups with VPC functions

2. Which of the following is the correct way to specify a target group protocol in AWS CLI?

easy

A. aws elbv2 create-target-group --protocol HTTP

B. aws elbv2 create-target-group --protocol FTP

C. aws elbv2 create-target-group --protocol SMTP

D. aws elbv2 create-target-group --protocol SSH

Solution

1. Step 1: Recall supported protocols for target groups

   AWS target groups support HTTP, HTTPS, TCP, TLS, UDP, and TCP_UDP protocols.

2. Step 2: Identify valid protocol in options

   HTTP is a valid protocol; FTP, SMTP, and SSH are not supported for target groups.

3. Final Answer:

   aws elbv2 create-target-group --protocol HTTP -> Option A

4. Quick Check:

   HTTP is valid protocol for target groups [OK]

Hint: Use HTTP or HTTPS for target group protocol [OK]

Common Mistakes:

• Using unsupported protocols like FTP or SSH
• Confusing target group protocol with security group rules
• Misspelling protocol names

3. Given a target group with 3 registered targets, two healthy and one unhealthy, how will the load balancer route traffic?

medium

A. It sends traffic only to the two healthy targets

B. It sends traffic equally to all three targets

C. It sends traffic only to the unhealthy target

D. It stops sending traffic until all targets are healthy

Solution

1. Step 1: Understand health check impact

   Load balancers route traffic only to targets marked healthy by health checks.

2. Step 2: Apply to given targets

   With two healthy and one unhealthy, traffic goes only to the two healthy targets.

3. Final Answer:

   It sends traffic only to the two healthy targets -> Option A

4. Quick Check:

   Traffic routes only to healthy targets [OK]

Hint: Load balancer skips unhealthy targets [OK]

Common Mistakes:

• Assuming traffic goes to all targets regardless of health
• Thinking traffic stops if any target is unhealthy
• Confusing unhealthy targets with backup targets

4. You created a target group but your load balancer shows no healthy targets. What is the most likely issue?

medium

A. Load balancer is in a different region

B. Security group blocks health check traffic

C. Targets are not registered with the target group

D. Target group protocol is set to FTP

Solution

1. Step 1: Check common causes for no healthy targets

   Targets not registered or security blocking health checks cause no healthy targets.

2. Step 2: Analyze options

   Security group blocking health check traffic prevents health checks from succeeding, causing targets to be unhealthy.

3. Final Answer:

   Security group blocks health check traffic -> Option B

4. Quick Check:

   Blocked health checks cause no healthy targets [OK]

Hint: Check security groups allow health check ports [OK]

Common Mistakes:

• Ignoring security group rules
• Assuming region mismatch causes health check failure
• Using unsupported protocols like FTP

5. You want to route HTTP traffic to two different target groups based on URL path: /images/* to one group and /api/* to another. Which AWS service feature should you use?

hard

A. Elastic IP addresses assigned to each target group

B. Network Load Balancer with static IP routing

C. Application Load Balancer with path-based routing rules

D. Classic Load Balancer with SSL termination

Solution

1. Step 1: Identify routing needs

   Routing based on URL path requires path-based routing rules.

2. Step 2: Match AWS service feature

   Application Load Balancer supports path-based routing to different target groups.

3. Step 3: Exclude incorrect options

   Network Load Balancer does not support path-based routing; Classic Load Balancer lacks this feature; Elastic IPs do not route traffic by path.

4. Final Answer:

   Application Load Balancer with path-based routing rules -> Option C

5. Quick Check:

   Path-based routing = Application Load Balancer [OK]

Hint: Use ALB for URL path routing to target groups [OK]

Common Mistakes:

• Using Network Load Balancer for path routing
• Confusing Classic Load Balancer features
• Thinking Elastic IPs control routing rules