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

Process Flow - Target groups concept

Client sends request

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Load Balancer receives request

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Load Balancer checks Target Group

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Select healthy target from Target Group

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Forward request to selected target

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Target processes request and responds

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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.