Why load balancing matters in AWS - Visual Breakdown

Process Flow - Why load balancing matters

User sends request

↓

Load Balancer receives request

↓

Check server health

↓

Distribute request to healthy server

↓

Server processes request

↓

Response sent back to user

Requests from users go to a load balancer, which checks server health and sends requests only to healthy servers, ensuring smooth and reliable service.

Execution Sample

AWS

User -> Load Balancer -> Server1 or Server2 -> Response

Shows how a load balancer directs user requests to one of multiple servers to share the work.

Process Table

Step	Action	Server1 Status	Server2 Status	Request Routed To	Reason
1	User sends request	Healthy	Healthy	Load Balancer	Request arrives at load balancer
2	Load balancer checks servers	Healthy	Healthy	Load Balancer	Both servers are healthy
3	Load balancer routes request	Healthy	Healthy	Server1	Server1 chosen to balance load
4	Server1 processes request	Healthy	Healthy	Server1	Server1 handles request
5	User sends another request	Healthy	Healthy	Load Balancer	New request arrives
6	Load balancer checks servers	Healthy	Healthy	Load Balancer	Both servers healthy
7	Load balancer routes request	Healthy	Healthy	Server2	Server2 chosen to balance load
8	Server2 processes request	Healthy	Healthy	Server2	Server2 handles request
9	Server1 becomes unhealthy	Unhealthy	Healthy	Load Balancer	Server1 fails health check
10	User sends request	Unhealthy	Healthy	Load Balancer	New request arrives
11	Load balancer checks servers	Unhealthy	Healthy	Load Balancer	Server1 unhealthy, Server2 healthy
12	Load balancer routes request	Unhealthy	Healthy	Server2	Only Server2 is healthy
13	Server2 processes request	Unhealthy	Healthy	Server2	Server2 handles request
14	Server1 recovers	Healthy	Healthy	Load Balancer	Server1 passes health check again
15	User sends request	Healthy	Healthy	Load Balancer	New request arrives
16	Load balancer routes request	Healthy	Healthy	Server1	Load balancing resumes
17	Server1 processes request	Healthy	Healthy	Server1	Server1 handles request

💡 Load balancer stops routing to unhealthy servers to keep service reliable.

Status Tracker

Variable	Start	After Step 9	After Step 14	Final
Server1 Status	Healthy	Unhealthy	Healthy	Healthy
Server2 Status	Healthy	Healthy	Healthy	Healthy
Request Routed To	Load Balancer	Load Balancer	Load Balancer	Load Balancer

Key Moments - 2 Insights

Why does the load balancer stop sending requests to Server1 at step 12?

Because Server1 failed the health check at step 9 and is marked unhealthy, so the load balancer only sends requests to healthy servers as shown in execution_table rows 9-12.

How does the load balancer decide which server to send requests to when both are healthy?

It distributes requests to balance the load, alternating between Server1 and Server2 as seen in steps 3 and 7 in the execution_table.

Visual Quiz - 3 Questions

Look at the execution table, at step 9 what happens to Server1 status?

A Server1 becomes healthy

B Server1 becomes unhealthy

C Server1 is removed

D Server1 processes a request

💡 Hint

Check the 'Server1 Status' column at step 9 in the execution_table.

At which step does the load balancer route a request to Server2 for the first time?

A Step 3

B Step 12

C Step 7

D Step 16

💡 Hint

Look at the 'Request Routed To' column in the execution_table to find when Server2 first receives a request.

If Server2 became unhealthy at step 10, what would happen at step 12?

A Load balancer cannot route request to any server

B Load balancer routes request to Server2 anyway

C Load balancer routes request to Server1

D Load balancer routes request to a new server

💡 Hint

Refer to how the load balancer only routes to healthy servers in the execution_table steps 9-12.

Concept Snapshot

Load balancing sends user requests to multiple servers to share work.
It checks server health to avoid sending requests to broken servers.
This keeps the service fast and reliable.
If a server fails, load balancer routes requests only to healthy ones.
When servers recover, they get requests again.
Load balancing improves uptime and user experience.

Full Transcript

Load balancing is important because it helps distribute user requests across multiple servers. When a user sends a request, it first goes to the load balancer. The load balancer checks which servers are healthy and sends the request to one of them. This sharing of work prevents any single server from getting overloaded. If a server becomes unhealthy, the load balancer stops sending requests to it until it recovers. This ensures users get fast and reliable responses. When the unhealthy server recovers, it starts receiving requests again. This process keeps the service running smoothly and improves user experience.

Practice

1. What is the main purpose of a load balancer in AWS?

easy

A. To create backups of servers automatically

B. To store user data securely

C. To distribute incoming traffic evenly across multiple servers

D. To monitor server CPU usage only

Solution

1. Step 1: Understand load balancer role

   A load balancer shares user traffic across servers to keep apps fast and reliable.

2. Step 2: Identify main function

   Its main job is to distribute incoming traffic evenly to avoid overloading one server.

3. Final Answer:

   To distribute incoming traffic evenly across multiple servers -> Option C

4. Quick Check:

   Load balancer = distribute traffic [OK]

Hint: Load balancer spreads traffic to avoid overload [OK]

Common Mistakes:

• Confusing load balancer with data storage
• Thinking it only monitors CPU
• Assuming it creates backups

2. Which AWS service is used to create a load balancer?

easy

A. Amazon S3

B. Elastic Load Balancing (ELB)

C. Amazon RDS

D. AWS Lambda

Solution

1. Step 1: Identify AWS services

   Amazon S3 stores files, RDS manages databases, Lambda runs code, ELB manages load balancing.

2. Step 2: Match service to load balancing

   Elastic Load Balancing (ELB) is the AWS service designed to create load balancers.

3. Final Answer:

   Elastic Load Balancing (ELB) -> Option B

4. Quick Check:

   Load balancer service = ELB [OK]

Hint: ELB is the AWS load balancer service name [OK]

Common Mistakes:

• Choosing storage or compute services instead of ELB
• Confusing Lambda with load balancing
• Selecting database services

3. Given this AWS setup code snippet, what does the load balancer do?

LoadBalancer -> TargetGroup -> EC2 Instances
Listener listens on port 80 and forwards to TargetGroup

medium

A. It forwards traffic on port 80 to all EC2 instances evenly

B. It blocks traffic on port 80

C. It stores data from EC2 instances

D. It only forwards traffic to one EC2 instance

Solution

1. Step 1: Understand listener role

   The listener listens on port 80 and forwards requests to the target group.

2. Step 2: Understand target group role

   The target group contains multiple EC2 instances and the load balancer distributes traffic evenly among them.

3. Final Answer:

   It forwards traffic on port 80 to all EC2 instances evenly -> Option A

4. Quick Check:

   Listener + TargetGroup = distribute traffic [OK]

Hint: Listener forwards traffic evenly to all targets [OK]

Common Mistakes:

• Thinking it blocks traffic
• Assuming it stores data
• Believing it sends traffic to only one instance

4. You set up a load balancer but all traffic goes to a single server. What is the likely mistake?

medium

A. Security group blocks all traffic

B. Listener is set to forward on wrong port

C. Load balancer is not created

D. Target group has only one instance registered

Solution

1. Step 1: Check target group configuration

   If only one instance is registered in the target group, all traffic goes to that instance.

2. Step 2: Verify listener and load balancer existence

   Listener forwarding on wrong port or missing load balancer would cause no traffic or errors, not single server traffic.

3. Final Answer:

   Target group has only one instance registered -> Option D

4. Quick Check:

   Single server traffic = one target instance [OK]

Hint: Check target group instance count first [OK]

Common Mistakes:

• Assuming listener port causes single server traffic
• Thinking load balancer absence sends traffic
• Blaming security group for traffic routing

5. You want to handle sudden traffic spikes without slowing your app. Which load balancing setup helps best?

hard

A. Use a load balancer with multiple target groups and auto-scaling EC2 instances

B. Use a single EC2 instance behind a load balancer

C. Use only one target group with fixed number of instances

D. Disable load balancer and rely on DNS round robin

Solution

1. Step 1: Understand traffic spike handling

   Handling spikes requires scaling resources automatically to meet demand.

2. Step 2: Match setup to scaling needs

   Using a load balancer with multiple target groups and auto-scaling EC2 instances allows dynamic resource adjustment and balanced traffic.

3. Final Answer:

   Use a load balancer with multiple target groups and auto-scaling EC2 instances -> Option A

4. Quick Check:

   Auto-scaling + load balancer = handle spikes [OK]

Hint: Combine load balancer with auto-scaling for spikes [OK]

Common Mistakes:

• Relying on single instance for heavy traffic
• Using fixed instances without scaling
• Disabling load balancer reduces reliability