Why load balancers distribute traffic in HLD - Architecture Impact

System Overview - Why load balancers distribute traffic

This system explains why load balancers distribute incoming user traffic across multiple servers. The goal is to ensure no single server is overwhelmed, improving system reliability, speed, and availability.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  +---------+---------+
  |         |         |
Server 1  Server 2  Server 3
  |         |         |
Database  Database  Database

Components

User

client

Sends requests to the system

Load Balancer

load_balancer

Distributes incoming requests evenly across servers to prevent overload

Server 1

service

Processes user requests

Server 2

service

Processes user requests

Server 3

service

Processes user requests

Database

database

Stores and retrieves data for servers

Request Flow - 6 Hops

User → Load Balancer

Load Balancer → Server 2

Server 2 → Database

Database → Server 2

Server 2 → Load Balancer

Load Balancer → User

Failure Scenario

Component Fails:Load Balancer

Impact:All incoming user requests fail because traffic cannot be distributed to servers

Mitigation:Use multiple load balancers with failover or clustering to avoid single point of failure

Architecture Quiz - 3 Questions

Why does the load balancer send requests to different servers?

A To confuse the servers

B To make the user wait longer

C To prevent any single server from getting too much traffic

D To reduce the number of users

Design Principle

Load balancers improve system reliability and performance by spreading user requests evenly across multiple servers, preventing overload and enabling scalability.

Practice

1. Why do load balancers distribute incoming traffic across multiple servers?

easy

A. To prevent any single server from becoming overloaded and improve system reliability

B. To increase the number of servers needed for the system

C. To slow down the response time for users

D. To reduce the total number of user requests

Solution

1. Step 1: Understand server overload

   When too many requests go to one server, it slows down or crashes.

2. Step 2: Role of load balancers

   Load balancers spread requests evenly to keep servers healthy and fast.

3. Final Answer:

   To prevent any single server from becoming overloaded and improve system reliability -> Option A

4. Quick Check:

   Load balancers prevent overload = C [OK]

Hint: Load balancers keep servers from getting too busy [OK]

Common Mistakes:

• Thinking load balancers reduce total requests
• Believing load balancers slow down responses
• Assuming load balancers increase server count unnecessarily

2. Which of the following is a common method used by load balancers to distribute traffic?

easy

A. Bubble sort

B. Binary search

C. Depth-first search

D. Round-robin

Solution

1. Step 1: Identify load balancing methods

   Common methods include round-robin and least connections.

2. Step 2: Match options to known methods

   Round-robin cycles through servers evenly; others are unrelated algorithms.

3. Final Answer:

   Round-robin -> Option D

4. Quick Check:

   Round-robin is a load balancing method = D [OK]

Hint: Round-robin cycles requests evenly [OK]

Common Mistakes:

• Confusing sorting/search algorithms with load balancing
• Choosing unrelated computer science terms
• Ignoring common load balancing strategies

3. Consider a load balancer using the least connections strategy. If Server A has 3 active connections and Server B has 1 active connection, where will the next request be sent?

medium

A. Server B

B. Server A

C. Both servers equally

D. To a new server not listed

Solution

1. Step 1: Understand least connections strategy

   It sends new requests to the server with fewer active connections.

2. Step 2: Compare active connections

   Server A has 3, Server B has 1, so Server B has fewer connections.

3. Final Answer:

   Server B -> Option A

4. Quick Check:

   Least connections picks server with fewer connections = B [OK]

Hint: Send to server with least active connections [OK]

Common Mistakes:

• Choosing server with more connections
• Assuming equal distribution regardless of load
• Thinking new servers are automatically chosen

4. A load balancer is configured to use round-robin but always sends requests to the same server. What is the most likely cause?

medium

A. The servers are all down

B. The load balancer is not tracking the last server used correctly

C. The network is too slow

D. The user requests are too few

Solution

1. Step 1: Understand round-robin behavior

   Round-robin cycles through servers in order to balance load.

2. Step 2: Identify cause of repeated server selection

   If it always picks the same server, it likely lost track of which server was last used.

3. Final Answer:

   The load balancer is not tracking the last server used correctly -> Option B

4. Quick Check:

   Round-robin needs correct tracking = A [OK]

Hint: Round-robin must remember last server used [OK]

Common Mistakes:

• Blaming server downtime without checking load balancer
• Assuming network speed affects server choice
• Thinking low request volume causes imbalance

5. You design a system with a load balancer distributing traffic to 5 servers. Suddenly, one server becomes much slower but still receives equal requests. What change to the load balancing strategy can improve overall system performance?

hard

A. Use round-robin but add more user requests

B. Increase the number of servers without changing strategy

C. Switch from round-robin to least connections to avoid slow server overload

D. Disable the load balancer and send requests to one server

Solution

1. Step 1: Identify problem with round-robin

   Round-robin sends equal requests regardless of server speed, causing slow server overload.

2. Step 2: Choose better strategy

   Least connections sends requests to less busy servers, reducing load on slow server.

3. Final Answer:

   Switch from round-robin to least connections to avoid slow server overload -> Option C

4. Quick Check:

   Least connections adapts to server speed = A [OK]

Hint: Use least connections to avoid slow servers [OK]

Common Mistakes:

• Adding servers without fixing load distribution
• Increasing requests worsens overload
• Disabling load balancer causes single point failure