Load balancing algorithms (round robin, least connections) in HLD - Architecture Diagram

System Overview - Load balancing algorithms (round robin, least connections)

This system demonstrates how load balancers distribute incoming user requests to multiple backend servers using two common algorithms: round robin and least connections. The goal is to efficiently balance traffic to ensure no single server is overwhelmed, improving response times and reliability.

Architecture Diagram

User
  |
  v
Load Balancer (Round Robin / Least Connections)
  |
  +-------------------+-------------------+-------------------+
  |                   |                   |
Server 1           Server 2           Server 3
  |                   |                   |
Database            Database            Database
  

Components

User

client

Sends requests to the system

Load Balancer

load_balancer

Distributes incoming requests to backend servers using round robin or least connections algorithm

Server 1

service

Processes requests and queries the database

Server 2

service

Processes requests and queries the database

Server 3

service

Processes requests and queries the database

Database

database

Stores persistent data accessed by servers

Request Flow - 6 Hops

User → Load Balancer

Load Balancer → Server (selected by algorithm)

Server → Database

Database → Server

Server → Load Balancer

Load Balancer → User

Failure Scenario

Component Fails:Load Balancer

Impact:All incoming user requests fail to reach backend servers, causing complete service outage.

Mitigation:Deploy multiple load balancers with health checks and failover to ensure high availability.

Architecture Quiz - 3 Questions

Which load balancing algorithm sends requests to servers in a fixed order, cycling through each one?

A Round Robin

B Least Connections

C Random Selection

D IP Hash

Design Principle

This architecture shows how load balancers use algorithms like round robin and least connections to distribute traffic evenly and efficiently among servers, preventing overload and improving system reliability.

Practice

1. Which load balancing algorithm sends requests to servers in a fixed, repeating order without considering current load?

easy

A. Round Robin

B. Least Connections

C. Random Selection

D. IP Hash

Solution

1. Step 1: Understand Round Robin behavior

   Round Robin cycles through servers one by one in order, ignoring current load or connections.

2. Step 2: Compare with other algorithms

   Least Connections considers active connections, Random Selection picks randomly, IP Hash uses client IP. Only Round Robin ignores load.

3. Final Answer:

   Round Robin -> Option A

4. Quick Check:

   Fixed order without load check = Round Robin [OK]

Hint: Round Robin cycles evenly, ignores load [OK]

Common Mistakes:

• Confusing Least Connections with Round Robin
• Thinking Random Selection is fixed order
• Assuming IP Hash ignores load

2. Which of the following is the correct pseudocode snippet for implementing Round Robin load balancing?

easy

A. server = server with minimum active connections

B. server = hash(client_ip) % total_servers

C. server = random.choice(servers)

D. server = servers[(current_index + 1) % total_servers]

Solution

1. Step 1: Identify Round Robin logic

   Round Robin selects the next server by incrementing an index and wrapping around using modulo.

2. Step 2: Match pseudocode to logic

   server = servers[(current_index + 1) % total_servers] uses (current_index + 1) % total_servers, which correctly cycles through servers.

3. Final Answer:

   server = servers[(current_index + 1) % total_servers] -> Option D

4. Quick Check:

   Modulo cycling index = Round Robin [OK]

Hint: Round Robin uses modulo to cycle servers [OK]

Common Mistakes:

• Choosing minimum connections for Round Robin
• Using random selection syntax
• Using hash function for Round Robin

3. Given 3 servers with active connections: Server A=5, Server B=2, Server C=4, which server will Least Connections algorithm select for the next request?

medium

A. Server B

B. Server C

C. Server A

D. Server A or Server C (tie)

Solution

1. Step 1: Identify server with fewest active connections

   Server B has 2 connections, fewer than A (5) and C (4).

2. Step 2: Apply Least Connections selection

   Least Connections chooses the server with the minimum active connections, so Server B is selected.

3. Final Answer:

   Server B -> Option A

4. Quick Check:

   Fewest connections = Server B [OK]

Hint: Least Connections picks server with minimum active connections [OK]

Common Mistakes:

• Picking server with most connections
• Choosing Round Robin instead of Least Connections
• Confusing ties with multiple servers

4. A load balancer using Least Connections algorithm always sends requests to Server A, even though Server B has fewer active connections. What is the most likely cause?

medium

A. Round Robin algorithm is being used instead

B. The connection count for Server B is not updated correctly

C. Server A has a lower IP address

D. The load balancer is using random selection

Solution

1. Step 1: Analyze behavior mismatch

   Least Connections should pick server with fewer active connections, but always picks Server A.

2. Step 2: Identify likely cause

   If Server B's connection count is stale or incorrect, the balancer wrongly thinks Server A has fewer connections.

3. Final Answer:

   The connection count for Server B is not updated correctly -> Option B

4. Quick Check:

   Incorrect connection tracking causes wrong Least Connections choice [OK]

Hint: Check if connection counts update properly for Least Connections [OK]

Common Mistakes:

• Assuming algorithm is Round Robin without checking
• Thinking IP address affects Least Connections
• Ignoring connection count accuracy

5. You have 4 servers with varying capacities and active connections: Server A (capacity 10, connections 5), Server B (capacity 20, connections 15), Server C (capacity 15, connections 10), Server D (capacity 10, connections 2). Which server should a capacity-aware Least Connections algorithm select to balance load best?

hard

A. Server C

B. Server B

C. Server D

D. Server A

Solution

1. Step 1: Calculate load ratio (connections/capacity) for each server

   Server A: 5/10 = 0.5, Server B: 15/20 = 0.75, Server C: 10/15 ≈ 0.67, Server D: 2/10 = 0.2

2. Step 2: Select server with lowest load ratio

   Server D has the lowest ratio (0.2), so it is selected in capacity-aware Least Connections.

3. Final Answer:

   Server D -> Option C

4. Quick Check:

   Lowest load ratio = Server D [OK]

Hint: Consider connections relative to capacity, not just counts [OK]

Common Mistakes:

• Picking server with fewest connections ignoring capacity
• Ignoring capacity differences
• Choosing server with lowest absolute connections only