Why Load balancing algorithms (round robin, least connections) in HLD? - Purpose & Use Cases

The Big Idea

What if your website could never slow down, no matter how many visitors arrive?

The Scenario

Imagine you run a busy restaurant with only one waiter taking orders and serving all tables. As more guests arrive, the waiter gets overwhelmed, orders get mixed up, and some tables wait too long while others get served quickly.

The Problem

Handling all requests manually means slow service and mistakes. The waiter can't remember who ordered what or who is waiting longest. Guests get frustrated, and the restaurant loses customers because the workload isn't shared fairly.

The Solution

Load balancing algorithms like round robin and least connections act like smart hosts who assign new guests to waiters evenly or based on who is least busy. This way, no waiter is overloaded, and guests get served faster and more fairly.

Before vs After

✗ Before

sendRequestTo(server1)
sendRequestTo(server1)
sendRequestTo(server1)

✓ After

sendRequestTo(roundRobin.next())
sendRequestTo(roundRobin.next())
sendRequestTo(roundRobin.next())

What It Enables

It enables smooth, fair, and efficient handling of many requests by spreading the load smartly across servers.

Real Life Example

When you visit a popular website, load balancers use these algorithms to send your request to the best server, so the site loads quickly even when millions are browsing.

Key Takeaways

Manual handling of requests causes delays and errors.

Load balancing algorithms distribute work evenly or by current load.

This improves speed, reliability, and user experience.

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