Load balancing algorithms (round robin, least connections) in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Round Robin load balancing algorithm?

Round Robin distributes incoming requests evenly across all servers in a fixed cyclic order, assigning each new request to the next server in the list.

beginner

How does the Least Connections load balancing algorithm work?

Least Connections sends each new request to the server with the fewest active connections, helping balance load dynamically based on current server usage.

intermediate

Which load balancing algorithm is better for servers with uneven request processing times?

Least Connections is better because it considers current load, sending requests to less busy servers, unlike Round Robin which ignores server load.

intermediate

What is a potential downside of Round Robin load balancing?

Round Robin can overload a slow server because it does not consider server capacity or current load, leading to uneven performance.

intermediate

Why might Least Connections load balancing require more overhead than Round Robin?

Because it must track the number of active connections on each server in real-time, which adds complexity and resource usage.

Which load balancing algorithm assigns requests in a fixed order without considering server load?

A Least Connections

B Weighted Round Robin

C Random

D Round Robin

Round Robin cycles through servers in order, ignoring current load.

Least Connections algorithm directs traffic to which server?

A Server chosen randomly

B Server with the most CPU power

C Server with the fewest active connections

D Server with the highest memory

Least Connections chooses the server with the fewest active connections to balance load.

What is a key advantage of Least Connections over Round Robin?

A Balances load based on current server usage

B Does not require tracking server state

C Always faster response time

D Simpler to implement

Least Connections balances load dynamically by considering active connections.

Which algorithm might cause uneven load if servers have different speeds?

A Round Robin

B Weighted Least Connections

C Least Connections

D IP Hash

Round Robin ignores server speed and load, possibly overloading slower servers.

What extra data does Least Connections need to maintain?

A Server IP addresses

B Number of active connections per server

C Server uptime

D Request size

Least Connections tracks active connections to decide where to send requests.

Explain how Round Robin and Least Connections load balancing algorithms differ in handling incoming requests.

Think about whether the algorithm tracks server usage or not.

Describe scenarios where Least Connections would be preferred over Round Robin for load balancing.

Consider how server load affects performance.

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