Load balancing algorithms (round robin, least connections) in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

How does the round robin load balancing algorithm distribute requests?

Imagine you have 4 servers and a load balancer using the round robin algorithm. How does it assign incoming requests?

A It sends each new request to the next server in order, cycling back to the first after the last.

B It sends all requests to the server with the fewest active connections.

C It sends requests randomly to any server without order.

D It sends requests only to the server with the highest CPU usage.

💡 Hint

Think about a simple way to share requests evenly without checking server load.

Round robin cycles through servers one by one, sending each new request to the next server in the list. This balances load evenly if all servers have similar capacity.

❓ Architecture

intermediate

Which load balancing algorithm is best when servers have uneven workloads?

You have 3 servers with different current loads. Which algorithm helps distribute new requests to the least busy server?

A Round robin, because it cycles evenly through servers.

B Random, because it picks any server without checking load.

C IP hash, because it sends requests based on client IP.

D Least connections, because it sends requests to the server with the fewest active connections.

💡 Hint

Consider which algorithm checks server load before assigning requests.

Least connections algorithm sends requests to the server with the fewest active connections, helping balance uneven workloads.

❓ scaling

advanced

What is a key limitation of round robin load balancing in a system with servers of different capacities?

Consider a system with 2 servers: one powerful and one weaker. What problem can round robin cause?

A It causes requests to be dropped randomly.

B It may overload the weaker server by sending equal requests to both servers regardless of capacity.

C It requires servers to share session state.

D It always sends requests to the most powerful server only.

💡 Hint

Think about how equal distribution affects servers with different power.

Round robin does not consider server capacity, so it can overload weaker servers by sending them the same number of requests as stronger ones.

❓ tradeoff

advanced

What is a tradeoff when using least connections load balancing compared to round robin?

Least connections balances load better but has a cost. What is a downside compared to round robin?

A It requires tracking active connections on each server, increasing complexity and overhead.

B It always sends requests to the same server, causing imbalance.

C It cannot handle more than 2 servers.

D It ignores server health status.

💡 Hint

Think about what extra information least connections needs to work.

Least connections needs to monitor active connections on each server, which adds complexity and overhead compared to simple round robin.

❓ estimation

expert

Estimate the maximum number of concurrent connections a load balancer can handle using least connections algorithm with 5 servers, each supporting 1000 connections.

Assuming the load balancer perfectly distributes connections using least connections algorithm, what is the maximum total concurrent connections supported?

A 1000 connections, limited by the weakest server.

B 2000 connections, assuming half the servers are used at a time.

C 5000 connections, as it sums the capacity of all servers.

D 10000 connections, doubling the total server capacity.

💡 Hint

Think about how least connections distributes load across all servers.

Least connections balances load so all servers are used up to their capacity, so total max connections is sum of all servers' capacities.

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