Load balancing algorithms (round robin, least connections) in HLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select the next server using round robin.

HLD

next_server = servers[[1] % len(servers)]

Drag options to blanks, or click blank then click option'

Acurrent_index

Bserver_count

Crandom_index

Dmax_connections

Attempts:

3 left

💡 Hint

Common Mistakes

Using random index instead of cycling

Using max connections for round robin

Round robin selects the next server by cycling through the list using the current index modulo the number of servers.

2fill in blank

medium

Complete the code to find the server with the least connections.

HLD

least_loaded_server = min(servers, key=lambda s: s.[1])

Drag options to blanks, or click blank then click option'

Aload

Bcapacity

Cconnections

Dlatency

Attempts:

3 left

💡 Hint

Common Mistakes

Using load or capacity instead of connections

Using latency which is unrelated

Least connections algorithm picks the server with the smallest number of active connections.

3fill in blank

hard

Fix the error in the round robin index update.

HLD

current_index = (current_index [1] 1) % len(servers)

Drag options to blanks, or click blank then click option'

A/

B-

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction which moves backward

Using multiplication or division which is incorrect

To move to the next server, increment the index by 1 and wrap around using modulo.

4fill in blank

hard

Fill both blanks to create a dictionary of servers with their connection counts, filtering servers with less than 10 connections.

HLD

{server.id: server.[1] for server in servers if server.[2] < 10}

Drag options to blanks, or click blank then click option'

Aconnections

Bload

Dcapacity

Attempts:

3 left

💡 Hint

Common Mistakes

Using different attributes for mapping and filtering

Using load or capacity instead of connections

This dictionary comprehension maps server IDs to their connection counts, filtering only servers with fewer than 10 connections.

5fill in blank

hard

Fill all three blanks to implement a function that selects a server using least connections algorithm.

HLD

def select_server(servers):
    return min(servers, key=lambda [1]: [2].[3])

Drag options to blanks, or click blank then click option'

Aserver

Bs

Dconnections

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent variable names

Accessing wrong attributes

The function uses a lambda with parameter 's' to find the server with the minimum number of connections.

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