Leader election in HLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the leader node in a ring-based election.

HLD

if node_id == [1]:
    leader = True

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

Amin_id

Bmax_id

Crandom_id

Dcurrent_id

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing the minimum ID as leader

Using a random ID instead of max ID

The leader is usually the node with the highest ID in ring-based leader election algorithms.

2fill in blank

medium

Complete the code to send election messages to the next node.

HLD

send_message(to=[1], message='election')

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

Aprevious_node

Bleader_node

Cnext_node

Drandom_node

Attempts:

3 left

💡 Hint

Common Mistakes

Sending messages to the previous node

Sending messages to a random node

In ring-based leader election, messages are passed to the next node in the ring.

3fill in blank

hard

Fix the error in the condition to check if the current node is the leader.

HLD

if [1] == max_id:
    declare_leader()

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

Anode_id

Belection_id

Ccurrent_leader

Dleader_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using leader_id which may not be set yet

Using election_id which is for messages

The current node's ID should be compared to the maximum ID to decide leadership.

4fill in blank

hard

Fill both blanks to complete the election message handling logic.

HLD

if received_id [1] node_id:
    forward_id = [2]
else:
    forward_id = node_id

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

A>

B<

Creceived_id

Dnode_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' in comparison

Forwarding node_id instead of received_id when received_id is larger

If the received ID is greater than the current node's ID, forward the received ID; otherwise, forward own ID.

5fill in blank

hard

Fill all three blanks to complete the leader announcement broadcast.

HLD

for node in [1]:
    send_message(to=node, message=[2])
leader = [3]

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

Anetwork_nodes

B'leader_elected'

CTrue

DFalse

Attempts:

3 left

💡 Hint

Common Mistakes

Sending message to only one node

Setting leader to False after election

The leader broadcasts to all nodes in the network, sends a leader announcement message, and sets its leader status to True.

Practice

1. What is the main purpose of leader election in a distributed system?

easy

A. To delete inactive nodes automatically

B. To increase the number of nodes in the system

C. To encrypt communication between nodes

D. To select one node as the main coordinator for tasks

Solution

1. Step 1: Understand the role of leader election

   Leader election is used to pick one node to coordinate tasks in a distributed system.

2. Step 2: Identify the correct purpose

   Among the options, only selecting a main coordinator matches the leader election goal.

3. Final Answer:

   To select one node as the main coordinator for tasks -> Option D

4. Quick Check:

   Leader election = select coordinator [OK]

Hint: Leader election picks one main node to coordinate [OK]

Common Mistakes:

• Confusing leader election with node addition
• Thinking it deletes nodes automatically
• Assuming it handles encryption

2. Which of the following is a correct step in a leader election algorithm?

easy

A. Nodes send messages to agree on the leader

B. Nodes duplicate leader roles simultaneously

C. Nodes ignore messages from others

D. Nodes randomly shut down to reduce load

Solution

1. Step 1: Recall leader election communication

   Nodes communicate by sending messages to agree on who will be leader.

2. Step 2: Match options with correct behavior

   Only sending messages to agree fits the leader election process.

3. Final Answer:

   Nodes send messages to agree on the leader -> Option A

4. Quick Check:

   Leader election = message agreement [OK]

Hint: Leader election needs message exchange between nodes [OK]

Common Mistakes:

• Thinking nodes shut down randomly
• Believing nodes ignore others' messages
• Assuming multiple leaders run at once

3. Consider a ring of 4 nodes (A, B, C, D) running a leader election where each node sends its ID clockwise. If node C has the highest ID, which node will be elected leader?

medium

A. Node B

B. Node C

C. Node A

D. Node D

Solution

1. Step 1: Understand ring leader election

   Nodes pass IDs around; the highest ID wins and becomes leader.

2. Step 2: Identify highest ID node

   Node C has the highest ID, so it will be elected leader after messages circulate.

3. Final Answer:

   Node C -> Option B

4. Quick Check:

   Highest ID node = leader [OK]

Hint: Highest ID node in ring wins leader election [OK]

Common Mistakes:

• Choosing first node instead of highest ID
• Confusing direction of message passing
• Assuming multiple leaders

4. In a leader election algorithm, a node fails to send its election message. What is the likely impact?

medium

A. All nodes become leaders simultaneously

B. The system immediately elects a new leader without delay

C. The election process may stall or fail to complete

D. The failed node automatically becomes leader

Solution

1. Step 1: Analyze message failure impact

   If a node fails to send its election message, other nodes may wait indefinitely or miss information.

2. Step 2: Understand election process dependency

   Leader election relies on message exchange; missing messages can stall or break the process.

3. Final Answer:

   The election process may stall or fail to complete -> Option C

4. Quick Check:

   Missing message = election stalls [OK]

Hint: Missing messages can stall leader election [OK]

Common Mistakes:

• Assuming instant new leader election
• Thinking all nodes become leaders
• Believing failed node becomes leader

5. You design a distributed system with 100 nodes using leader election. To improve fault tolerance, you want to avoid single leader failure. Which approach is best?

hard

A. Use a leader and backup leaders that take over if leader fails

B. Use a single leader with frequent heartbeat checks and automatic re-election

C. Elect multiple leaders simultaneously to share tasks equally

D. Avoid leader election and let all nodes act independently

Solution

1. Step 1: Understand fault tolerance needs

   Single leader failure risks system downtime; backups improve reliability.

2. Step 2: Evaluate options for fault tolerance

   Using leader plus backups allows quick failover without multiple leaders conflicting.

3. Step 3: Reject unsafe or inefficient options

   Multiple leaders cause conflicts; no leader risks coordination issues; single leader alone is risky.

4. Final Answer:

   Use a leader and backup leaders that take over if leader fails -> Option A

5. Quick Check:

   Leader + backups = fault tolerance [OK]

Hint: Leader with backups prevents single point failure [OK]

Common Mistakes:

• Electing multiple leaders causing conflicts
• Relying on single leader without backups
• Skipping leader election causing chaos