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HLDsystem_design~5 mins

Leader election in HLD - Cheat Sheet & Quick Revision

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Recall & Review
beginner
What is leader election in distributed systems?
Leader election is the process where nodes in a distributed system choose one node as the coordinator or leader to manage tasks and coordinate actions.
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beginner
Why is leader election important in distributed systems?
It ensures coordination, avoids conflicts, and helps manage shared resources by having a single node act as the leader.
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intermediate
Name a common algorithm used for leader election.
Bully algorithm and Ring algorithm are common leader election algorithms.
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intermediate
What happens if the leader node fails in a distributed system?
A new leader election process starts so that another node becomes the leader to maintain system coordination.
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advanced
How does the Bully algorithm select a leader?
Nodes with higher IDs bully lower ID nodes by sending election messages. The highest ID node becomes the leader.
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What is the main goal of leader election?
ATo increase network traffic
BTo delete nodes from the system
CTo select one node as coordinator
DTo slow down the system
Which algorithm uses node IDs to elect a leader by 'bullying' lower ID nodes?
ARing algorithm
BPaxos
CRaft
DBully algorithm
What triggers a new leader election in a distributed system?
AUser request
BLeader node failure
CSystem reboot
DAdding a new node
Which of these is NOT a property of a good leader election algorithm?
AHigh latency
BFault tolerance
CScalability
DFairness
In the Ring algorithm, how is the leader chosen?
ANode with highest ID in a ring passes messages
BRandom node selected
CNode with lowest ID in a ring passes messages
DCentral server picks leader
Explain the leader election process and why it is critical in distributed systems.
Think about how nodes agree on one leader and what happens if that leader stops working.
You got /3 concepts.
    Describe the Bully algorithm steps for electing a leader.
    Imagine nodes with numbers competing to be the boss by sending messages.
    You got /4 concepts.

      Practice

      (1/5)
      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