Explain Sequence Numbers vs Version Vectors.
Sequence numbers provide a global order for operations across replicas, while version vectors track causal relationships between updates to detect conflicts in distributed or replicated databases.
When to Use
Use sequence numbers when a single leader or coordinator can assign monotonically increasing numbers — e.g., in leader-based replication or message queues. Use version vectors in multi-master systems (like DynamoDB or Cassandra) to detect concurrent updates without a single point of ordering.
Example
In a message queue, each message gets a sequence number for ordered delivery.
In a distributed key-value store, each node maintains a version vector to identify and resolve conflicting writes.
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Why Is It Important
Choosing between them impacts consistency, conflict resolution, and scalability in distributed databases. Sequence numbers simplify ordering; version vectors preserve causality in concurrent systems.
Interview Tips
Explain the difference between total order (sequence numbers) and causal order (version vectors). Use real systems like Kafka vs DynamoDB to illustrate.
Trade-offs
Sequence numbers: +Simple ordering, –Need coordination. Version vectors: +Concurrency detection, –Metadata grows with replicas.
Pitfalls
- Assuming sequence numbers detect conflicts—they only ensure order.
- Ignoring vector growth or misinterpreting causality in large clusters.
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