Strong Eventual Consistency Fundamentals Quiz Quiz

Discover how Strong Eventual Consistency (SEC) ensures reliable data synchronization in distributed systems. This quiz covers SEC basics, its distinctions, guarantees, and real-life scenarios, ideal for learners keen on distributed data consistency concepts.

1. SEC Definition

   What does Strong Eventual Consistency (SEC) guarantee in distributed data systems?

   1. Nodes can hold different versions indefinitely
   2. All nodes will eventually reach the same state if no new updates are made
   3. All updates are instantly visible on all nodes
   4. Data is always consistent and locked during updates

   Explanation: SEC assures that in the absence of new updates, all replicated data copies converge to the same state. Instant visibility on all nodes is impossible due to network delays, so that's incorrect. ‘Nodes can hold different versions indefinitely’ contradicts SEC’s convergence guarantee. Locking data during updates is not part of SEC, which supports availability without global locks.

2. SEC vs. Eventual Consistency

   How does Strong Eventual Consistency differ from basic eventual consistency in a system with concurrent updates?

   1. SEC makes updates visible faster
   2. SEC prevents concurrent updates from happening
   3. SEC allows permanent divergence for conflicting updates
   4. SEC guarantees convergence regardless of update order

   Explanation: SEC ensures all replicas converge, even when updates happen concurrently and in any order. It does not prevent concurrent updates nor does it speed up update visibility. Allowing permanent divergence goes against the principle of SEC.

3. Update Delivery in SEC Systems

   In a distributed database using SEC, what must occur for replicas to reach the same state after a network partition?

   1. Replication only occurs within a single data center
   2. All updates are delivered to all replicas eventually
   3. Replicas must ignore conflicting updates
   4. Every update must be delivered instantly

   Explanation: SEC requires that every update eventually reaches all replicas, ensuring convergence over time. Instant delivery is not required or always possible. Ignoring conflicting updates or limiting replication to one data center goes against SEC’s aim of eventual global agreement.

4. Application Scenario

   Which of the following is a suitable scenario for using Strong Eventual Consistency?

   1. An online banking system where transactions must never conflict
   2. A ticket reservation system needing strict one-at-a-time seat updates
   3. A messaging app where messages may be read out of order but must not disappear
   4. A real-time multiplayer game where latency is unacceptable

   Explanation: A messaging app can tolerate temporary ordering issues and relies on all messages being eventually visible, fitting SEC well. Banking and ticketing require stronger consistency to prevent conflicts or double spending. Real-time games often need tighter latency or consistency than SEC provides.

5. Causal Consistency Comparison

   Compared with Strong Eventual Consistency, what extra guarantee does Causal Consistency provide?

   1. It makes all reads strongly consistent
   2. It preserves the order of causally related operations
   3. It removes the need for update delivery
   4. It relaxes convergence requirements

   Explanation: Causal consistency ensures operations with a cause-and-effect relationship are seen in the correct order, which SEC does not. Strongly consistent reads are stricter than both SEC and causal consistency. Causal consistency does not remove update delivery or relax convergence; it adds ordering guarantees.

6. SEC and Conflict Resolution

   How are conflicting updates handled in most SEC-based systems?

   1. They are resolved using deterministic conflict resolution rules
   2. Conflicting updates are always rejected automatically
   3. Conflicts are ignored, causing permanent divergence
   4. Users are manually prompted to resolve all conflicts

   Explanation: SEC relies on automatic, deterministic conflict resolution (for example, by merging updates or using rules), guaranteeing convergence. Ignoring conflicts would break SEC. Manual resolution and automatic rejection are not scalable or suitable for asynchronous systems using SEC.

7. State Convergence Example

   If two users make different changes to the same document at the same time in an SEC system, what is expected after all updates are delivered?

   1. Every replica will maintain its unique version forever
   2. One user's changes will be lost without exception
   3. The system will prevent any simultaneous edits
   4. All replicas will reflect the same resolved state

   Explanation: SEC guarantees that, despite conflicts and timing, all replicas will ultimately become identical through conflict resolution. Unique versions persisting would violate SEC. Automatically losing a user’s changes or blocking simultaneous edits are not part of SEC mechanisms.

8. Consistency Spectrum Placement

   Where does Strong Eventual Consistency fit within the data consistency spectrum?

   1. Stronger than eventual, weaker than strong consistency
   2. Weaker than basic eventual consistency
   3. Stronger than linearizability
   4. Equivalent to strong consistency

   Explanation: SEC sits between basic eventual consistency (which doesn’t always guarantee convergence) and strong consistency (which guarantees instant agreement and strict ordering). It is not equivalent to strong consistency or linearizability, and is not weaker than basic eventual.

9. SEC with Network Delays

   If a network delay causes a replica to receive updates late in an SEC system, what is the expected outcome?

   1. The replica will eventually synchronize and match others
   2. The system requires manual synchronization
   3. The replica will lose all late updates
   4. Permanent inconsistency will result

   Explanation: SEC ensures that, no matter how late updates arrive, all replicas will ultimately match after all updates are exchanged. Losing updates or permanent inconsistency would violate this guarantee. Automatic, not manual, reconciliation is part of SEC’s design.

10. Real-world Technology Analogy

    Which day-to-day tool best illustrates how SEC works in a simple analogy?

    1. A single shared hard drive with exclusive access
    2. A calculator storing values locally only
    3. Encrypted email where only recipients see the message
    4. Collaborative note-taking where every edit is eventually synced

    Explanation: Collaborative note-taking tools sync edits across users, even if some work offline, mimicking SEC’s eventual convergence. Encrypted email and local calculators do not involve multiple sources or convergence. A shared hard drive with exclusive access enforces immediate consistency, not SEC.