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hardSystem Design

What is the difference between strong and eventual consistency?

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Compare consistency models.

consistency
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Strong vs. Eventual Consistency: What Is the Difference?

Target Audience: Junior & Senior Software Engineers preparing for System Design Interviews — no prior system design knowledge assumed. Difficulty: Medium

How to read this doc: Every concept is explained in plain language first. Right after, you'

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Strong vs eventual consistency: replica read simulation
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class ReplicatedStore {
  constructor(initialValue) {
    this.primaryValue = initialValue;
    this.replicaLog = {
      replica1: [[0, initialValue]],
      replica2: [[0, initialValue]],
    };
  }

  write(value, replicaArrivalTimes) {
    this.primaryValue = value;
    for (const [replicaId, arrivalTime] of Object.entries(replicaArrivalTimes)) {
      this.replicaLog[replicaId].push([arrivalTime, value]);
    }
  }

  strongRead(now) {
    return this.primaryValue;
  }

  eventualRead(replicaId, now) {
    let latest = this.replicaLog[replicaId][0][1];
    for (const [arrivalTime, value] of this.replicaLog[replicaId]) {
      if (arrivalTime <= now) latest = value;
    }
    return latest;
  }
}

const store = new ReplicatedStore('v0');
store.write('v1', { replica1: 5, replica2: 10 });

console.log('strong_read(t=1):', store.strongRead(1));
console.log('strong_read(t=100):', store.strongRead(100));
console.log('eventual_read(replica1, t=3):', store.eventualRead('replica1', 3));
console.log('eventual_read(replica1, t=6):', store.eventualRead('replica1', 6));
console.log('eventual_read(replica2, t=6):', store.eventualRead('replica2', 6));
console.log('eventual_read(replica2, t=11):', store.eventualRead('replica2', 11));
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