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SAA-C03 Practice Question 45: DynamoDB Global Tables Consistency

AWS Certified Solutions Architect – Associate (SAA-C03) · Test 5 · question 5 of 10

Database Resiliencyhard

A payments service will run active-active on a DynamoDB global table, serving writes from us-east-1 and eu-west-1. All replicas live in one AWS account. The requirement is strict: a write acknowledged in either Region must never be lost or silently overwritten by a concurrent write to the same item in the other Region.

Which design satisfies this requirement?

Select one answer
Show answer and explanation

Answer:

  • C. Create the global table with multi-Region strong consistency, adding a third Region as either a full replica or a witness, because MRSC requires exactly three Regions

Global tables offer two consistency modes, chosen when the table is created and fixed thereafter. The default, multi-Region eventual consistency (MREC), replicates asynchronously and resolves concurrent writes to the same item with last-writer-wins; one of two simultaneous writes is discarded, which the requirement forbids. Multi-Region strong consistency (MRSC) removes that outcome: a write is synchronously replicated to at least one other Region before it is acknowledged, and a strongly consistent read from any replica returns the latest item. MRSC must be deployed in exactly three Regions, either as three full replicas or as two replicas plus a *witness*, a third Region that stores replicated data to support the availability quorum but serves no reads or writes. So the two serving Regions stay as planned and a witness supplies the third. MRSC is also limited to same-account configurations, which this deployment already is.

Option A is wrong twice over. The default mode is eventually consistent, and its conflict resolution is last-writer-wins by timestamp, not "keep the highest value", so an acknowledged write can be silently discarded.

Option B improves matters without meeting the requirement. A conditional write is evaluated against the local replica, so two Regions can each satisfy their condition against stale local state; under MREC the losing write is then dropped during replication.

Option D avoids write conflicts by giving up the active-active design the service is being built for. Single-Region writes mean eu-west-1 cannot accept the traffic the premise requires.

Reference: How DynamoDB global tables work

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