Eventual consistency allows replicas or derived views to temporarily return older data after a write. Later reads converge when updates propagate. section 12.4.6
DynamoDB applies the model in several practical ways:
- Returns an older item version after an update when a different replica answers the request. section 12.4.6
- Updates global secondary indexes asynchronously, so index queries are always eventually consistent. section 12.4.3 section 12.4.6
- Uses ConsistentRead: true with getItem, query, or scan when a strongly consistent read is required; this increases latency, cost, and read-capacity use. section 12.4.6
Retrying an eventually consistent external update
A fault-tolerant application retries a social-media status update, but an immediate lookup may not reveal the first post. It must choose whether to tolerate a missing update or duplicate updates. section 16.2.2
Figure 12.5 Eventually consistent reads can return old values after a write operation until the change is propagated to all machines.
Shows how reads can return old values after a write until the change reaches all machines. section 12.4.6
