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Multi-Region Strong Consistency for Amazon DynamoDB Global Tables
Overview
- Amazon DynamoDB's Global Tables feature provides multi-region, multi-active replication, allowing data to be replicated across multiple AWS regions.
- The new "Multi-Region Strong Consistency" feature, currently in preview, allows building globally distributed applications with a recovery point objective (RPO) of zero.
DynamoDB Consistency Models
Single-Region Tables
- In a single-region table, updates made using the
UpdateItem
API are eventually consistent by default.
- To ensure strong consistency, the
ConsistentRead
parameter can be set to true
for GetItem
calls.
Global Tables (Eventual Consistency)
- In a Global Table, replicas are eventually consistent across regions.
- Strongly consistent reads in one region may return stale data if an update was made in another region.
- Conflict resolution uses a "last writer wins" approach based on timestamp.
Global Tables (Multi-Region Strong Consistency)
- With multi-region strong consistency, replication between regions is strongly consistent.
- Strongly consistent reads in any region will always return the latest data, regardless of where the update was made.
- Conflict resolution is handled differently - if two regions try to update the same item concurrently, a "replicated write conflict" exception is thrown, which can be retried by the application.
- This ensures an RPO of zero, as the data is always available and consistent across regions.
Use Cases
- Multi-region strong consistency is particularly useful for applications that cannot tolerate any data loss, such as financial transactions.
- It also allows getting data closer to users while still maintaining strong consistency, although with higher read and write latencies compared to eventual consistency.
Preview Limitations and Considerations
- Multi-region strong consistency is currently in public preview and not recommended for production use.
- The preview supports only three regions: US-East-1, US-East-2, and US-West-2.
- Replicas within a Global Table must all use the same consistency mode (strong or eventual).
Demo
- The presenter demonstrates the differences between eventually consistent and multi-region strongly consistent Global Tables, showing how concurrent writes and strongly consistent reads behave in each case.
Replication Internals
- DynamoDB uses a "write-ahead log" approach for regional replication within a single DynamoDB table.
- For Global Tables, each region has a replication engine that asynchronously replicates changes to the other regions.
- Multi-region strong consistency uses a "multi-region journal" to serialize writes across regions and ensure strong consistency.
Failure Scenarios
- In case of regional failures or network partitions, eventually consistent Global Tables remain highly available, but strongly consistent reads may return stale data.
- Multi-region strongly consistent Global Tables maintain high availability for both reads and writes, even in failure scenarios, by leveraging the multi-region journal.
Confidence in the System
- The presenters used formal modeling, chaos testing, and integration with AWS Fault Injection Service to validate the correctness and behavior of the multi-region strong consistency feature.
- They also discussed the importance of anti-entropy processes to ensure replica convergence.
Conclusion
- Eventually consistent and multi-region strongly consistent Global Tables each have their own trade-offs in terms of latency, consistency, and availability.
- Customers should evaluate their application requirements and choose the appropriate Global Table consistency mode accordingly.