AWS Database Services: DynamoDB Global Tables

For the SAA-C03 exam, understanding Amazon DynamoDB Global Tables is crucial for scenarios requiring multi-region availability and low-latency access for globally distributed users. It is the premier “Active-Active” multi-region database solution in the AWS ecosystem.

Core Concepts & Architecture

Global Tables build upon the foundation of standard DynamoDB tables. They provide a fully managed, multi-region, and multi-active database. When you write data to a table in one region, DynamoDB automatically replicates that data to all other participant regions.

Replication Mechanism

• DynamoDB Streams: Global Tables rely on DynamoDB Streams. You must enable “New and Old Images” for the stream to allow replication across regions.
• Latency: Replication typically completes within a second, though it is “eventually consistent” across regions.
• Conflict Resolution: DynamoDB uses a “Last Writer Wins” (LWW) mechanism based on the timestamp of the write operation to resolve concurrent updates to the same item.

Comparison: Global Tables vs. Aurora Global Database

Decision Matrix: If-Then Guide

• If you need sub-millisecond local latency for users in multiple continents, Then use Global Tables.
• If your application requires 99.999% availability (five nines), Then Global Tables is the recommended choice.
• If you need to write to multiple regions simultaneously, Then choose Global Tables (Aurora Global only allows writes in one region).
• If you must use complex SQL joins across regions, Then Global Tables is NOT the right choice; stick to Aurora.

Exam Tips and Gotchas

• Streams Requirement: You cannot enable Global Tables without first enabling DynamoDB Streams.
• Consistency: While local reads can be Strongly Consistent, cross-region replication is always Eventually Consistent.
• WCU/RCU Management: You must ensure that the Write Capacity Units (WCU) and Read Capacity Units (RCU) are configured identically across all regions to avoid throttling during replication.
• Last Writer Wins: Be aware that if two users update the same record in different regions at the exact same time, the one with the later timestamp persists.

Topics covered:

Summary of key subtopics covered in this guide:

• Multi-region Active-Active architecture
• DynamoDB Streams dependency
• Conflict resolution (Last Writer Wins)
• Consistency models (Local vs. Global)
• Comparison with Aurora Global Database
• Capacity management across regions