Cross-region replication (Global Tables) in DynamoDB - Step-by-Step Execution

Concept Flow - Cross-region replication (Global Tables)

Create Global Table

↓

Write Data in Region A

↓

Replication Service Detects Change

↓

Replicate Data to Region B

↓

Data Available in Region B

↓

Read/Write in Any Region

↓

Conflict Resolution if Needed

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Consistent Data Across Regions

Data written in one region is automatically copied to other regions, allowing reads and writes anywhere with conflict resolution to keep data consistent.

Execution Sample

DynamoDB

1. Create Global Table with Region A and B
2. Write item {id:1, name:'Alice'} in Region A
3. Replication service copies item to Region B
4. Read item from Region B returns {id:1, name:'Alice'}

Shows how writing data in one region replicates it to another region automatically.

Execution Table

Step	Action	Region A Data	Region B Data	Replication Status	Notes
1	Create Global Table	{}	{}	Not started	Table created in both regions, empty data
2	Write item {id:1, name:'Alice'} in Region A	{id:1, name:'Alice'}	{}	Pending	Data written only in Region A
3	Replication service detects change	{id:1, name:'Alice'}	{}	In progress	Change detected, replication started
4	Replicate data to Region B	{id:1, name:'Alice'}	{id:1, name:'Alice'}	Completed	Data replicated to Region B
5	Read item from Region B	{id:1, name:'Alice'}	{id:1, name:'Alice'}	Completed	Data available and consistent in Region B
6	Write item {id:1, name:'Alice Smith'} in Region B	{id:1, name:'Alice'}	{id:1, name:'Alice Smith'}	Pending	Update made in Region B
7	Replication service detects change	{id:1, name:'Alice'}	{id:1, name:'Alice Smith'}	In progress	Change detected, replicating back to Region A
8	Conflict resolution if needed	{id:1, name:'Alice Smith'}	{id:1, name:'Alice Smith'}	Completed	Conflict resolved, data consistent
9	Final data in both regions	{id:1, name:'Alice Smith'}	{id:1, name:'Alice Smith'}	Completed	Data synchronized across regions

💡 Replication completes and data is consistent across all regions

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	After Step 8	Final
Region A Data	{}	{id:1, name:'Alice'}	{id:1, name:'Alice'}	{id:1, name:'Alice'}	{id:1, name:'Alice Smith'}	{id:1, name:'Alice Smith'}
Region B Data	{}	{}	{id:1, name:'Alice'}	{id:1, name:'Alice Smith'}	{id:1, name:'Alice Smith'}	{id:1, name:'Alice Smith'}
Replication Status	Not started	Pending	Completed	Pending	Completed	Completed

Key Moments - 3 Insights

Why does Region B data remain empty immediately after writing in Region A (Step 2)?

Because replication happens asynchronously, the data is first written only in Region A. The replication service detects the change next (Step 3) before copying it to Region B (Step 4).

How is data consistency maintained when updates happen in both regions?

The replication service detects changes in each region and uses conflict resolution rules to synchronize data, as shown in Steps 6 to 8 where an update in Region B is replicated back to Region A and conflicts are resolved.

What happens if replication is delayed or interrupted?

Data in regions may temporarily differ, but the replication service retries until data is consistent, ensuring eventual consistency as seen by the 'Replication Status' moving from 'Pending' to 'Completed'.

Visual Quiz - 3 Questions

Look at the execution_table at Step 4. What is the replication status?

A Pending

B Completed

C Not started

D In progress

💡 Hint

Check the 'Replication Status' column at Step 4 in the execution_table.

At which step does Region B first have the item {id:1, name:'Alice'}?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Region B Data' column in the execution_table to see when data appears.

If the replication service failed after Step 3, what would be the Region B data at Step 5?

A {} (empty)

B {id:1, name:'Alice'}

C {id:1, name:'Alice Smith'}

D Replication status would be 'Completed'

💡 Hint

Refer to the replication status and Region B data columns in the execution_table for Step 5.

Concept Snapshot

Cross-region replication (Global Tables) in DynamoDB:
- Create a global table spanning multiple regions.
- Writes in any region replicate asynchronously to others.
- Reads can be served locally from any region.
- Conflict resolution ensures data consistency.
- Enables low-latency global access and disaster recovery.

Full Transcript

Cross-region replication with DynamoDB Global Tables means when you write data in one region, it automatically copies to other regions. This lets you read and write data anywhere with low delay. The replication service detects changes and copies them asynchronously. If updates happen in multiple regions, conflict resolution keeps data consistent. The execution table shows steps from creating the table, writing data in Region A, replicating to Region B, and updating data in Region B with replication back to Region A. Variables track data states in each region and replication status. Key moments clarify why data appears later in Region B and how conflicts are resolved. The quiz tests understanding of replication timing and data consistency. This helps build globally available, fault-tolerant applications.

Practice

1. What is the main purpose of DynamoDB Global Tables in cross-region replication?

easy

A. To keep data copied and synchronized across multiple AWS regions

B. To create backups of tables in the same region

C. To increase the size limit of a single DynamoDB table

D. To encrypt data only in one region

Solution

1. Step 1: Understand Global Tables concept

   Global Tables replicate data automatically across multiple AWS regions to keep them in sync.

2. Step 2: Identify the main purpose

   This replication helps applications stay fast and available worldwide by having data close to users.

3. Final Answer:

   To keep data copied and synchronized across multiple AWS regions -> Option A

4. Quick Check:

   Global Tables = cross-region data sync [OK]

Hint: Global Tables = data sync across regions [OK]

Common Mistakes:

• Confusing backups with replication
• Thinking Global Tables increase table size
• Assuming encryption is the main feature

2. Which AWS CLI command is used to create a DynamoDB Global Table?

easy

A. aws dynamodb delete-global-table --table-name MyTable

B. aws dynamodb create-table --table-name MyTable --region us-east-1

C. aws dynamodb create-global-table --global-table-name MyTable --replication-group RegionName=us-east-1

D. aws dynamodb update-global-table --table-name MyTable

Solution

1. Step 1: Identify the correct command for creating Global Tables

   The command to create a Global Table is create-global-table with replication group specifying regions.

2. Step 2: Check options

   aws dynamodb create-global-table --global-table-name MyTable --replication-group RegionName=us-east-1 uses create-global-table with replication group, which is correct. Others are for creating normal tables, updating, or deleting.

3. Final Answer:

   aws dynamodb create-global-table --global-table-name MyTable --replication-group RegionName=us-east-1 -> Option C

4. Quick Check:

   Create Global Table = create-global-table command [OK]

Hint: Use create-global-table to make Global Tables [OK]

Common Mistakes:

• Using create-table instead of create-global-table
• Confusing update or delete commands
• Missing replication group parameter

3. Given a DynamoDB Global Table replicated in us-east-1 and eu-west-1, what happens if you write an item in us-east-1?

medium

A. The item is immediately available in eu-west-1 after replication

B. The item is only stored in us-east-1 and not replicated

C. The write fails because cross-region writes are not allowed

D. The item is stored in eu-west-1 but not in us-east-1

Solution

1. Step 1: Understand replication behavior of Global Tables

   Global Tables replicate writes made in one region to all other regions automatically and quickly.

2. Step 2: Apply to the scenario

   Writing an item in us-east-1 will replicate it to eu-west-1, making it available there.

3. Final Answer:

   The item is immediately available in eu-west-1 after replication -> Option A

4. Quick Check:

   Write in one region = replicate to others [OK]

Hint: Writes replicate automatically across regions [OK]

Common Mistakes:

• Thinking writes stay local only
• Assuming writes fail across regions
• Confusing replication direction

4. You tried to add a new region to an existing Global Table but got an error. What is a common cause?

medium

A. You forgot to enable encryption on the new region

B. The new region's table has a different primary key schema

C. Global Tables do not support more than two regions

D. You used the update-table command instead of update-global-table

Solution

1. Step 1: Check schema requirements for Global Tables

   All replicas in Global Tables must have the same primary key schema to replicate correctly.

2. Step 2: Identify cause of error

   If the new region's table has a different primary key schema, adding it will fail.

3. Final Answer:

   The new region's table has a different primary key schema -> Option B

4. Quick Check:

   Matching primary keys required for Global Tables [OK]

Hint: All regions must share the same primary key schema [OK]

Common Mistakes:

• Assuming encryption causes add-region errors
• Believing Global Tables limit regions to two
• Confusing update-table with update-global-table commands

5. You want to design a Global Table for a worldwide app with users in Asia, Europe, and America. Which strategy best reduces latency and ensures data consistency?

hard

A. Create replicas in Asia, Europe, and America with strongly consistent reads enabled

B. Create a single table in one region and route all traffic there

C. Create replicas only in Asia and America to reduce costs

D. Create replicas in Asia, Europe, and America regions with eventual consistency reads

Solution

1. Step 1: Consider latency and availability

   Replicas in all user regions reduce latency by serving data locally.

2. Step 2: Understand consistency trade-offs

   Strongly consistent reads across regions are not supported; eventual consistency is used to balance latency and consistency.

3. Step 3: Evaluate options

   Create replicas in Asia, Europe, and America regions with eventual consistency reads creates replicas in all needed regions with eventual consistency, which is best for performance and availability.

4. Final Answer:

   Create replicas in Asia, Europe, and America regions with eventual consistency reads -> Option D

5. Quick Check:

   Global Tables + eventual consistency = low latency worldwide [OK]

Hint: Use replicas in all regions with eventual consistency [OK]

Common Mistakes:

• Expecting strong consistency across regions
• Using single region causing high latency
• Skipping regions to save cost but hurting users