AWS - Dynamo DB Eventual Vs Strong Consistency

 1. Eventually Consistent Reads (Default)

🔹 Definition:

When you perform a read immediately after a write, you may not get the latest data right away — but it will become consistent after a short delay (typically a fraction of a second).

🔹 Characteristics:

• Low latency

• Higher throughput (uses fewer read capacity units)

• Suitable for most applications where data staleness of a few milliseconds is acceptable.

🔹 Use Case Example:

You’re displaying a product catalog on an e-commerce site where slight delay in updated inventory count is okay.

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2. Strongly Consistent Reads

🔹 Definition:

When you request a strongly consistent read, DynamoDB returns the most up-to-date value — including all successful writes before the read.

🔹 Characteristics:

• Higher latency than eventually consistent reads

• Lower throughput (uses more read capacity units)

• Guaranteed to return the latest data from a single region

🔹 Use Case Example:

Banking app displaying account balance immediately after a transaction.

🔐 Note: Strongly consistent reads are only supported in the same AWS region. Cross-region replication introduces eventual consistency.

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Additional Concepts Related to Consistency:

🔸 Transactions:

• DynamoDB supports ACID transactions, which ensure serializable isolation — the highest consistency level.

• You can group multiple reads/writes to succeed or fail as a unit.

🔸 DynamoDB Global Tables:

• Allow multi-region, active-active replication.

• Eventually consistent by design across regions.

• Great for high availability and low-latency access globally, but writes in one region may not be immediately visible in another.

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Summary Table:

Feature	Eventually Consistent	Strongly Consistent
Read Latency	Lower	Higher
Read Capacity Usage	0.5 units per KB	1 unit per KB
Data Freshness Guarantee	No	Yes (within region)
Availability	Higher	Slightly Lower
Use Case	Read-most apps, low risk of staleness	Critical reads needing up-to-date data