Eventually consistent reads is a data consistency model used by Amazon DynamoDB that prioritizes high availability and performance over immediate data accuracy. When you perform an eventually consistent read, DynamoDB returns data from any available replica, which may not reflect the most recent wr…Eventually consistent reads is a data consistency model used by Amazon DynamoDB that prioritizes high availability and performance over immediate data accuracy. When you perform an eventually consistent read, DynamoDB returns data from any available replica, which may not reflect the most recent write operation.
In DynamoDB, data is replicated across multiple Availability Zones for durability and high availability. When a write occurs, DynamoDB acknowledges the write after it has been stored on at least two Availability Zones. However, propagating this change to all replicas takes a small amount of time, typically within one second.
During this brief propagation window, if you perform an eventually consistent read, you might receive data that does not include the latest modifications. This is perfectly acceptable for many use cases where reading slightly stale data does not impact application functionality.
Key characteristics of eventually consistent reads include:
1. **Higher throughput**: Eventually consistent reads consume half the read capacity units compared to strongly consistent reads, making them more cost-effective.
2. **Lower latency**: Since DynamoDB can return data from any replica, response times are typically faster.
3. **Default behavior**: Eventually consistent reads are the default read consistency model in DynamoDB operations like GetItem, Query, and Scan.
When to use eventually consistent reads:
- Displaying product catalogs or content that updates infrequently
- Analytics and reporting where real-time accuracy is not critical
- Caching scenarios where slight delays are acceptable
- High-traffic applications prioritizing performance
For scenarios requiring the most up-to-date data, such as financial transactions or inventory management, you should use strongly consistent reads by setting the ConsistentRead parameter to true. This ensures you receive data reflecting all successful write operations prior to the read, though at twice the read capacity cost.
Understanding when to apply each consistency model helps optimize both application performance and AWS costs.
Eventually Consistent Reads in AWS
What is Eventually Consistent Reads?
Eventually consistent reads is a read consistency model used by AWS services, most notably Amazon DynamoDB and Amazon S3. When you perform an eventually consistent read, the response might not reflect the results of a recently completed write operation. The data will become consistent across all storage locations, typically within one second, but there is no guarantee that your read will return the most up-to-date data.
Why is Eventually Consistent Reads Important?
Understanding eventually consistent reads is crucial for several reasons:
• Cost Optimization: Eventually consistent reads in DynamoDB consume half the read capacity units compared to strongly consistent reads, making them more cost-effective.
• Higher Throughput: Applications can achieve better performance and handle more requests when using eventually consistent reads.
• Distributed Systems Design: AWS services are distributed across multiple Availability Zones, and eventual consistency allows for better availability and partition tolerance.
• Application Architecture Decisions: Choosing the right consistency model affects how you design your application logic.
How Does Eventually Consistent Reads Work?
When data is written to a distributed system like DynamoDB:
1. The write is acknowledged after being stored on the primary node 2. The data is then replicated asynchronously to other nodes across Availability Zones 3. During this replication window (usually under one second), reads from replica nodes may return stale data 4. Once replication completes, all nodes return consistent data
In DynamoDB specifically: • Eventually consistent reads are the default behavior for GetItem, Query, and Scan operations • They consume 0.5 RCU per 4KB of data (compared to 1 RCU for strongly consistent reads)
In Amazon S3: • S3 now provides strong read-after-write consistency for all operations as of December 2020 • Previously, S3 offered eventual consistency for overwrite PUTs and DELETEs
When to Use Eventually Consistent Reads
• When your application can tolerate slightly stale data • For read-heavy workloads where cost optimization is important • When displaying data that does not require real-time accuracy (dashboards, analytics) • For applications where high availability is more important than immediate consistency
When NOT to Use Eventually Consistent Reads
• Financial transactions requiring accurate balance information • Inventory systems where overselling must be prevented • Any scenario where reading stale data could cause business logic errors • Leader election or distributed locking mechanisms
Exam Tips: Answering Questions on Eventually Consistent Reads
• Remember the default: DynamoDB uses eventually consistent reads by default. If the question does not specify consistency requirements, assume eventual consistency.
• Know the RCU calculation: Eventually consistent reads use half the RCUs of strongly consistent reads. For 4KB items: 0.5 RCU for eventually consistent, 1 RCU for strongly consistent.
• Look for keywords: Questions mentioning 'cost reduction', 'optimize read capacity', or 'high throughput' often point toward eventually consistent reads as the answer.
• Identify use cases: If the scenario describes analytics, reporting, or situations where slight delays in data freshness are acceptable, eventually consistent reads are appropriate.
• Watch for traps: If a question mentions a user seeing outdated data after a write, the cause is likely eventually consistent reads being used when strongly consistent reads are needed.
• S3 Update: Remember that S3 now provides strong consistency. Questions about S3 eventual consistency may be outdated or testing historical knowledge.
• ConsistentRead parameter: To enable strongly consistent reads in DynamoDB API calls, set ConsistentRead to true. The absence of this parameter means eventual consistency.
• Global Tables consideration: DynamoDB Global Tables only support eventually consistent reads for cross-region replicas.