Strongly consistent reads in Amazon DynamoDB ensure that you receive the most up-to-date data reflecting all successful write operations that occurred prior to the read request. This is a critical concept for AWS Certified Developer - Associate exam candidates to understand when working with Dynamo…Strongly consistent reads in Amazon DynamoDB ensure that you receive the most up-to-date data reflecting all successful write operations that occurred prior to the read request. This is a critical concept for AWS Certified Developer - Associate exam candidates to understand when working with DynamoDB.
By default, DynamoDB uses eventually consistent reads, which may not reflect the results of a recently completed write operation. However, when your application requires the latest data with guaranteed accuracy, strongly consistent reads become essential.
When you perform a strongly consistent read, DynamoDB returns a response with the most current data, reflecting updates from all prior write operations that were successful. This is particularly important for applications where data accuracy is paramount, such as financial transactions, inventory management systems, or any scenario where stale data could cause issues.
To request a strongly consistent read, you set the ConsistentRead parameter to true in your GetItem, Query, or Scan API calls. For example, when using the AWS SDK, you would include this parameter in your request configuration.
There are important trade-offs to consider. Strongly consistent reads consume twice the read capacity units compared to eventually consistent reads. They also have higher latency and are only available within a single AWS Region. Additionally, strongly consistent reads are not supported on global secondary indexes.
From a practical standpoint, you should use strongly consistent reads when your application cannot tolerate reading outdated information. For read-heavy workloads where slight delays in data consistency are acceptable, eventually consistent reads provide better performance and cost efficiency.
For the AWS Developer Associate exam, remember that strongly consistent reads guarantee the latest data, require double the read capacity units, work only in the same Region, and must be explicitly requested through the ConsistentRead parameter in your API calls.
Strongly Consistent Reads in AWS DynamoDB
What are Strongly Consistent Reads?
Strongly consistent reads in AWS DynamoDB are read operations that return the most up-to-date data, reflecting all writes that received a successful response prior to the read. When you perform a strongly consistent read, DynamoDB returns data from the leader node of the partition, ensuring you always get the latest version of your data.
Why are Strongly Consistent Reads Important?
Understanding strongly consistent reads is crucial for the AWS Developer Associate exam because:
• Data Accuracy: Some applications require absolute certainty that they are reading the most current data, such as financial transactions, inventory management, or any system where stale data could cause problems.
• Trade-offs: Strongly consistent reads come with performance and cost implications that developers must understand when designing applications.
• Use Case Selection: Knowing when to use strongly consistent reads versus eventually consistent reads is a key architectural decision.
How Strongly Consistent Reads Work
DynamoDB replicates data across multiple Availability Zones. Here is how the two read types differ:
Eventually Consistent Reads (Default): • Data is read from any available replica • May not reflect the most recent write • Consistency is typically achieved within one second • Uses half the read capacity units (RCUs)
Strongly Consistent Reads: • Data is read from the leader replica • Always returns the most recent data • Consumes twice the read capacity units compared to eventually consistent reads • May have higher latency • Not supported on Global Secondary Indexes (GSIs)
How to Request Strongly Consistent Reads
To perform a strongly consistent read, set the ConsistentRead parameter to true in your API call:
• GetItem: Set ConsistentRead to true • Query: Set ConsistentRead to true • Scan: Set ConsistentRead to true • BatchGetItem: Set ConsistentRead to true for each table
Capacity Consumption
• One strongly consistent read of an item up to 4 KB consumes 1 RCU • One eventually consistent read of an item up to 4 KB consumes 0.5 RCU • Items larger than 4 KB require additional RCUs, rounded up to the nearest 4 KB
Exam Tips: Answering Questions on Strongly Consistent Reads
1. Default Behavior: Remember that DynamoDB uses eventually consistent reads by default. If a question does not mention consistency requirements, assume eventually consistent.
2. Cost Implications: Strongly consistent reads cost twice as much in terms of RCUs. Questions about optimizing costs often involve switching to eventually consistent reads where appropriate.
3. GSI Limitation: Global Secondary Indexes do NOT support strongly consistent reads. If a question asks about strongly consistent reads on a GSI, the answer involves querying the base table instead.
4. LSI Support: Local Secondary Indexes DO support strongly consistent reads.
5. Use Case Identification: When questions describe scenarios requiring the absolute latest data (banking, inventory counts, real-time dashboards), strongly consistent reads are likely the correct answer.
6. Latency Consideration: Strongly consistent reads may have slightly higher latency because they must communicate with the leader node.
7. API Parameter: Know that ConsistentRead is the parameter name used to enable strongly consistent reads.
8. Calculation Questions: For RCU calculations, always check if the question specifies strongly or eventually consistent reads, as this affects the math significantly.
Common Exam Scenarios
• An application needs to read the most current data after a write - use strongly consistent reads • An application needs to minimize read costs - use eventually consistent reads • Reading from a GSI requires the latest data - query the base table with strongly consistent reads instead • Calculating provisioned capacity - remember to double RCUs for strongly consistent reads