DynamoDB Consistency

• AWS has a Region, which is a physical location around the world where we cluster data centers, with one or more Availability Zones which are discrete data centers with redundant power, networking, and connectivity in an AWS Region.
• Amazon automatically stores each DynamoDB table in the three geographically distributed locations or AZs for durability.
• DynamoDB consistency represents the manner and timing in which the successful write or update of a data item is reflected in a subsequent read operation of that same item.

DynamoDB Consistency Modes

Eventually Consistent Reads (Default)

• Eventual consistency option maximizes the read throughput.
• Consistency across all copies is usually reached within a second
• However, an eventually consistent read might not reflect the results of a recently completed write.
• Repeating a read after a short time should return the updated data.
• DynamoDB uses eventually consistent reads, by default.

Strongly Consistent Reads

• Strongly consistent read returns a result that reflects all writes that received a successful response prior to the read
• Strongly consistent reads are 2x the cost of Eventually consistent reads
• Strongly Consistent Reads come with disadvantages
  • A strongly consistent read might not be available if there is a network delay or outage. In this case, DynamoDB may return a server error (HTTP 500).
  • Strongly consistent reads may have higher latency than eventually consistent reads.
  • Strongly consistent reads are not supported on global secondary indexes.
  • Strongly consistent reads use more throughput capacity than eventually consistent reads.

• DynamoDB allows the user to specify whether the read should be eventually consistent or strongly consistent at the time of the request
• Read operations (such as GetItem, Query, and Scan) provide a ConsistentRead parameter, if set to true, DynamoDB uses strongly consistent reads during the operation.
• Query, GetItem, and BatchGetItem operations perform eventually consistent reads by default.
  • Query and GetItem operations can be forced to be strongly consistent
  • Query operations cannot perform strongly consistent reads on Global Secondary Indexes
  • BatchGetItem operations can be forced to be strongly consistent on a per-table basis

AWS Certification Exam Practice Questions

• Questions are collected from Internet and the answers are marked as per my knowledge and understanding (which might differ with yours).
• AWS services are updated everyday and both the answers and questions might be outdated soon, so research accordingly.
• AWS exam questions are not updated to keep up the pace with AWS updates, so even if the underlying feature has changed the question might not be updated
• Open to further feedback, discussion and correction.

1. Which of the following statements is true about DynamoDB?
   1. Requests are eventually consistent unless otherwise specified.
   2. Requests are strongly consistent.
   3. Tables do not contain primary keys.
   4. None of the above
2. How is provisioned throughput affected by the chosen consistency model when reading data from a DynamoDB table?
   1. Strongly consistent reads use the same amount of throughput as eventually consistent reads
   2. Strongly consistent reads use variable throughput depending on read activity
   3. Strongly consistent reads use more throughput than eventually consistent reads.
   4. Strongly consistent reads use less throughput than eventually consistent reads

References

DynamoDB_Read_Consistency