Saturday, March 30, 2019

DynamoDB Read/Write Capacity

Read Request Units and Write Request Units

  • One read request unit represents one strongly consistent read request, or two eventually consistent read requests, for an item up to 4 KB in size. Transactional read requests require 2 read request units to perform one read for items up to 4 KB. If you need to read an item that is larger than 4 KB, DynamoDB needs additional read request units. The total number of read request units required depends on the item size, and whether you want an eventually consistent or strongly consistent read. For example, if your item size is 8 KB, you require 2 read request units to sustain one strongly consistent read, 1 read request unit if you choose eventually consistent reads, or 4 read request units for a transnational read request.
  • One write request unit represents one write for an item up to 1 KB in size. If you need to write an item that is larger than 1 KB, DynamoDB needs to consume additional write request units. Transactional write requests require 2 write request units to perform one write for items up to 1 KB. The total number of write request units required depends on the item size. For example, if your item size is 2 KB, you require 2 write request units to sustain one write request or 4 write request units for a transnational write request. 
    Read Consistency

    Eventually Consistent Reads
    When you read data from a DynamoDB table, the response might not reflect the results of a recently completed write operation. The response might include some stale data. If you repeat your read request after a short time, the response should return the latest data.
    Strongly Consistent Reads
    When you request a strongly consistent read, DynamoDB returns a response with the most up-to-date data, reflecting the updates from all prior write operations that were successful. A strongly consistent read might not be available if there is a network delay or outage. Consistent reads are not supported on global secondary indexes (GSI).

    DynamoDB uses eventually consistent reads, unless you specify otherwise. Read operations (such as GetItem, Query, and Scan) provide a ConsistentRead parameter. If you set this parameter to true, DynamoDB uses strongly consistent reads during the operation.