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Any REST call that writes data will always be in a single database transaction. This includes writes where data is stored in S3. For such cases, data is first written to S3 using a key containing a UUID. The key is then stored in RDS as part of the single transaction. This means any S3 data or RDS data will always have read/write consistency. All other data sources will have eventual consistency.
Asynchronous processing
All write to data sources other than RDS and S3 are done with an asynchronous messages processing system making data in these sources eventually consistent. The following sequence diagram shows how this message system starts:
In the Above example, a create entity call is made, resulting in the start of a database transaction. The entity is then inserted into RDS. A change message containing meta data about this new entity (including ID and Etag) is also inserted into RDS as part of this initial transaction. The change message object is also bound to the transaction in an in-memory queue. When this initial transaction is committed, a transaction listener is notified, and all messages bound to that transaction are sent to an SNS Topic. The topic servers as a message syndication, and pushes the message to a queue for each process type.