Data Sources
At the highest level, the Synapse REST API is supported from a single war file deployed to Amazon's Elastic Beanstalk call the repository.war. While the repository services reads data from many sources including RDS (MySQL), S3, CloudSearch, Dynamo, SQS, it will only write to RDS and S3.
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.