I'm building an offline-first mobile application using AWS Amplify, using the local DataStore and cloud sync. So far, I'm following the documentation without any variation (I think.)
As of now, I only have one model, lets call it at Thing. I noticed that after running amplify push, my environment contains not one, but two DynamoDB tables:
Thing-<app-id>-<env>AmplifyDataStore-<app-id>-<env>Whenever I save a Thing entity, it appears to be persisted redundantly in both tables. This effectively doubles my DynamoDB storage costs.
Is there a sound technical reason for this, or any way to avoid it? Or am I just making a mistake somewhere that is causing it to persist twice?
Assuming you have k models, then the Amplify DataStore will provision k + 1 tables. The extra table you're noticing is called the "delta sync table." It used to store incremental changes that have occurred since the last time the client synchronized fully with AppSync. The Delta Sync table carries a short TTL on the records, and they will get dropped if not utilized within that window of time.
To learn more about Delta Sync and DataStore generally, I recommend Ed Lima's AWS AppSync offline reference architecture – powered by the Amplify DataStore. See particularly the section labeled "The Delta Sync table."
Source: I'm an engineer on this product team.