What is the REST API Transaction/Commit behaviour?
By default, each record is treated as a separate transaction and committed separately. Failure of one record change doesn’t cause a rollback of other record changes.
This behaviour can be altered to an “all or nothing” behaviour. Use the REST API composite resources to make a series of updates in one API call.
What is the recommended data set size for Rest API?
For bulk data operations (more than 500,000 records), use the REST-based Bulk API.