In computing, Extract, Transform and Load (ETL) refers to a process in database usage and especially in data warehousing that:

Since the data extraction takes time, it is common to execute the three phases in parallel. While the data is being pulled, another transformation process executes. It processes the already received data and prepares it for loading. As soon as there is some data ready to be loaded into the target, the data loading kicks off without waiting for the completion of the previous phases.