The data transformation process will vary from data warehouse to data warehouse, and it is possible that you will have no transformation at all to perform, If data transformation is required it will be necessary to complete the transformation before the data can be loaded, even though some of the transformation may be performed by the load process itself. This is another serialization; which cannot be proceeded with until the transformation is complete. But, consideration needs to be given to the design of the process, to allow some data to be transformed and made read for load, even if other data is delayed.