The ODS is optional – some organizations need them and other organizations do not. An ODS is a place where you can do online transaction processing against integrated data. The ODS is fed from ETL processing. There are three classes of ODS – type I, type II, and type III. Type II is the most common ODS. The ODS typically feeds data to the data warehouse. On occasions, the data warehouse can feed data to the ODS. When the data warehouse feeds data to the ODS, it is called “backflow.” The ODS can be updated from external sources of data.