what is Real-Time Data Warehouse?

A real-time data warehouse is a data warehouse that is updated (by the ETL) the
moment the transaction happens in the source system.
For example, you can put triggers on the sales transaction table in the source system so that whenever there is a transaction inserted into the database, the trigger fires and sends the new record to the data warehouse as a message. The data warehouse has an active listenerthat captures the message the moment it arrives, cleanses it, DQs it, transforms it, and inserts it into the fact table immediately. I’m talking about a two-second time difference here, between the moment a customer purchased a product on the web site and the moment data is available in the fact table.