You have spend the last set of exercises firmly founded on the Target System. Working in the Target Map, you exposed an interface of Business Objects and their Interface Fields, and - based on this interface - you have specified the mapping necessary to produce all the data to be delivered to the Target System. When you worked in the Target Map you imported the metadata describing the structures in the Target System and you built the Business Object hierarchies by hand.
Now you are leaving the Target map for a little while, the only thing you bring with you to the Source Map is the interface defined in the Target Map. In fact, the entire point of the Source Map is to produce the data that your Target Map needs - as described in the Target interface. Have a look at this video placing the Source Map in the big picture:
So on one hand in the Source Map you are going to import the Target interface you have just published from the Target Map. And on the other hand you are going to import the metadata describing the structures in the Source System. In fact, let us draw a line back to the figure presented in first exercise introducing the Target Map. In the figure below, the Source side of things (on the left) are added to the previous figure (on the right).
Regarding the Source System metadata it is worth the while to look a little ahead and into how the things are going to work down the line.
The Source Map you are working on in Studio will produce the input to generate the Source Engine, that in turn will do this part of the job when the migration is running. And here's the point: As part of doing the job, the generated Source Engine will actually create and maintain a staging database with a table for each structure in the Source System metadata and load the data received from the Source System into these staging tables.
Once data is loaded, the Source Engine will then perform the Export part of the migration by applying the Source Map to these staging tables in order to produce the Interface Data that conforms to the Target interface. And then the generated Target Engine will take it from there
It is beneficial to keep the above insight in mind when you start working in the Source Map. You will in fact be specifying how to extract the data from the staging tables in order to deliver the data conforming to the Target interface published by the Target Map. As an extra little twist, the Source Map enables you to create Views. A view is nothing more than extracting data from one or more staging tables and place these data in yet another staging table.
Ok, let's get on with it! All that remains is to define how to get from the metadata structures in the Source System to the Taget interface published by the Target Map. Piece of cake!