The logic is actually quite simple. Once 2 cells are connected via a line, their values are kept in-sync. If one cell changes the value, the other cell will be informed of the change, and the module will react accordingly, very much like a reactive spreadsheet.

During the initial connection, we have to “pick a winner” and determine which of the 2 cells’ values to use. The logic is that the empty cell will be overridden. If both cells have values, the direction of how you connect them determines the winner. Dragging A to B will have B’s value overridden by A’s value.

However, the developer of the module, or the creator have the ability to specify the direction flow when necessary. For example, in the Github example, the Github module populates the (issues) cell via fetch call and is expected to be a read-only cell. This means that any connection to the cell will become a directional data flow out.

The simple big picture is that we are drawing lines to tell the system how we want data to flow among the different modules, very much like how we pass variables around in code.