http://worrydream.com/MagicInk/#p350

The programs must somehow exchange information without knowing anything about each other—without even knowing the other exists.

As it happens, such a mechanism has long existed for manipulation software—copy-and-paste. This mechanism uses the platform as an intermediary. When the user “copies” a picture in a drawing program, the program hands data off to the platform. When the user then “pastes” the picture into a word processing document, the program requests data from the platform, and handles it according to its type. The drawing and word processing programs know nothing of each other—they know only of the platform and standard data exchange formats.

[...]

The components below the platform are translators. The platform gives them information objects, which they convert from one type to another and return to the platform. Sometimes this involves decomposing an object into constituent parts (“digesting” it); other times, it involves enriching the object with additional information.

The platform itself acts as an intermediary between components, attempting to fulfill requests by constructing a chain of translators to convert topics into the requested types.