Three etudes on bufferisation

Three tasks are considered, all inspired by practical application development and general enough to make generic solutions valuable. The tasks are a buffer between a message source and a handler; a buffer to keep data up-to-date between a single source and multiple consumers; and a buffer of queries waiting to be executed. All three tasks can be easily solved ad hoc, for a particular application. Interesting and complicated issues arise when one tries to invent a generic solution that can be tailored to a wide variety of possible applications.