Impedance mismatch
In electrical and electronic systems impedance is a property that is used to work out the amount of resistance over varying frequencies that components like capacitors and inductors create. An impedance mismatch infers an inefficient transfer of power. For time varying signals in an electrical or electronic system, an impedance mismatch can result in reflection of the signal back to the source - a potentially catastrophic situation. This commonly happens where two different types of cable are joined together. An impedance mismatch in software can have similar implications.
"In fact impedance is very similar to refraction index, the difference being that they are practically relevant in completely different frequency domains. An example of mismatch is: you look through a piece of glass (let's say you are admiring a new notebook in a store) and you see, vaguely, your face. This happens because air and glass have different refractive indexes so part of the light bounces back on the separation surface between air and glass. Exactly the same thing happens to signals when they cross from one type of cable (let's say 50ohms) to another type of cable (let's say 75 ohms).
Using "impedance mismatch" for the troubles you get when you try to fit a relational DB with an OO application is merely an analogy (while the two phenomenons presented above are actually the _same_ at a basic level). "[1]
What it Means to Software
Where two or more software boundaries intersect, the interfaces exposed by these boundaries may not interact cohesively.
In general this maxim refers to the problem that occurs when connecting a relational database with an object oriented program. As the properties objects in the context of OO design are quite different to the representation that in used in the database, the transfer of data from one side of the connection to another can be problematic. One example is, if the granularity of the objects is too big or too small then data transfer is not as easy or efficient as it could be.
