A Philosophy of Software Design

by John Ousterhout

Exception handling code is inherently more difficult to write than normal-case code. An exception disrupts the normal flow of the code; it usually means that something didn’t work as expected. When an exception occurs, the programmer can deal with it in two ways, each of which can be complicated. The first approach is to move forward and complete the work in progress in spite of the exception. For example, if a network packet is lost, it can be resent; if data is corrupted, perhaps it can be recovered from a redundant copy. The second approach is to abort the operation in progress and report the exception upwards. However, aborting can be complicated because the exception may have occurred at a point where system state is inconsistent (a data structure might have been partially initialized); the exception handling code must restore consistency, such as by unwinding any changes made before the exception occurred. Furthermore, exception handling code creates opportunities for more exceptions. Consider the case of resending a lost network packet. Perhaps the packet wasn’t actually lost, but was simply delayed. In this case, resending the packet will result in duplicate packets arriving at the peer; this introduces a new exceptional condition that the peer must handle.