Race Condition

A '''race condition''' can be defined as an anomalous behavior 
due to unexpected unexpected ordering of events and critical 
dependence on the relative timing of events. It characterizes an 
undesirable situation in a concurrent or distributed system where 
the cause for the output of a process depends on the arbitrary 
timing of other events. There are many participants (messages, 
threads, etc.) which have different speed, and each of 
the participants can win. Only the winner determines the resulting 
behavior.

In other words, the output of the process is unexpectedly and dependents 
critically on a sequence or timing of other events. For instance, a computation 
in a [[Distributed System|distributed system]] may depend on the
arrival order of two different contradictory messages, the first says 
"maintain object", whereas the second says "release object", and the message 
which arrives first - the winner of the race - determines the result.
A race condition can also occur when multiple processes access and manipulate 
the same data concurrently, and the outcome of the execution depends on the 
particular order in which the access takes place.

Race conditions are not only undesirable because the program doesn't 
work as it's supposed to do. In debugging they are also very undesirable, 
because you have to "run the race" many times before you can reproduce a 
situation which a certain fault, error or failure. The error may also 
vanish if you slow the processes down by debugging the computation
step-by-step.