The actions that are taken in a distributed system to ensure that the system is not left in an inconsistent state when transactions are carried out in parallel. For example, a transaction could be in the middle of carrying out some transformation on data when another parallel transaction uses this data before the first transaction has finished with it. This can lead to the data being given an incorrect value. The aim of concurrency control is to ensure that the result of a series of transactions which are carried out in parallel is the same as if the transactions were carried out one after another. This is a property known as serial equivalence.