The most successful MPP applications have been for problems that can be broken down into many separate, independent operations on vast quantities of data. In data mining , there is a need to perform multiple searches of a static database. In artificial intelligence , there is the need to analyze multiple alternatives, as in a chess game. Often MPP systems are structured as clusters of processors. Within each cluster the processors interact as in a SMP system. It is only between the clusters that messages are passed. Because operands may be addressed either via messages or via memory addresses, some MPP systems are called NUMA machines, for Non-Uniform Memory Addressing.