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Directory-based coherence is a mechanism to handle Cache coherence problem in Distributed shared memory a.k.a. Non-Uniform Memory Access. Another popular way is to use a special type of computer bus between all the nodes as a "shared bus". Directory-based coherence uses a special directory to serve instead of the shared bus in the bus-based coherence protocols. Both of these designs use the corresponding medium as tool to facilitate the communication between different nodes, and to guarantee that the coherence protocol is working properly along all the communicating nodes. In directory based cache coherence, this is done by using this directory to keep track of the status of all cache blocks, the status of each block includes in which cache coherence "state" that block is, and which nodes are sharing that block at that time, which can be used to eliminate the need to broadcast all the signals to all nodes, and only send it to the nodes that are interested in this single block.
Following are a few advantages and disadvantages of the directory based cache coherence protocol:
According to the above discussion, it is clear that using bus based systems seems more attractive for relatively small systems. However, directory based systems become crucial when the system scale up and the number of nodes grows. So there is a kind of trade-off between the simplicity and the scalability when comparing between Bus-based and Directory-based cache coherence designs.