When you register interest, you must specify an entry in the local cache vector (VECTORINDEX keyword) that you have assigned to the data item. The system uses the vector entry to indicate the validity of the associated data item in your local cache buffer. Figure 1, shows data item X in the local storage buffer of the connecting user A. The data item is valid because vector entry 2 — the vector entry that connection A assigned to data item X, indicates that the data is valid.

The system keeps track of users, the validity of copies of their data items, and the vector entries for each user in the directory entry for each data item in the cache structure. In Figure 1, the directory entry for data item Z shows that connecting users A and B have registered interest in data item Z (that is, the connections have valid copies of data item Z). If a third connection updates Z in the cache structure, the system uses the assigned vector entries (entry 5 for connection A and entry 4 for connection B) to invalidate the local copies belonging to connections A and B.

Figure 1. Registered Interest in Data Items