The Cache Consistency Problem

The cache consistency problem is best illustrated with two clients and a single server. Imagine client C1 reads a file F, and keeps a copy of the file in its local cache. Now imagine a different client, C2, overwrites the file F, thus changing its contents; let’s call the new version of the file F (version 2), or F[v2] and the old version F[v1] so we can keep the two distinct (but of course the file has the same name, just different contents). Finally, there is a third client, C3, which has not yet accessed the file F.

Update visibility

You can probably see the problem that is upcoming (from the figure above). In fact, there are two subproblems. The first subproblem is that the client C2 may buffer its writes in its cache for a time before propagating them to the server; in this case, while F[v2] sits in C2’s memory, any access of F from another client (say C3) will fetch the old version of the file (F[v1]). Thus, by buffering writes at the client, other clients may get stale versions of the file, which may be undesirable; indeed, imagine the case where you log into machine C2, update F, and then log into ...