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Pessimistic

My definition of pessimism is:

Definition 3.1   A pessimistic replication strategy employed by a client, C, with regards to a file, f, ensures that C's reads from or writes to f are performed on the newest version seen by the server and the updates are guaranteed to reach the server successfully (under normal circumstances).

Note, giving such guarantees requires some sort of locking (on the server).

The normal definition of a pessimistic replication strategy is that it allows an update--to any (logical) file at any given time--to be made to at most one of the replicas (of the file) throughout the system. This is a more restrictive definition than mine; with my definition two clients cannot be pessimistic about the same file, but it is possible for a client to do optimistic updates (see Section 3.3.3) to a file even if the file is locked due to another client being pessimistic (but the optimistic clients are almost certain to fail). This could be the case if a client could not get in contact with the server and decided to do updating anyway (being optimistic).

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