Readers / Writers Problem

Readers/Writers problem models access to a database. It is acceptable to have multiple processes reading the database at the same time, but if one process is updating (writing) the database, no other processes may have access to the database, no even readers.


P.J. Corbato, F. Heymans, and D.L. Parans, Concurrent Control with Readers and Writers, Commun. of the ACM, vol. 10, pp. 667-668. Oct. 1971

Jean Bacon, Conncurrent systems Operating Systems, Database and Distributed Systems: An integrated Approach, 2nd Edition, Addison-Wesley, pp. 286-289

Andrew S. Tanenbaum and Albert S. Woodhull, Operating Systems: Design and Implementation, 2nd Edition, Prentice-Hall, pp. 77-80

Bradford Nichols, Dick Buttlar and Jacqueline Proulx Farrell, Pthreads Programming, O'Reilly & Associates, Inc., pp. 84-89

Solution 1 use at your own risk

We demand of our solution that no reader be kept waiting unless a writer has already obtained permission to use the resources; i.e. no reader should wait simply because a writer is waiting for other readers to finish. It is possible that a writer could wait indefinitly while a stream of readers arrived.

Solution 2 use at your own risk

We give priority to writers in which once a writer is ready to write, he performs his "write" as soon as possible. Readers are allowed to wait indefinitely while a stream of writers is working.

Solution 3 use at your own risk

When a reader arrives and a writer is waiting, the reader is suspended behind the writer instead of being admitted immediately. In this way, a writer has to wait for readers that were active when it arrived to finish but does not to wait for readers that came along after it.