Read/Write Lock

Origin:

Lea 97

Reason:

To allow concurrent read access to an object, but exclusive write access.

Synopsis:

Use a specialized Scheduler whose method enter is replaced by two methods: readLock and writeLock.

Example:

An on-line auction system where remote users get and set bids.

Solution:

In a very simple form, a Read/Write Lock is sketched as follow.

Method readLock looks like:

void synchronized readLock () {
    while (pendingWriters + activeWriters > 0) wait ();
    activeReaders++:     // being reading
}

Method writeLock looks like:

void synchronized writeLock () {
    pendingWriters++;    // block readers
    while (activeReaders + activeWriters > 0) wait ();
    pendingWriters--;
    activeWriters++;     // being writing
}

Method done looks like:

void synchronized done () {
    if (activeWriters > 0)  activeWriters--;
    else if (activeReaders > 0)  activeReaders--;
    else error ();
    notifyAll ();
}

Note that activeWriters is either 0 or 1.