The algorithm works as follows:

When a process wants to enter a critical region, it builds a message containing the name of the critical region it wants to enter, its process number and the current time. It then sends the message to all the other processes including itself. When a process receives a request message from another process, the action it takes depends on its state with respect to the critical region named in the message. There are three possible states:

• If the receiver is not in the critical region and does not want to enter it, it sends back an OK message to the sender (shown as Ready state in workbench).
• If the receiver is already in the critical region, it does not reply. Instead, it queues the request (shown as In CS state in workbench).
• If the receiver wants to enter the critical region, but has not yet done so, it compares the timestamp in the incoming message with the one contained in the message that it has sent everyone. The lowest one wins. If the incoming message is lower, the receiver sends back an OK message. If its own message has a lower timestamp, the receiver queues the incoming message and sends nothing (shown as Waiting state in workbench).

After sending out requests asking permission to enter a critical region, a process sits back and waits until everyone else has given permission. As soon as all the permissions are in, it may enter the critical region. When it exits the critical region, it sends OK messages to all processes on its queue and deletes them all from the queue.

This workbench displays a distributed system with four nodes, each node having a request queue and an acknowledge queue. The request queue displays the queued requests along with the timestamps. The status of each node is displayed at the top of the node. Sending a request is animated by using colored balls and sending acknowledgements by using colored rectangles. In the interactive mode, you can generate requests for nodes by pushing corresponding Node buttons at the bottom. You can also execute the workbench in the continuous mode by pushing the Animate button. The animation speed can be varied using Speed scroll bar.

Workbench