Earlier in the Summer I took a rock climbing class at Stone Gardens in Ballard. I left the gym pretty fired up thinking this would be a great way to explain some our WS-* technology to the not so technologically savvy. I spent a couple of hours that evening talking with a climber about the metaphor.
We have all seen reliable messaging when ordering our grande soy latte at Starbucks. After we order the drink the cashier blurts the order out loud only to have the person making the drink acknowledge the order by repeating it back. Clearly, this is not 100% reliable because the human making the drink may still forget the order if they do not proceed to write it onto the cup immediately; however, the business folks at Starbucks recognize that the human mind is more likely to remember and execute the drink order correctly if they repeat the order out loud. Thus, they established a protocol for drink ordering. The Policy on the drink maker is to receive the drink order At Most Once. The drink maker responds Exactly Once. In the end, reliability is subject to the corresponding infrastructure implementing the protocol; in this case, the reliability of the system is measured by the consciousness of the humans involved in the interaction. Reliable messaging is often required in more mission critical environments and at times failure of the system could be life threatening. I recently spoke with a group of folks at Siemens who builds systems that have human machine interfaces (hmi) where human operators are constantly waiting to take over manual control of the machines if the computer systems fail. Their policy is that within 10 seconds of failure a human operator is required to take over control or thousands of lives can be affected.
In rock climbing, there are usually two people. The person on belay must be attentive and not make mistakes to ensure the climber's safety. When rock climbing, protocol usage can help mitigate potential physical harm. The protocol is very simple. What is interesting to note, is most of the time, the messages are At Least Once. At least once means the endpoint (in this scenario, person) receiving the message must receive it at least once to react i.e. if the person does not hear the message, the message must be resent.
When beginning a climb from one pitch to the next
Both individuals give each other safety checks. Both the climber and the person on belay must have all straps double backed. The climbers knot must be tight and well dressed. The person belaying must have their carabiner locked and have the rope properly threaded through the belay device.
rock climber: on belay
person on belay: belay on
rock climber gets on the wall
rock climber: climbing
person on belay: climb on
rock climber begins climbing
once the climber reaches the next pitch, they anchor in and belay the climber below
To begin rappelling
rock climber: slack
person on belay removes all of the slack from the rope
rock climber: Lower
person on belay begins to lower rock climber
One way messages the climber may send to the person on belay
rock climber: Falling!
person on belay puts on the brake
In memory of William Tharpe
