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Author:halw

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Date:2011-03-04T23:02:02.000000Z


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2011-03-04 23:02:02 +00:00
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documentation/current/solutions/concurrent-computing/concurrent-eiffel-scoop/scoop-examples/baboon-crossing.wiki
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[[Property:title|Baboon crossing]]
[[Property:weight|-10]]
[[Property:uuid|450867dd-7e90-c385-a081-6dacab5d210c]]
[[Property:uuid|8b48b7bb-baa8-41a2-2d93-1d9667b05323]]
{{UnderConstruction}}
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=Description=
The baboon crossing problem is based on a situation in which a number of baboons are located on the edge of a deep canyon. There are baboons on both sides of the canyon and, as you have probably already guessed, some of the baboons on the left side of the canyon want to get to the right side, and vice versa. Fortunately, a large rope has been stretched across the abyss allowing the baboons to cross by brachiation, that is, by hanging from the rope and swinging hand-over-hand to the other side.
The system may be workable, but it is certainly not perfect. You can easily imagine the problems:
# If two baboons meet in the middle of the rope, then all activity stops, and no other baboons can cross (deadlock). So, at any given time, all the baboons on the rope must be going the same direction.
# The rope can hold only a certain number of baboons at a time. A baboon cannot be allowed on the rope if the rope is already at full capacity.
# A continuous stream of baboons from one direction could prevent baboons wanting to go the opposite direction from ever being able to cross (starvation).
=Highlights=
The rope is modeled in this example by class <code>ROPE</code>, and is the primary shared resource. The fact that, at any given moment, the rope is considered unavailable to baboons wanting to cross the canyon in the direction opposite the current flow of baboons adds an interesting twist ... as does the maximum limit of baboon flesh that the rope can support. The <code>ROPE</code> class has features to manage its state:
: <code>capacity</code> is an integer representing the maximum number of baboons that can be supported at one time.
: <code>baboons</code> is an integer which is the number of baboons currently traversing the rope.
: <code>direction</code> is the current direction of baboon flow, represented as a boolean.
: <code>changing</code> is an indicator of whether the direction of the rope is currently in the process of changing.
: <code>is_secure</code> is a boolean indicating whether the rope is in such a state that a baboon may be allowed to mount the rope.
<code>ROPE</code> also includes procedures <code>mount</code> and <code>dismount</code> which a baboon uses to get on and off the rope.
There are two more interesting features of <code>ROPE</code>, <code>directions</code> and <code>announce</code> which will be discussed below.
The baboons (modeled by class <code>BABOON</code>), when they are created are determined at random to want to go either left or right. As each baboon is created by the root class of the example, it is launched into life. For the purposes of this example, the baboon life is short and consists of four steps:
: 1) Announcing one's desired direction to the rope. Doing this involves the feature <code>{ROPE}.announce</code> that was mentioned above. The rope keeps a queue of these desired directions as announced by the baboons in its <code>directions</code> feature. It is by keeping this queue that the rope can make its decisions to change the direction of the flow of baboons.
: 2) Mounting the rope. The baboon calls its own <code>mount</code> feature (which in turn calls <code>{ROPE}.mount</code>. The baboon's <code>mount</code> procedure includes two wait conditions. One makes sure the rope is safe (<code>{ROPE}.is_secure</code>), and the second make sure that the direction of the baboons is the same as that of the baboon wanting to mount. Whenever these conditions are met, the baboon is able to grab a spot on the rope and start to cross the canyon.
: 3) Traversing the canyon. Once on the rope, the baboon takes a short time to make it to the other side.
: 4) Dismounting the rope. After crossing the canyon, the baboon gets off the rope ... and apparently dies immediately after satisfying his only life's ambition.