2005-01-24 19:52:04 +01:00
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
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<title>Design rules - Madeleine</title>
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<link type="text/css" href="docs.css" rel="stylesheet">
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<body>
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<h1>Design rules</h1>
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<p>This is a summary of the design rules your application has to
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follow to work with Madeleine.
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<h2>The Prevalent System</h2>
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<h3>Your objects have to fit into memory</h3>
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<p>All of them. At the same time.
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<h3>Your objects have to be marshallable</h3>
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<p>Snapshots are taken of the system by marshalling the whole system to a
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file. If your classes can't be marshalled/unmarshalled then Madeleine
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won't be able to store/restore the system.
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<h3>Your objects have to be deterministic</h3>
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<p><em>Deterministic</em> means that, given the same commands, they have
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to always give the same results.
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<p>For the much of your code this won't
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be a problem, but there are a few common issues:
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<h4>The system clock</h4>
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<p>You can't use the system clock (see instead ClockedSystem and TimeActor).
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<h4>Random numbers</h4>
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<p><code>Kernel.rand()</code> uses the system clock internally by
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default. Use <code>Kernel.srand()</code> to seed the random number
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generator before using <code>rand()</code>.
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<h4>Files, network and other IO</h4>
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<p>You generally can't access the outside world from within your
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prevalent system. Instead do IO outside of the prevalent system and
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call into the system when needed.
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<h3>Changes to the system have to be done through command
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objects</h3>
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<p>Everything that modifies the prevalent system must be done through a
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<em>command object</em> sent to the Madeleine instance, using
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<code>execute_command(aCommand)</code>. Queries that don't modify the
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system can be done either through direct method calls or through
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command objects.
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<h2>Command Objects</h2>
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<p>A command object is an object that implements the method
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<code>execute(system)</code>. They are an example of the "Command"
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design pattern.
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<h3>The command objects also have to be marshallable</h3>
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<p>Madeleine keeps track of changes between snapshots by logging
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marshalled commands.
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<h3>The command must raise errors before modifying the system</h3>
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<p>Unlike a RDBMS, Madeleine can't roll back a command (yet). This means
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that your commands will have to do their error checking and raise any
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errors before modifying the system. Failing to do this will cause an
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inconsistent command log.
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<h3>Command objects can't hold references to the system's objects</h3>
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<p>Unmarshalling such a command would create clones of the original
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objects, which would then be modified instead of the real
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objects. The commands must <i>find</i> the objects to modify.
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<hr>
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<tt>$Id: designRules.html,v 1.1 2005/01/07 23:03:27 alexeyv Exp $</tt>
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