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| <title>Avalon for Apache Cocoon</title> |
| <link href="http://purl.org/DC/elements/1.0/" rel="schema.DC"> |
| <meta content="Tom Klaasen" name="DC.Creator"> |
| <meta content="Berin Loritsch" name="DC.Creator"> |
| <meta content="Carsten Ziegeler" name="DC.Creator"> |
| <meta content="This document tries to give the basic knowledge of Avalon that is |
| necessary to understand Apache Cocoon." name="DC.Description"> |
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| <body> |
| |
| <h1>Goal</h1> |
| |
| <p>This document tries to provide you with the basic knowledge of Avalon |
| that is necessary to understand Cocoon.</p> |
| |
| <p>People trying to understand Avalon in depth, will probably not be much |
| helped by this document. But, if you want to understand Cocoon, you have |
| to have a basic grasp of Avalon.</p> |
| |
| <p>The document also outlines the basic steps for configuring Avalon |
| components within Cocoon.</p> |
| |
| <p>Much of this document is copied and pasted from original Avalon |
| documentation. However, I hope that by putting all things relevant to |
| Cocoon together in one place I can help you to understand Cocoon more |
| quickly.</p> |
| |
| <p>For people wishing to learn Avalon in-depth, |
| <a class="external" href="http://avalon.apache.org/developing/index.html">this |
| is your starting point</a>.</p> |
| |
| |
| <h1>Overview</h1> |
| |
| <p>For a mission statement of Apache Avalon, please read |
| <a class="external" href="http://avalon.apache.org/index.html">the Avalon |
| homepage</a>.</p> |
| |
| <p>In short, Avalon tries to take design efforts away from server-side |
| programmers by providing a framework that </p> |
| |
| <ul> |
| |
| <li>provides basic working classes;</li> |
| |
| <li>provides interfaces to allow different efforts to be integrated |
| more easily.</li> |
| |
| </ul> |
| |
| |
| <h1>The classes and interfaces</h1> |
| |
| <p>These classes and interfaces are extensively used by Cocoon:</p> |
| |
| <h2>ServiceManager</h2> |
| <p> |
| |
| <span class="codefrag">org.apache.avalon.framework.service.ServiceManager</span> |
| |
| </p> |
| <p>A <span class="codefrag">ServiceManager</span> selects <span class="codefrag">Component</span>s based |
| on a role. The contract is that all the <span class="codefrag">Component</span>s |
| implement the differing roles and there is one <span class="codefrag">Component</span> |
| per role. If you need to select one of many <span class="codefrag">Component</span>s |
| that implement the same role, then you need to use a |
| <span class="codefrag">ServiceSelector</span>. Roles are the full interface name.</p> |
| <p>To understand roles better let's use the the analogy of a play. There |
| are many different roles in a script. Any actor or actress can play |
| any given part and you get broadly the same results (same phrases |
| spoken, same movements made, etc.), but with each actor the exact |
| nuances of the performance are different.</p> |
| |
| <h2>Serviceable</h2> |
| <p> |
| |
| <span class="codefrag">org.apache.avalon.framework.service.Serviceable</span> |
| |
| </p> |
| <p>A <span class="codefrag">Serviceable</span> is a class that needs to connect to |
| software components using a "role" abstraction, thus not depending on |
| particular implementations but on behavioral interfaces. This means, |
| if you need to use other components in your implementation, you |
| have to implement <span class="codefrag">Serviceable</span> to be able to get other |
| components.</p> |
| |
| <h2>Configuration</h2> |
| <p> |
| |
| <span class="codefrag">org.apache.avalon.framework.configuration.Configuration</span> |
| |
| </p> |
| <p> |
| |
| <span class="codefrag">Configuration</span> is an interface encapsulating a |
| configuration node used to retrieve configuration values. This is a |
| "read only" interface preventing applications from modifying their own |
| configurations. The contract surrounding the |
| <span class="codefrag">Configuration</span> is that once it is created, information |
| never changes. The <span class="codefrag">Configuration</span> is built by the |
| <span class="codefrag">ConfigurationBuilder</span>. |
| </p> |
| |
| <h2>Configurable</h2> |
| <p> |
| |
| <span class="codefrag">org.apache.avalon.framework.configuration.Configurable</span> |
| |
| </p> |
| <p> |
| |
| <span class="codefrag">Configurable</span> is an interface describing a component which |
| can be configured. This component gets a <span class="codefrag">Configuration</span> |
| object as input.</p> |
| |
| <h2>ConfigurationBuilder</h2> |
| <p> |
| |
| <span class="codefrag">org.apache.avalon.ConfigurationBuilder</span> |
| |
| </p> |
| <p>A <span class="codefrag">ConfigurationBuilder</span> builds |
| <span class="codefrag">Configuration</span>s.</p> |
| |
| |
| <h1>Configuration</h1> |
| |
| <p>Most available Avalon components are configured in the cocoon.xconf.</p> |
| |
| <h2>Pooling configuration</h2> |
| <p>Avalon now incorporates a couple of modifiers for a Component |
| definition that allows you to control the number of Components in a |
| pool. This is especially helpful in Cocoon |
| where the defaults don't always work well.</p> |
| <p>The magic attribute is "pool-max". |
| The defaults are:</p> |
| <ol> |
| |
| <li>pool-max: 8</li> |
| |
| </ol> |
| <p>What this means is that the pool for the default component initially |
| contains zero instances. If demand exceeds this then the pool will |
| increase, one component at a time, up to 8 instances. Beyond that the |
| pool turns into a factory. That is, new <span class="codefrag">Component</span> |
| instances are created, but destroyed when they are returned. This is a |
| performance issue - but it does manage the number of instances |
| available at one time.</p> |
| |
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