Configuration Hierarchy Design |  |
Author: Douglas Voet (dvoet at sapient.com)
Version: $Revision: 1.2 $($Author: araman $ / $Date: 2003/05/05 07:59:09 $)
Created: May 2003
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This document details the design of the configuration hierarchy system. This system
defines the naming service in which all Carbon Configurations exist. All relevant
classes used to implement this can be found in the
org.sape.carbon.core.config.node package.
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At its most fundamental level, the configuration hierarchy system is a tree-graph
of nodes. There are 2 main types of nodes, Folders and ConfigurationDocuments
(see Node Interfaces Class Diagram below).
Folders can contain children that are other Folders or ConfigurationDocuments. ConfigurationDocuments
can contain children that are NestedConfigurationDocuments. ConfigurationDocuments also
contain the actual configuration data.
The basic functions of the nodes are implemented in a set of abstract classes:
AbstractNode, AbstractFolder, AbstractConfigurationDocument
(see Node Implementations Class Diagram below). These classes are
extended to implement functionality that is specific to particular configuration
storage mechanisms. As of Carbon 2.0 existing storage mechanisms include file system,
jars, classloader, and JNDI.
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Link nodes allow links within the configuration hierarchy (e.g. /foo points to /bar)
or to other configuration hierarchies (e.g. /jndi points to a hierarchy of JNDI nodes).
The LinkNode interface extends the Node interface by adding methods to read and
write the ConfigurationDocument that defines the target of the link. The LinkNode interface
is further extended by FolderLink and ConfigurationDocumentLink. These 2 interfaces
allow links to be manipulated as if they were their targets. The only exception
to this pass-through behavior is node removal. Calling remove on a link node removes the link,
not the target.
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The actual implementations of link nodes are independent of the technology in which the
nodes are stored. LinkNodes are implemented as aggregations of the target node and the
link's ConfigurationDocument which are already specific to the storage technology.
The target node is in the technology of the target system while the link
ConfigurationDocument is in the technology of the system that contains the link.
The implementation classes are
AbstractLinkNode,
FolderLinkImpl and
ConfigurationDocumentLinkImpl
(see Node Implementations Class Diagram below)
in the org.sape.carbon.core.config.node.link package.
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All nodes are created by node factories. The root of the configuration hierarchy is itself a
link. It is created by a LinkNodeFactory. The LinkNodeFactory determines the factories that
are used to create sub-nodes. That is, until another link is encountered, at which time that
link determines which factories will be used to create sub-nodes. These factories are specific
to the storage mechanism (e.g. file system, jndi, etc.) of the nodes. It is in this fashion
that links control the specific implementations of nodes that are instantiated for sub-nodes.
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