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JustInTime Configuration
This pattern can be used as alternative to Observer or Mediator pattern, in the situations where the number of objects that require dynamic configuration is large and the configuration keeps changing very frequently at runtime. In such situation the Observer or Mediator pattern can become bottleneck due to large number of objects to be updated with the configuration.
Participants:
Configuration Manager: object contains the latest configuration along with its version, Configuration Manager can also contain logic to accept new configuration, validate it and keep track of current valid configuration by assigning a version id.
Configuration Manager should also provide method to get current version and get latest configuration.
Configuration: An abstraction of configuration (may not be required in simple implementation).
Configured Object: This is the object which needs the configuration update at runtime. This object must have an attribute to keep reference of Configuration Manager. When an operation is invoked on configured object, it checks its current configuration version with the version at Configuration Manager and decides if needs updated configuration from Configuration Manager.
Object Factory: The object factory creates instances of Configured Object initialized with correct Configuration Manager.
Consequences:
This pattern allows lazy update of shared configuration at runtime, without forcing update configuration for all the objects at once. It also reduces burden of configuration update further, as each object decides when to update its configuration (at the time when an operation is invoked on it). The advantages of lazy configuration update become more pronounced as only update of configuration parameter at Configuration Manager requires synchronized access.
The Object Factory can create instance of configured objects setting other attributes and reference to Configuration Manager leaving configuration for lazy update by Configuration Manager.
Configuration Manager can implement logic to accept and validate configuration as required, thus reducing complexity of Configured Object. It can also keep history of configuration changes if such logs are required, this can also enable Configuration Manager to rollback to a particular version of configuration.
Implementation:
In simple implementation the Configuration Manager can also take up role of Object Factory.
You may not need an abstraction of configuration; the concrete Configuration Manager can use setters on Configured Objects to update configuration attributes.
Known Uses:
I have used this pattern for developing a decision matrix based rule engine API. Rules for decision can be updated at runtime (i.e. Add, delete and update) even when rule engine is executing. Also there can be multiple instances of rule engine using same rule set (configuration) especially in multithreaded applications.
Related Patterns:
Observer pattern and Mediator pattern.
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EJB Design Patterns is now available for free download in PDF format. The book contains a catalog of twenty advanced EJB patterns and provides strategies for mapping application requirements to patterns-driven design, J2EE development best practices, and more. EJB Design Patterns was the #2 book at Java One 2002, and held the #1 Java book position on Amazon.com for weeks since the book was released in March. Download your PDF here.
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| Useful patterns around the webUseful patterns around the webUseful patterns around the web |
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This
essential pattern describes how to model your entity beans.
How to make an entity bean a facade to a set of dependent objects.
Every guru should use unit testing.
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| Other Patterns sitesOther Patterns sitesOther Patterns sites |
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The original reference
site for patterns. Frequented by the gang of 4 and their mentors (Kent Beck, Ward Cunningham).
A catalogue of J2EE design patterns from Suns Consulting Division.
A catalogue of
high level business, architectural and topological patterns for large scale systems.
Design Patterns from the J2EE Blueprints, Suns authoritative guide to J2EE development.
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