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• Modules make it easier for developers to implement new technologies to be used with VCL easily
• Core VCL code does not need to be altered in order to support additional technologies or functionality
• Increased flexibility for different configurations
• Consistent methods to access data stored in the database
• Code maintainability is increased because each module focuses on a distinct task and the core code does not need to check for numerous different conditions based on the technology being used

Architecture 

A module is comprised of a Perl module file.

Object Orientation and Inheritance

Module.pm (VCL::Module)

• Provides a constructor for all derived objects to use
  • Objects which inherit from VCL::Module do not need to implement their own new() subroutines
  • Objects which inherit from VCL::Module do not need to deal with "blessing" themselves
• Provides access to the database data for the reservation via the data() subroutine implemented by Module.pm

  • Wiki Markup
    Any module derived from VCL::Module can call $self->data->\[get_something\] or $self->data->\[set_something\]

State.pm (VCL::Module::State)

• Supports the core VCL state modules such as new.pm, image.pm, reclaim.pm, and others
• Provides an initialize() subroutine which performs common tasks whenever a state object is created
  • initialize() creates the provisioning and OS objects

• Wiki Markup
  Provides an os() subroutine which allows the state objects to interact with the resource's operating system by calling $self->os->\[subroutine\]

• Wiki Markup
  Provides a provisioner() subroutine which allows the state objects to interact with the provisioning engine that has been configured for the resource by calling $self->provisioner->\[subroutine\]

• Provides other subroutines such as reservation_failed() which performs a consistent set of tasks when a reservation fails

Provisioning.pm (VCL::Module::Provisioning)

• Provisioning engine modules should inherit from VCL::Module::Provisioning
• Modules which are a subclass of VCL::Module::Provisioning also receive the functionality provided by VCL::Module through inheritance
• VCL::Module::Provisioning provides provisioning modules with access to the OS object created for the reservation

OS.pm (VCL::Module::OS)

• OS modules should inherit from VCL::Module::OS
• Modules which are a subclass of VCL::Module::OS also receive the functionality provided by VCL::Module through inheritance
• VCL::Module::OS provides OS modules with access to the provisioning object created for the reservation
  • This is useful if the OS needs to perform a task such as power cycling the computer if a reboot fails

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