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A requested service is performed in a computational system by executing code that operates upon some data. The data represents
a component of the state of the computational system. The code performs the requested service, which may change the state
of the system.
Code that is executed to perform a service is called a method. A method is an immutable description of a computation that
can be interpreted by an execution engine. A method has an immutable attribute called a method format that defines the set
of execution engines that can interpret the method. An execution engine is an abstract machine (not a program) that can interpret
methods of certain formats, causing the described computations to be performed. An execution engine defines a dynamic context
for the execution of a method. The execution of a method is called a method activation.
When a client issues a request, a method of the target object is called. The input parameters passed by the requestor are
passed to the method and the output and input-output parameters and return result value (or exception and its parameters)
are passed back to the requestor.
Performing a requested service causes a method to execute that may operate upon an object’s persistent state. If the persistent
form of the method or state is not accessible to the execution engine, it may be necessary to first copy the method or state
into an execution context. This process is called activation; the reverse process is called deactivation.