App domains themselves are subdivided into contexts. Contexts can be thought of as boundaries within which objects share usage rules. These usage rules include synchronization transactions, and so forth.

Context-Bound and Context-Agile Objects

Objects are either context-bound or they are context-agile. If they are context-bound, they exist in a context, and to interact with them the message must be marshaled. If they are context-agile, they act within the context of the calling object; that is, their methods execute in the context of the object that invokes the method and so marshaling is not required.

Suppose you have an object A that interacts with the database and so is marked to support transactions. This creates a context. All method calls on A occur within the context of the protection afforded by the transaction. Object A can decide to roll back the transaction, and all actions taken since the last commit are undone.

Suppose that you have another object, B, which is context-agile. Now suppose that object A passes a database reference to object B and then calls methods on B. Perhaps A and B are in a call-back relationship, in which B will do some work and then call A back with the results. Because B is context-agile, B’s method operates in the context of the calling object; thus it will be afforded the transaction protection of object A. The changes B makes to the database will be undone if A rolls back the transaction, because B’s methods execute within the context ...

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