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Contexts

• A context is a non-wildcardable component of,
  • the message label,
  • the communication scope.
• A context is identified by an integer ID.
• Context are used to create independent message streams.
• Contexts are used to disambiguate message selection when an application calls a library routine that performs message passing. Indeterminacy may arise,
  • if processes enter the library routine asynchronously,
  • if processes enter the library routine synchronously, but there are outstanding communication operations.

Use of Contexts

• Different phases of an application are assigned different contexts to avoid their messages being confused.
• Library routines need to be passed a group and an array of contexts so they can construct the communicators that they need.