# 5.7. Caching

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MPI provides a ``caching'' facility that allows an application to attach arbitrary pieces of information, called attributes, to communicators. More precisely, the caching facility allows a portable library to do the following:

- pass information between calls by associating it with an MPI intra- or inter-communicator,
- quickly retrieve that information, and
- be guaranteed that out-of-date information is never retrieved, even if the communicator is freed and its handle subsequently reused by MPI.

The caching capabilities, in some form, are required by built-in MPI routines such as collective communication and application topology. Defining an interface to these capabilities as part of the MPI standard is valuable because it permits routines like collective communication and application topologies to be implemented as portable code, and also because it makes MPI more extensible by allowing user-written routines to use standard MPI calling sequences.

[] Advice to users.

The communicator MPI_COMM_SELF is a suitable choice for posting process-local attributes, via this attributing-caching mechanism. ( End of advice to users.)

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