12.9. Establishing Communication

This section provides functions that establish communication between two sets of MPI processes that do not share a communicator.

Some situations in which these functions are useful are:

1. Two parts of an application that are started independently need to communicate.
2. A visualization tool wants to attach to a running process.
3. A server wants to accept connections from multiple clients. Both clients and server may be parallel programs.

Establishing contact between two groups of processes that do not share an existing communicator is a collective but asymmetric process. One group of processes indicates its willingness to accept connections from other groups of processes. We will call this group the (parallel) server, even if this is not a client/server type of application. The other group connects to the server; we will call it the (parallel) client.

Advice to users.

While the names client and server are used throughout this section, MPI does not guarantee the traditional robustness of client/server systems. The functionality described in this section is intended to allow two cooperating parts of the same application to communicate with one another. For instance, a client that gets a segmentation fault and dies, or one that does not participate in a collective operation may cause a server to crash or hang. ( End of advice to users.)