## 5.4.4. Name Publishing

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The routines in this section provide a mechanism for publishing names. A ( service_name, port_name) pair is published by the server, and may be retrieved by a client using the service_name only. An MPI implementation defines the scope of the service_name, that is, the domain over which the service_name can be retrieved. If the domain is the empty set, that is, if no client can retrieve the information, then we say that name publishing is not supported. Implementations should document how the scope is determined. High quality implementations will give some control to users through the info arguments to name publishing functions. Examples are given in the descriptions of individual functions.

MPI_PUBLISH_NAME(service_name, info, port_name)
IN service_name | a service name to associate with the port (string)
IN info | implementation-specific information (handle)
IN port_name | a port name (string)

`int MPI_Publish_name(char *service_name, MPI_Info info, char *port_name)
` `MPI_PUBLISH_NAME(SERVICE_NAME, INFO, PORT_NAME, IERROR)
INTEGER INFO, IERROR
CHARACTER*(*) SERVICE_NAME, PORT_NAME
` `void MPI::Publish_name(const char* service_name, const MPI::Info& info, const char* port_name)
`

This routine publishes the pair ( port_name, service_name) so that an application may retrieve a system-supplied port_name using a well-known service_name.

The implementation must define the scope of a published service name, that is, the domain over which the service name is unique, and conversely, the domain over which the (port name, service name) pair may be retrieved. For instance, a service name may be unique to a job (where job is defined by a distributed operating system or batch scheduler), unique to a machine, or unique to a Kerberos realm. The scope may depend on the info argument to MPI_PUBLISH_NAME.

MPI permits publishing more than one service_name for a single port_name. On the other hand, if service_name has already been published within the scope determined by info, the behavior of MPI_PUBLISH_NAME is undefined. An MPI implementation may, through a mechanism in the info argument to MPI_PUBLISH_NAME, provide a way to allow multiple servers with the same service in the same scope. In this case, an implementation-defined policy will determine which of several port names is returned by MPI_LOOKUP_NAME.

Note that while service_name has a limited scope, determined by the implementation, port_name always has global scope within the communication universe used by the implementation (i.e., it is globally unique).

port_name should be the name of a port established by MPI_OPEN_PORT and not yet deleted by MPI_CLOSE_PORT. If it is not, the result is undefined.

Advice to implementors.

In some cases, an MPI implementation may use a name service that a user can also access directly. In this case, a name published by MPI could easily conflict with a name published by a user. In order to avoid such conflicts, MPI implementations should mangle service names so that they are unlikely to conflict with user code that makes use of the same service. Such name mangling will of course be completely transparent to the user.

The following situation is problematic but unavoidable, if we want to allow implementations to use nameservers. Suppose there are multiple instances of ``ocean'' running on a machine. If the scope of a service name is confined to a job, then multiple oceans can coexist. If an implementation provides site-wide scope, however, multiple instances are not possible as all calls to MPI_PUBLISH_NAME after the first may fail. There is no universal solution to this.

To handle these situations, a high quality implementation should make it possible to limit the domain over which names are published. ( End of advice to implementors.)

MPI_UNPUBLISH_NAME(service_name, info, port_name)
IN service_name | a service name (string)
IN info | implementation-specific information (handle)
IN port_name | a port name (string)

`int MPI_Unpublish_name(char *service_name, MPI_Info info, char *port_name)
` `MPI_UNPUBLISH_NAME(SERVICE_NAME, INFO, PORT_NAME, IERROR)
INTEGER INFO, IERROR
CHARACTER*(*) SERVICE_NAME, PORT_NAME
` `void MPI::Unpublish_name(const char* service_name, const MPI::Info& info, const char* port_name)
`

This routine unpublishes a service name that has been previously published. Attempting to unpublish a name that has not been published or has already been unpublished is erroneous and is indicated by the error class MPI_ERR_SERVICE.

All published names must be unpublished before the corresponding port is closed and before the publishing process exits. The behavior of MPI_UNPUBLISH_NAME is implementation dependent when a process tries to unpublish a name that it did not publish.

If the info argument was used with MPI_PUBLISH_NAME to tell the implementation how to publish names, the implementation may require that info passed to MPI_UNPUBLISH_NAME contain information to tell the implementation how to unpublish a name.

MPI_LOOKUP_NAME(service_name, info, port_name)
IN service_name | a service name (string)
IN info | implementation-specific information (handle)
OUT port_name | a port name (string)

`int MPI_Lookup_name(char *service_name, MPI_Info info, char *port_name)
` `MPI_LOOKUP_NAME(SERVICE_NAME, INFO, PORT_NAME, IERROR)
CHARACTER*(*) SERVICE_NAME, PORT_NAME
INTEGER INFO, IERROR
` `void MPI::Lookup_name(const char* service_name, const MPI::Info& info, char* port_name)
`

This function retrieves a port_name published by MPI_PUBLISH_NAME with service_name. If service_name has not been published, it raises an error in the error class MPI_ERR_NAME. The application must supply a port_name buffer large enough to hold the largest possible port name (see discussion above under MPI_OPEN_PORT).

If an implementation allows multiple entries with the same service_name within the same scope, a particular port_name is chosen in a way determined by the implementation.

If the info argument was used with MPI_PUBLISH_NAME to tell the implementation how to publish names, a similar info argument may be required for MPI_LOOKUP_NAME.

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