6.4 Communicationlayer implementation

The current implementation of the distribution library uses TCP/IP for communication between shells. On first communication between two shells, a socket connection is established, and this connection is reused for future communications.

The socket connections opened may be closed in response to one of the following conditions:

• An error occurs on the connection, leading to the conclusion that the connection is not sound anymore. In most situations this occurs when trying to communicate with a dead shell process.
• No more file descriptors are available in the current process. In that case the least recently used connection is closed down to make it possible to open a new connection to some other shell. The closed connection may be reopened if it is later needed.
• The connection is to an ensemble. Since any distributed BETA program that exports object references need to communicate with the ensemble, it is expected that the ensemble will communicate with a large number of processes during its lifetime. Thus, the ensemble automatically closes down connections that have not been used for some time.