Combine messages

In order to conserve network bandwidth and to provide support for atomic operations, the OS supports combine messages. A combine message is constructed by the client's C library and consists of a number of I/O and/or connect messages packaged together into one.

For example, the function readblock() allows a thread to atomically perform an lseek() and read() operation. This is done in the client library by combining the io_lseek and io_read messages into one. When the resource manager shared library receives the message, it will process both the io_lseek and io_read messages, effectively making that readblock() function behave atomically.

Combine messages are also useful for the stat() function. A stat() call can be implemented in the client's library as an open(), fstat(), and close(). Instead of generating three separate messages (one for each of the component functions), the library puts them together into one contiguous combine message. This boosts performance, especially over a networked connection, and also simplifies the resource manager, which doesn't need a connect function to handle stat().

The resource manager shared library takes care of the issues associated with breaking out the individual components of the combine message and passing them to the various handler functions supplied. Again, this minimizes the effort associated with writing a resource manager.