Message types

Architecturally, there are two categories of messages that a resource manager will receive:

• connect messages
• I/O messages

A connect message is issued by the client to perform an operation based on a pathname (e.g., an io_open message). This may involve performing operations such as permission checks (does the client have the correct permission to open this device?) and setting up a context for that request.

An I/O message is one that relies upon this context (created between the client and the resource manager) to perform subsequent processing of I/O messages (e.g., io_read).

There are good reasons for this design. It would be inefficient to pass the full pathname for each and every read() request, for example. The io_open handler can also perform tasks that we want done only once (e.g., permission checks) rather than with each I/O message. Also, when the read() has read 4096 bytes from a disk file, there may be another 20 megabytes still waiting to be read. Therefore, the read() function would need to have some context information telling it the position within the file it's reading from.