UNIX domain socket: is there such a thing as a “busy” signal?

Question

Can a Client pushing data through a UNIX domain socket ( AF_UNIX type ) be signaled busy if the receiving end cannot cope with the load?

OR

Must there be a Client-Server protocol on top of the socket to handle flow control?

Answer 1

Unless you include this in the protocol, there is no way for the server to tell the client to pause sending the information.

Answer 2

You can definitely do a blocking send to a UNIX Domain socket. If the receiving side's receive buffer is full, or if the number of outstanding (undelivered) send socket buffers is too high, the sender will block.

SOCK_STREAM UNIX Domain Sockets work like TCP sockets. SOCK_DGRAM UNIX Domain Sockets work like UDP, except that UNIX Domain datagrams have guaranteed, in-order delivery, whereas UDP sockets can be re-ordered or dropped. (Also, UNIX Domain Sockets can be used to send file descriptors and pass user credentials between processes, neither of which can be done with TCP, UDP, or pipes.)

So, because in-order delivery is guaranteed by all types of UNIX Domain Sockets, the receiver can just stop receiving when it is busy doing other things, and the sender will be automatically blocked when there's no more buffer space available (or will be notified that there's no more buffer space, if they requested non-blocking operation on their socket). Then, when the receiver starts receiving again, the sender will be allowed to send more.

Answer 3

Other than the server having some knowledge of when it is 'busy' and sending a specific signal back (e.g. HTTP's 503 Service Unavailable). You can also set up the client side connection to timeout after a certain length of time, and if you get a timeout event, interpret that as the server is busy.