[lug] socket programming/kernel question

[Bear Giles]

Michael J. Hammel wrote:
> You have two processes that know about each other, correct?  Or does
> your process know about the others, but the others don't know about
> you?  I'm guessing the former, based on your description.

There's a key server daemon, which knows the pid of callers from 
the SCM_CREDENTIALS information attached to the Unix socket 
message.  The clients who know nothing about the daemon - they 
just open the socket connection and send stuff through it.

There's no logical connection between the processes, so having the 
server act as a factory isn't a practical solution.  (The server 
will probably be in C, the clients java applets triggered by 
asynchronous messages.  That also makes it difficult for the 
client to use a gatekeeper.)

> If both know about each other but you want asyncronous message handling,
> then TCP is the best way to go about it.  EPIPE can be used to know when
> the opposite end goes away.  Trying to do it by process id is far more
> difficult.

The more I think about it, the problem isn't detecting when the 
connection drops, it's detecting that the other process has died. 
  Information needs to go down the rabbit hole.  Stream protocols 
would tell me this, but require me to keep the stream open for the 
lifetime of the process even if days pass without use.

> I find that UDP is better for situations where the sender doesn't care
> about the receivers status.  This doesn't sound like the case here.

It's also useful for cases where the requests are idempotent and 
have no logical connection to other requests, and that's the case 
here.

I haven't worked through all of the details (since I'm also 
considering other approaches), but the idea is that you hit the 
server with a one object and it echos a modified object, or it 
remains silent and the client times out.  If the client misses the 
response (unlikely on Unix sockets :-), they can resubmit the 
request without causing any problems to the server.

The twist is that authentication is necessary, but nothing the 
client provides can be trusted.  Hence Unix sockets and the 
SCM_CREDENTIALS - it's not much, but it's a start.