Applications Messaging Standard

[Patrick Dowler]

On Friday 16 February 2007 04:20, Alasdair Allan wrote:
> > My feeling is still that the file-based approach, while inelegant,
> > presents fewer problems in practice than the socket-based one, but
> > I am prepared to have my opinion changed.
>
> I don't see any way around using a file to solve this problem, I'd
> welcome persuasion, but a file based approach to communicate this one
> piece of vital information seems to be the only way around it.
> Everything else, of course, should go down the wire.

Why not have binding/listening to the port be the thing that says a hub is 
running. Then clients just have to try to connect to the well known port (on 
localhost) to find the hub. Hubs need to try to open that port for listening 
to see if a hub is already running; that fails if the port is in use. 

On a multi-user machine, the initial hand-shake could exchange the user-name 
to make sure client and hub are running under the same user (spoofable, but 
we aren't trying to make this overly secure, just simple for users).

If there is a collision on the port in a multi-user environment, just 
increment the port number and try again. So clients and hubs start by 
connecting to port N and checking the user; if they connect and find the same 
user is running the hub, the client proceeds and the (2nd) hub terminates 
quietly (or not). If the new hub doesn't find anything, it tries to take the 
port (starts listening). If the client or hub finds a hub but user doesn't 
match, they try again on port N+1. If the client doesn't find a hub, it 
probably has to check higher port numbers (in case that users hub was not the 
first to start and hence is on port N+something). For sanity and quick 
failure, one would want relatively short socket connection timeouts and to 
limit the number of times one would increment the port number and try again.

The nice thing about this is that if a hub dies horribly, it isn't leaving any 
trail behind that will block the startup of another hub (the .plastic file). 
And all you have to agree on is some arbitrary port number, which is pretty 
standard in many internet applications. 

The little user-matching handshake may well not be "independent of wire 
protocol" but I really have very little idea of what people actually mean by 
that. In the end, both sides of the socket have to be talking the same wire 
protocol.

In future, this may well extend to the LAN environment simply by the user 
knowing which host his hub is on, but there are several magical ways to do it 
too (IP multicast, eg), but that's for the future.

my 2c,

-- 

Patrick Dowler
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