Python 7.0pl5 Manuale Utente Scaricare il pdf (Pagina 76)

Message Exchange

When an application is distributed across multiple machines, it is often the case that even if one ma-

chine were to be shutdown, the processes on a different machine might be able to quite happily keep

operating so long as they could still communicate. To support this, a means of setting up a distributed

version of the message exchange server is provided.

In this arrangement, each machine has its own message exchange server, with each message exchange

server connected to all others. If a machine is now shutdown or connections to one machine lost, other

machines will still be able to communicate with processes on any machines which are still accessible.

That is, loss of the message exchange server on one machine will only directly impact that machine.

To setup a distributed exchange server, the message exchange server endpoint is created as before. The

difference is that as well as listening on a port for new connections, client like connections are created

to the other message exchange servers. The aim here is to effectively create a star connected network

between the message exchange servers. That is, each message exchange server has a connection to all

other message exchange servers.

port = 11111

exchange = Exchange(netsvc.EXCHANGE_SERVER)

exchange.listen(port)

delay = 5

for host in hosts:

exchange.connect(host,port,delay)

Note that since connections are bidirectional, it is not necessary for each message exchange server to

mutually connect to each other. That is, if you have two message exchange servers, it is only necessary

for one to connect to the other. In other words, the list of remote hosts in one would be empty, where

as the list of the remote hosts in the other would be the reciprocal host. If two message exchange serv-

ers do connect to each other, this will be detected and one connection will be ignored, however it

should be avoided.

Multiple Exchange Groups

When creating a service agent, the default service audience is "*", indicating that knowledge of the

service should be distributed as widely as possible. One alternative is to set the service audience to the

empty string, which will always result in the service only being visible within its own process. What

occurs for other values of the service audience property depends on the exchange group assigned to a

message exchange endpoint.

By default, the exchange group of a message exchange endpoint is empty, but may be set by an option-

al argument when initialising the class. A message exchange endpoint is only able to be connected to

a complimentary message exchange endpoint which is a member of the same group. That is, a message

exchange client endpoint can only connect to a message exchange server endpoint with the same ex-

change group.

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Python 7.0pl5 Manuale Utente Pagina 76

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