This is a simple in-memory message broker (or "queue" or "switch"). A deployment consists of one switch server process and multiple clients. A low-level protocol allows clients to connect to the switch server process and:

1. create named message queues
2. enqueue messages in named queues
3. subscribe to new messages in queues

Messages contain:

1. a unique "correlation id", allowing replies to be associated to requests
2. an optional "reply to" queue name
3. a string payload

The client library currently contains a single high-level pattern for synchronous RPC clients and servers. Other patterns (e.g. for pub/sub) could be added.

git clone git://github.com/xapi-project/message-switch
cd message-switch
make

Run the server (by default it will bind to 127.0.0.1:8080)

_build/install/default/bin/message-switch

Use the CLI to list the available queues (there will be none):

_build/install/default/bin/message-cli list

Use the CLI to execute an RPC against a service with queue name "foo" (this will block):

_build/install/default/bin/message-cli call foo --body hello

Use the diagnostics command to see the queued message:

_build/install/default/bin/message-cli diagnostics

Use the CLI to start an RPC server servicing the queue name "foo"

_build/install/default/bin/message-cli serve foo --program /bin/cat

At this point the original RPC call will unblock and print "hello".

Use the diagnostics command to see the message has been removed:

_build/install/default/bin/message-cli diagnostics

(I'd like to improve this, if you have any ideas then let me know)

The low-level protocol is currently using HTTP. Perhaps in future we should use something a bit quicker: a binary protocol using ZeroMQ? Clients make the following requests:

• Login <client identifier>: allows the switch to associate the socket with a single client. When the last socket is closed resources associated with the client are deallocated. I'm currently using a Unix PID as an identifier.
• Create <Some name>: creates a persistent queue with name <name>
• Create <None>: creates a queue with a fresh name which will be deallocated when the last connection opened by the client closes. These queues are created temporarily for RPC replies, the idea being that a client that has closed all connections has given up waiting for the reply and so the queue is not needed.
• Send <string, Message.t>: enqueue a message in a particular queue
• Ack <message id>: signal that a message has been processed and should be deleted. A service should presumably only Ack a message once it has committed its effects to disk.
• Subscribe <name>: subscribe to new messages in a named queue
• Transfer <id, timeout>: blocking poll for new messages in subscribed queues

A client will:

• Login <some client id>
• Reply_to <- Create <None> (used for the RPC reply)
• Subscribe <reply_to>
• Create <service name> (in case it doesn't exist)
• Send <payload, service name>
• repeatedly poll <Transfer> until a reply turns up

The server will:

• Login <client id>
• Create <service name> (in case it doesn't exist)
• Subscribe <service name>
• repeatedly poll <Transfer> until a request turns up
• process a request, Send <reply, reply_to>
• Ack <message id> to mark the message as processed and delete it