zeromq - advantages of the router dealer pattern

Question

Could anyone please provide a real-world example of using zmq with the router/dealer pattern, and explain its advantage over the more simple publish/subscribe pattern? Thanks.

Runnerup

Answer 1

A 'real-world' example: Stock market simulation

Router socket is the server (or 'Market'), Dealer sockets are the clients (or 'Traders').

1. Traders place buy and sell orders by sending relevant 'order' messages to the Market.
2. The Market responds immediately with an 'acknowledge' message
3. Sometime later when an order is fulfilled the Market sends 'order complete' messages to all involved traders.

This sort of behavior would be quite cumbersome to implement with pub/sub as you would need both Market and Traders to run a Publisher and a Subscriber socket to allow the 2 way communication. There would also be privacy concerns if all completed transactions were 'published' rather than sent direct to the relevant traders. (Trader B should not get to know that Trader A bought or sold something).

What makes Router sockets different

Sending and receiving from Router sockets are slightly more complicated to allow the asynchronous responses:

1. Any incoming message has an identity frame prepended to the incoming message upon receipt, this indicates which client the message came from.
2. The first frame of any sent message is removed and used to identify which client to send the response to.

The 'identity' is a string and will be set to something unique per connected client by default, but you can set custom identities on client sockets via socket options.

Barcellona

but take this example from zeromq website: zguide.zeromq.org/js:rtdealer . How does it ensure that only the relevant dealers get the message from the router? from what I can see, all the dealers are bound to the same socket and get the same messages. – Runnerup

I've updated my answer to hopefully explain better. In your link you can see the behavior described above at this line: broker.send([identity, '', 'Work harder']); – Barcellona

The broker indeed sends an identity parameter, but all the clients in the example are listening to the same TCP socket. How can you prevent other clients from getting the message? – Runnerup

Because ZeroMQ ensures it only gets sent to one client. Remember TCP is a 1-1 communication, in the case of a Publisher on TCP the message is actually replicated and sent to multiple addresses. But then the whole point in using something like ZeroMQ is so you don't have to worry about low level sockets. – Barcellona

Answer 2

Dealers send their client ID with each message. Router is able to accept connections from multiple dealers and each time a message is received, it is able to discern which client sent the message. It is also able to send messages to specific clients by referring to their IDs.

Real life example: game server lets clients join a room until it is full. It keeps track of each client ID in the room. When room is full, it loops over each client ID within that room and sends them a "game started" message.

Tother

You saved my day thnx a lot I want to smash the upvote :P "it is able to discern which client sent the message. It is also able to send messages to specific clients by referring to their IDs" – Incudes