ZeroMQ

Shared Queue (DEALER and ROUTER Sockets)

In the “Hello World” client/server application, we have one client that talks to one service. However, in real cases we usually need to allow multiple services as well as multiple clients. This lets us scale up the power of the service (many threads or processes or nodes rather than just one). The only constraint is that services must be stateless, with all state being in the request or in some shared storage, such as a database.

There are two ways to connect multiple clients to multiple servers. The brute-force way is to connect each client socket to multiple service endpoints. One client socket can connect to multiple service sockets, and the REQ socket will then distribute requests among these services. Let’s say you connect a client socket to three service endpoints: A, B, and C. The client makes requests R1, R2, R3, R4. R1 and R4 go to service A, R2 goes to B, and R3 goes to service C (Figure 2-7).

Figure 2-7. Request distribution

This design lets you add more clients cheaply. You can also add more services. Each client will distribute its requests to the services. But each client has to know the service topology. If you have 100 clients and then you decide to add three more services, you need to reconfigure and restart all 100 clients in order for the clients to know about the three new services.

That’s clearly not the kind of ...