ZeroMQ

Group Messaging

Group messaging is a common and very useful pattern. The concept is simple: instead of talking to a single node, you talk to a “group” of nodes. The group is just a name, a string that you agree on in the application. It’s precisely like using the publish-subscribe prefixes in PUB and SUB sockets. In fact, the only reason I say “group messaging” and not “pub-sub” is to prevent confusion, since we’re not going to use PUB-SUB sockets for this.

PUB-SUB sockets would almost work. But we’ve just done such a lot of work to solve the late joiner problem. Applications are inevitably going to wait for peers to arrive before sending messages to groups, so we have to build on the Harmony pattern rather than start again beside it.

Let’s look at the operations we want to do on groups:

• We want to join and leave groups.

• We want to know what other nodes are in any given group.

• We want to send a message to (all nodes in) a group.

These will look familiar to anyone who’s used Internet Relay Chat (IRC), except we have no server. Every node will need to keep track of what each group represents. This information will not always be fully consistent across the network, but it will be close enough.

Our interface will track a set of groups (each an object). These are all the known groups with one or more member node, excluding ourselves. We’ll track nodes as they leave and join groups. Since nodes can join the network at any time, we have to tell new peers what groups we’re ...