Pipeline pattern 管道模式。
这种模式描述的场景是数据被散布到以管道方式组织的各个节点上。管道的每一步都连接一个或多个节点，连接多个节点时数据以RR方式往下流。
注意是流，意味着数据跟发布模式一样是单向的。这个模式对应的socket是ZMQ_PUSH和ZMQ_PULL.
ZMQ_PUSH
用来向下游节点发消息。下游多个节点时采取RoundRobin分发，zmq_recv()对于这个socket也是无效的。
与Pub不同的是,当下游节点达到高水位(HWM)或者根本没有下游节点时，zmq_send()就阻塞了，消息并不丢失。
| Summary of ZMQ_PUSH characteristics | |
|---|---|
| Compatible peer sockets | ZMQ_PULL |
| Direction | Unidirectional |
| Send/receive pattern | Send only |
| Incoming routing strategy | N/A |
| Outgoing routing strategy | Round-robin |
| ZMQ_HWM option action | Block |
| Summary of ZMQ_PULL characteristics | |
|---|---|
| Compatible peer sockets | ZMQ_PUSH |
| Direction | Unidirectional |
| Send/receive pattern | Receive only |
| Incoming routing strategy | Fair-queued |
| Outgoing routing strategy | N/A |
| ZMQ_HWM option action | N/A |
