如果你是希望通过0MQ来做缓冲队列，非阻塞的效果，那你就必须清楚 0MQ Socket是会阻塞，你要搞清楚0MQ Socket与队列的关系。
官方协议文档规定了，一部分类型的 0MQ Socket为不阻塞发送，而另一部分类型则是阻塞发送的。不阻塞发送同时往往也意味着丢弃消息，相反不能丢弃消息的则要求阻塞发送。这里的阻塞发送，不是说像tcp那样确保送到目的地，而是必须有目的地可以发送。而队列的作用就是架在发送端和目的地之间作缓冲。如果连目的地也没有，那么队列也就失去存在的意义了，所以只好将发送阻塞起来。到这里你可能会撇嘴，不是发送端缓冲在队列，在接收端建立起连接后，接收端从队列中取消息。这是你相像出来的 0MQ，而非人家设计出来 0MQ。还是要看人家的（协议设计）文档。
各种类型的详细行为责任定义：
- http://rfc.zeromq.org/spec:28/REQREP defines the semantics of REQ, REP, DEALER and ROUTER sockets.
- http://rfc.zeromq.org/spec:29/PUBSUB defines the semantics of PUB, XPUB, SUB and XSUB sockets.
- http://rfc.zeromq.org/spec:30/PIPELINE defines the semantics of PUSH and PULL sockets.
- http://rfc.zeromq.org/spec:31/EXPAIR defines the semantics of exclusive PAIR sockets.
The PAIR Socket Type
- SHALL create a double queue when initiating an outgoing connection to a peer, and SHALL maintain the double queue whether or not the connection is established.
- SHALL create a double queue when a peer connects to it. If this peer disconnects, the PAIR socket SHALL destroy its double queue and SHALL discard any messages it contains.
The PULL Socket Type
- SHALL create this queue when initiating an outgoing connection to a peer, and SHALL maintain the queue whether or not the connection is established.
- SHALL create this queue when a peer connects to it. If this peer disconnects, the PULL socket SHALL destroy its queue and SHALL discard any messages it contains.
The PUSH Socket Type
- SHALL create this queue when initiating an outgoing connection to a peer, and SHALL maintain the queue whether or not the connection is established.
- SHALL create this queue when a peer connects to it. If this peer disconnects, the PUSH socket SHALL destroy its queue and SHALL discard any messages it contains.
从上面一般行为规定可以看到，队列是与底层socket连接对应的，当底层没有任何socket连接时，队列就不存在。所以发送行为就有下面这样的规定。
The PAIR Socket Type
- SHALL consider its peer as available only when it has a outgoing queue that is not full.
- SHALL block on sending, or return a suitable error, when it has no available peer.
- SHALL not accept further messages when it has no available peer.
- SHALL NOT discard messages that it cannot queue.
The PUSH Socket Type
- SHALL consider a peer as available only when it has a outgoing queue that is not full.
- SHALL route outgoing messages to available peers using a round-robin strategy.
- SHALL block on sending, or return a suitable error, when it has no available peers.
- SHALL not accept further messages when it has no available peers.
- SHALL NOT discard messages that it cannot queue.
The DEALER Socket Type
- SHALL consider a peer as available only when it has a outgoing queue that is not full.
- SHALL route outgoing messages to available peers using a round-robin strategy.
- SHALL block on sending, or return a suitable error, when it has no available peers.
- SHALL not accept further messages when it has no available peers.
- SHALL NOT discard messages that it cannot queue.
The REQ Socket Type
- SHALL prefix the outgoing message with an empty delimiter frame.
- SHALL route outgoing messages to connected peers using a round-robin strategy.
- SHALL block on sending, or return a suitable error, when it has no connected peers.
- SHALL NOT discard messages that it cannot send to a connected peer.
1. 红色字眼 block on，说明了 0MQ Socket 在发送过程有可以阻塞。
2. ”consider a peer as available only when it has a outgoing queue that is not full“， 特别要注意peer，首先要有连接，只要这个连接关联的发送队列不满也就可以视作可以发送。
3. "block on sending, or return a suitable error, when it has no connected/available peers"， 当没有可以发送的peer，（意思是a. 有连接但发送队列满；b. 无任何连接，也就没有队列了。）要么阻塞发送，要么就返回错误给调用者，由调用者自己决定丢弃消息或其它方式处理。
4. "not accept further messages when it has no available peers"，这里是指 0MQ Socket 层不接受 0MQ Socket使用者发送的消息，不是说底层连接不接收消息。
现在是否清楚使用 0MQ时，在什么情况下可能会被阻塞了。