0MQ是会阻塞的，不要字面上看到队列就等同非阻塞。

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。