Yes, the old data still in the queue would be sent once the dealer starts receiving messages again.


On 10/18/2016 6:08 PM, 白木 勇矢 wrote:
Thank you for your response Joshua. So I cannot avoid queue drop when
sending packet from Pub to Sub or from Dealer to Router unless I
create some logic to solve this issue.

According to what you explained, when I call zmq_send continuously on
Router to send messages to specific Dealer, if Dealer is in mute
state, because the Router socket send is blocked and cannot dequeue a
message, it's going to reach high water mark in Router. After it
reached high water mark in Router, rest of the message I tried to
enqueue using zmq_send in Router is dropped. But do the previous data
enqueued already in ZMQ queue of Router would be able to reach Dealer
for sure if Dealer move out from mute state?


On Tue, Oct 18, 2016 at 10:48 AM, Joshua Foster <> wrote:
Lets see if I can explain it. Each socket has its own queue/buffer. This is
what is used to track how many messages. PUB/SUB uses a drop approach when
its queue gets full. If a subscriber is slow (has a full queue), the
publisher continues publishing and does not block. The slow subscriber would
then miss all messages until it was able to clear some of its queue.

I don't know much about PGM, but I believe that it just provides more
reliability at the packet level so the behavior of PUB/SUB would be the

PUSH/PULL block when their queues get full. This puts back-pressure on the
producers. Router and Dealers are sockets with a pair of Push/Pull sockets
so they behave the same. So if a Dealer is blocked, the router socket would
block on send until the dealer can accept.

The correct answer to avoiding queue drops is that you don't. The correct
approach is to create you solution that allows messages to drop. There are
MANY reasons for messages to drop and it is really just a tradeoff as to
what can drop and what performance is required. One simple way to handle
dropped messages is to use idempotent messages. The basic idea is that you
create messages that contain the most recent state of whatever you need.
That way if all previous messages get dropped, you still have all the data
needed. In contrast, transactional messages are not able to be dropped.

Idempotent Example:

Transactional Example:


On 10/18/2016 1:21 PM, 白木勇矢 wrote:
Hello everyone,

I'm trying to use ZeroMQ Pub/Sub and Router/Dealer for my school
project. I have a question about reliability of these sockets.
According to the API Page(, when queue reached
a high water mark and move to mute state, Dealer is going to block,
Router is going to drop, Sub I'm not sure, and Pub is going to drop
the message. But I cannot clearly see the behavior of each socket.

When Dealer send message to Router via TCP and Router is in mute
state, Dealer would be able to know this and resend message again
after some interval? If Router sends the message and Dealer is in mute
state and blocked, how Router is going to act?

Also when Pub sends the message to Sub via TCP and if Sub is in mute
state, how Pub is going to behave? If Pub wants to know whether the
message is correctly enqueued in Sub, PGM is going to solve this?

If someone would be able to answer these questions and tell me how to
avoid queue drops, I appreciate it so much.

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