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
same.
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:
{1}
{5}
{10}
Transactional Example:
{+1}
{+4}
{+5}
Joshua
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(http://api.zeromq.org/), 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.
Thanks,
Yuya
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