how to replicate or synchronize messages between nsqd automatically？

[gzliudan]

How to ensure high availability of nsqd when anyone nsqd is offline, and prevent message loss ? Can one topic is existed in all nsqd? Can one message is saved in all nsqd?

[ploxiln]

Nope, nsqd currently does help you with this. If you lose an nsqd, you lose the messages that were only on that nsqd. http://nsq.io/overview/features_and_guarantees.html#messages-are-not-durable-by-default

What nsqd does do, is avoid any messages from being lost or even slowed-down on all other nsqd instances.

A common pattern is to use nsq_to_file to save a copy of all messages from a separate channel to hourly "stream log" files. This way, all messages should end up in a "stream log" very quickly (less than one second usually) so there is a very small window where that message could be lost. If an nsqd instance hard-crashes and important messages were not consumed from some channels, the last few minutes or hours of messages could be re-sent manually based on the "stream log".
http://nsq.io/deployment/topology_patterns.html#persistence

[mreiferson]

Also see #510