1/9/2024 0 Comments Pub sub message queue![]() ![]() Publishers can’t tell when subscribers are successfully connected, both on initial connections, and on reconnections after network failures. But we also remove any possibility to coordinate senders and receivers. When we remove back-chatter, our overall message flow becomes much simpler, which lets us make simpler APIs, simpler protocols, and in general reach many more people. In other words, subscribers don’t connect to the publisher at all, they connect to a multicast group on the switch, to which the publisher sends its messages. With pub-sub, it’s how the pattern can map cleanly to the PGM multicast protocol, which is handled by the network switch. Killing back-chatter is essential to real scalability. There are some exceptions, e.g., SUB sockets will send subscriptions to PUB sockets, but it’s anonymous and infrequent. This means that recipients don’t talk back to senders. To get scalability, pub-sub uses the same trick as push-pull, which is to get rid of back-chatter. If you need millions of messages per second sent to thousands of points, you’ll appreciate pub-sub a lot more than if you need a few messages a second sent to a handful of recipients. This means large volumes of data, sent rapidly to many recipients. More profoundly, pub-sub is aimed at scalability. This bears repeating because people seem to quite often suggest doing this. You cannot simply replace PUSH with PUB or vice versa and hope that things will work. It’s worth understanding the trade-offs that pub-sub makes, how these benefit us, and how we can work around them if needed.įirst, PUB sends each message to “all of many”, whereas PUSH and DEALER rotate messages to “one of many”. It has that unique mix of meticulous simplicity and brutal indifference that characterizes ZeroMQ. Pub-sub addresses an old messaging problem, which is multicast or group messaging. ZeroMQ’s low-level patterns have their different characters. How to use the Binary Star pattern to add failover to a server.How to use reactors to simplify complex servers.How to build a shared key-value store (the Clone pattern).How to monitor a pub-sub network (the Espresso pattern). ![]() How to design high-speed subscribers (the Black Box pattern).How to handle too-slow subscribers (the Suicidal Snail pattern).In this chapter we’ll focus on publish-subscribe and extend ZeroMQ’s core pub-sub pattern with higher-level patterns for performance, reliability, state distribution, and monitoring. If you managed to digest all that, congratulations. Chapter 3 - Advanced Request-Reply Patterns andĬhapter 4 - Reliable Request-Reply Patterns we looked at advanced use of ZeroMQ’s request-reply pattern. ![]()
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