[zeromq-dev] Design for a Pub/Sub system

Yanone post at yanone.de
Tue Nov 3 12:50:51 CET 2020

Hello friends,

I'm new to the world of messaging systems. I'm developing an open source app in the world of graphic design that aims to change the way fonts are installed on people’s computers and with that tool help small and independent publishers to a better market position against the big evil corporations in our industry.

I need the GUI app to react to when either the user data or the font subscription data has changed. The central server will inform the GUI app of these changes and the app needs to react to them somewhat instantly. A few seconds lag are no problem, neither are missing messages as I can record the update timestamp for all datasets on the central server and have the GUI app ask the central server for the timestamps on startup or network-reconnect for all datasets that the GUI app instance is concerned with.

Back to ZeroMQ. 
So I need to design a pub/sub system that is horizontally scalable if necessary. At first I'm sure I can do with a few tens of thousands of subscribers, but later that number could grow dramatically. The thing doesn't need to scale now, but eventually it may need to. So I'd rather make the right decisions now.

I read the guide (discarded non-pub/sub topics), and from what I understand, I can do with the simple-most pub/sub pattern. I don't need to store anything as the central server can keep the update timestamps as described. Messages can go lost. Only as long as the client is running and connected, I want updates to trigger.

I'm expecting extremely little throughput (could be one update per topic in many months), but I'm expecting high number of topics and *very* high number of subscribers. Theoretically, two topics are sufficient (user accounts + subscriptions) when applying filters. This will mean even higher number of subscribers per topic. Otherwise, each user account and each subscription gets their own topic. In that case, normal number of subscribers per topic will be very low, except in rarer cases of free font subscriptions, when a single subscription could still get a very high number of subscribers.

I haven't found *any* information anywhere on ZeroMQ limitations. So I need to ask you for clarifications.

Here are a precise set of questions:
1. What are the limitations of number of topics and number of subscribers, either per topic or in total?
2. Which setup scales better? Only two topics with all subscribers in them and filters, or subscribers spread out across a high number of topics? In any case, the total number of subscribers stays the same.
3. In case of hard limitations on the above, how do I scale a cluster horizontally? How are the subscribers and topics spread across containers and brokered between them?
4. What is a good environment in the Google Cloud for that? I was eyeing Kubernetes. I have no experience with it as I come from a Google App Engine (Python) background, but I can learn it. The real question is: Will it scale?

Thank you so much for your input! 🙏 


Master of Design in Type & Media
Twitter/Instagram: @yanone
+49 176 24023268 (Germany)

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