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Comment by SanderMak | Hacker News Reader

Exactly, that's one of the tradeoffs discussed in the article (author here).

Another pattern that might work, which I didn't include in the article, is to scale-out the modular application (assuming it's 'stateless') into several clusters. Each node in the clusters still has the whole modular application. However, each cluster will be responsible for handling a certain part of the API. Then, put a load-balancer/API gateway in front that can route different functional parts of your API to different clusters. Scale up the individual clusters as required by load. Even though all nodes contain all modules, depending on which cluster they're in, only a certain subset of modules really takes up CPU cycles. There's still no node-to-node communication necessary, since all nodes contain all logic.

Certainly not a pattern that's always applicable, but I've used it with success several times for webapps with REST backends.

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For us scaling was never an issue; We did the stateless scale-out thing. Even solving statefull is relatively easy (at least in Java) with solutions like Hazelcast. The main drawback is there are usually only a few services in your monolith that are required to scale out. But you have to deploy the entire thing when scaling out.

As for OSGi, OSGi is complex; I dare to say that the complexity of OSGi rivals that of a microservice setup. If I had a nickle for every classloader issue I debugged... ORM was especially fun (For example wrote this piece way back: http://www.datanucleus.org/products/datanucleus/jdo/osgi.htm... ). But I must admit that I don't think we could have created (and maintained) such a large modular application without OSGi. Debugging itself is also way more complex. When an issue arises you spent a lot more time tracking down which module is misbehaving. Even though we had inserted lot's of probes (which ended up in graphite) and log statements (which ended up in graylog) to counter that.

In my experience writing smaller, simpler applications (which I acknowledge also have their own complexity with distributed debugging) are still easier to understand then an modularized application.

I'm with you on the complexity of OSGi, though some of the complexity plainly arises because it truly forces you to modularise vs. just winging it. In that regard, the new Java 9 module system has less of the service dynamics and classloading tricks going on. Very curious to see how the community will pick up the new module system.

This is a cool pattern which deserves to be more well-known.

I once worked on an e-commerce system where we deployed many copies of a monolith. Most were serving user requests. Two were running scheduled jobs. One was something to do with a distributed cache (this was a while ago). One was processing events off a queue. Having one codebase and one binary made development and releasing easy. Having multiple roles made production simpler to understand and more reliable (i think).

Another time, i worked on business web app. Again, one codebase, one binary, three instances serving users, two instances serving a data export API, and some number running scheduled jobs. When the API went down - which it often did - users were not affected. That company also tried splitting services out of that monolith; one was pretty successful, but one was a chronic failure, because so many features involved coordinated changes to both codebases.

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