Leveraging Elixir's hot code loading capabilities to modularize a monolithic app

Summary

Leveraging Elixir's hot code loading capabilities to modularize a monolithic app Jun 21 2025 My “services startup” Alzo is an Elixir monolithic app that gets deployed with 1 instance per client. In this post we will see how Elixir’s and the Erlang VM’s hot code loading capabilties help me build client-specific features while maintaining a coherent codebase and avoiding a microservices-like situation with cascading failures or complex testing situations. If you come from outside Elixir/Erlang, the BEAM VM (Erlang’s virtual machine) lets you load compiled modules into a running system, and also provides the ability to compile files at runtime. This allows to either add code and behaviour at runtime, or even replace it, without stopping a running system. I’ve wanted to write this one for a long time now, but had to face the difficulty of not writing everything I wanted to, because this post would be too long to be even considered for reading. I guess I really wore into Elixir and the BEAM with time and feel the desire to talk about it, showcase interesting parts of it… This syndrom seems widespread in our community. 1 instance per client The rationale behind that choice comes from the following goals I set for myself : I want Alzo to be able to easily run on-prem at clients that desire it I want to open-source it when it has stabilized enough, and I would like people to be able to use it without the hassle of managing a multi-tenant platform I want to be able to easily build client-specific apps (what clients actually want of my platform) on top of Alzo without going into an authorization / isolation nightmare. I want people that would use the open-source version to be able to build their own tools on top of it, without building a full-fledged separate service, nor changing the mainline code. Client-specific apps Client specific apps are mostly live multiplayer document editors built to accelerate existing workflows. See this presentation at the Belgian Elixir meetup f...