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

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

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