Open Source Summit Korea 2026

One controller at 846m CPU. The other at 6m. Both running. Both supposedly doing the same job.

We had two ArgoCD application controllers. Scaling looked solved on paper. Except -- one was doing all the work and the other was just... sitting there.

The thing nobody tells you when you first set up ArgoCD HA is that sharding works at the cluster level, not the application level (sadly). So when you only have one cluster -- it doesn't matter how many controller replicas you spin up. The work doesn't split. It all piles onto one pod.

We were running 300+ apps across 11 EKS clusters in a banking environment. Dropping availability was not an option. So we went digging.

What we found was not in the official docs. It was a trick - something so counterintuitive that the first time I thought of it, I laughed. Then I tried it. Then it worked.

This talk is the story of that fix -- how we found it, why it works, and how we shipped it safely to production using Terraform without touching a single app config.