30K~ DAU, I have full ownership over an infra under AWS/multiple on premise servers.

we use ECS, terraform, instead of ansible I use a deploy.sh that i give to my boxes with terraform, no graceful deploys. we have our own orchestrator (K8S like) for stateful containers that we have to spin up and down very often (median 1 min of uptime per container) with unsafe code coming from AI/users.

I took the place of someone who left that company and explained me nothing, we are 2 devs, nobody understands how this entire infra works expect me. have been adding observability a lot using cloudwatch, multiple alerts so I page myself at the right times, it's stable but we need to scale to at least 5x (an objective the hierarchy gave me). it currently costs 3.5k/month in aws/on premise servers. btw it's not a classical rest api where you run stateless apis and that's it.

I just discovered how redis worked under a distributed system like this, it's quite good :) I have been a postgresql guy but redis has been a saver for our threat model.

I often read here that you should put everything : EKS + K8S, ansible, terraform, make HA, grafana etc.. but since I manage this entire infra I have been resilient to all of this. If I refactor everything right now they have nobody to do the backend (which is also what I do here) -> "startup" role where one guy has multiple roles.

for the terraform it's all AI, I have been doing a lot of modifications and have created a terraform plan under ci cd so I can review myself, I removed a lot of "destruction" like the database, i versioned the s3, versioned the terraform state, put it to a private s3 etc...

there have been big big security issues (xss, csrf, cookie stealing, untrusted code that owns the entire box, shared passwords, untrusted s3 location so you could overwrite other users...) that were there before I got here

nothing rly interesting maybe for you guys in this experience but I have learned a lot, it has been a crescendo experience where i'm getting to understand more and more which tools to put there because I am the one who owns it 100%, and have to also do features (backend) so not much time either.