Building an instant messaging app — what are the real challenges?

[Kathrininfanta]

I was reading about how to build instant messaging app (including this guide on architecture and components), and I’m trying to understand the practical side of it.

From what I’ve seen, building a real-time chat app involves things like WebSockets, message queues, presence handling, push notifications, media sharing, and scaling for concurrent users.

For those who’ve actually built something like this from scratch:

• What were the biggest challenges you faced in production (not just the initial build)?
• At what point did you realize you needed a more scalable architecture or third-party services?
• How do you typically handle things like message delivery guarantees and offline sync?

I’m trying to understand where people usually draw the line between building in-house vs using existing messaging infrastructure.