[jcoby]

You will know when it becomes a problem. The helpdesk gets flooded. Servers go down. You get white pages and 500 internal server error pages. The entire site folds because a mouse farts.

You add new servers and they get crushed. You scrutinize every line of code and fix algorithms and cache whatever is possible and you still can't keep up. You look at your full stack configuration and tune settings. You are afraid of growth because it will bring the site down. THEN you know it's time to consider switching frameworks. And even then I would try to find the core of the problem and rewrite that one piece.

Rewriting an app in a new framework can kill a company. Be leery of starting over. I personally know of one company who started over some 4+ years ago because the old app was too hard to maintain and only started rolling out the new app last year to extremely poor reception (even with about 1/2 the features of the old app). The reception was so bad that they had to stop rolling it out until it was fixed. They could have easily spent a year improving the old app and would be miles ahead.

That's not to say that all problems are due to the framework either. I've had web servers go unresponsive for 20+ minutes because apache went into the swap of death because KeepAlive was set to 15s and MaxClients was set to a value that would exceed available RAM. The quickest solution was to cycle the box. This was 10 years ago though and I think I had a total of 1GB of ram to work with.