I already had question about this but obviously it was not accepted very well, apparently too long when it's actually more information so you could have given me better answer.

Ok, I will be much clearer now. Best possible logic to develop scalable chat in terms of stability, storing/reading messages on chat, updating chat on new message for all users etc.? I have most of this developed, the logic I think I miss is --> check if there is new message and show it for all users. I have this implemented but it crashes the site due to its traffic of 300k-400k people, so that's my main question.

The chat is PHP based and uses Pusher (www.pusher.com) for instant messaging but it lacks what I need because it's more like a websocket.

I'm using hardcoded files to keep messages (want to avoid database as much as possible). It's a no extension type of file, I'm sure you know. I'm getting crash with

$fp = fopen(..., "w"); // pretend ... is the path and filename
fwrite($fp, $msg); //hardcode the message
fclose($fp);

where $msg is the message itself. I'm having 1 file per message. I show last 150 messages = 150 file accesses and reads, yeah it's too much I guess. I have better logic now which I'm pursuing and that is 1 file with last 50-100 messages at all time. Sure it should be much better.

How does it crash, that's the trickiest part because everything seems ordinary, believe me it is difficult to determine what exactly crashes the site, but in like 5 minutes when I try to open the site it's gone, then I put the old content without chat and is back online again.

I'm having jquery post every 1 second to check if there is new message. I'm using timestamp in a special file where I keep the time last message was sent and if ((time() - time in file) <= 2) = reload last 150 messages including the last one. Too much input/output, write/read or however to say it I think is what crashes the site.