How to store information about new private messages in a chat?

Question

Chats

For "self-learning" I decided to write a chat using PHP, MYSQL, JS, AJAX. In general, there are "dialogues", roughly speaking like Vkontakte. The server is polled with an ajax request.
I store information about new messages in a file on the server. That is, when a message is sent and when it is read, the file is overwritten. I'm afraid with a large online performance drop and errors.
I think you can increase the number of files, each for a specific group. Or is it better to connect to the database every request and look for new messages there?

Answer 1

Write as quickly as possible, the main thing to keep in mind is that if you want to transfer to a database or somewhere else, you didn’t have to rewrite the entire code, but only the part of the code that is responsible for storing data.