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When I browse SO site I refresh page many times (enter new question, etc.) However, when someone comment my answer I can see the top-bar-envelope-notification 15 minutes after comment was made. Why I can't see this notification immediately after the comment was made? Is there some delay time fixed, or is it just mine browser-cache-related problem?

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I stopped paying attention to the envelope thing ages ago. I don't know exactly how it works, but it almost never lights up for me, even when I have new badges, upvotes, or comments. – Graeme Perrow Sep 21 '09 at 17:58

No, it's not your browser-cache. It is how the system internally caches data from the database.

What you see on the screen isn't always the absolute latest data stored in the database. For performance reasons, Stack Overflow caches various types of data in memory, only fetching new data from the database periodically... the refresh frequency depending on how often the data changes and how important it is that the data be up-to-date.

Every so often, the system will re-check the comments database to see if you need to be notified, but it does not happen in real-time.

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