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Why does it take around 30 secs for the new post to appear on home page of stackexchange?

I just posted my previous post and got back to home page refreshing it.. howver, there was 30 seconds delay in displaying the question.

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This is because StackExchange uses caching extensively to reduce the number of database queries and improve performances. As far as I remember, StackExchange (and all the other trilogy sites) uses Memcached.

Some sections of the site, such as the question listing, are cached for a specific amount of time. The first user causes the queries to be executed and the view fragment to be cached. Subsequent users will hit the cache as long as the cache doesn't expire.

If you post a question or make changes to an existing question, your entry won't be displayed as long as the existing cache doesn't expire and the query to list all questions is executed again. I don't know the exact cache TTL for StackExchange question list, but I suppose it's about 1 minute.

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