Why some websites delay database updates?

Why some websites such as Wikipedia and some advertising networks delaying updating their database after user changes?

For example: A user add the keyword "Experts Exchange", why do it take time to affect the online campaign while they can update the database the same time?
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Mohamed AbowardaSoftware EngineerAsked:
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vopConnect With a Mentor Commented:
ISPs generally cache quite a bit of the web traffic passed by them as this can significantly reduce backbone bandwidth requirements.

The delay is proably a combination of this and the phenomena mentioned by other posters.

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Guy Hengel [angelIII / a3]Connect With a Mentor Billing EngineerCommented:
replication might be 1 reason.
=> some sites are using several spread-over the world databases, so the "update" is not spread immediately to "all" those databases ...
validation another reason
another reason could be that (some) pages are being (re)generated based on content (changes), scheduled or triggered (but not realtime)
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ghodderConnect With a Mentor Commented:
It can also be due to content caching. Lots of larger sites use caches to reduce the load on their databases and improve the speed the pages load when you visit them.

Some pages could be configured to have a longer expiry timeout (e.g. if it is not anticipated that the page would be updated more than once per day) so the caching proxy will continue to serve the out-dated page until it reaches the expiry time.
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khairilConnect With a Mentor Commented:
Other possibility, page like Wikipedia might have to be evaluate first by someone else before it can release the updated information to the mass. After validated all the changes then only the user can access the updated content.
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Ray PaseurConnect With a Mentor Commented:
When unauthenticated clients post comments, a moderator might want to review the comments.  I have a church web site that has an open calendar - anybody that wants to can post anything into the calendar.  But for obvious reasons we have a "gatekeeper" that may delete or modify inappropriate content.

Another possibility would make sense if it takes a long time to generate a dynamic page.  Example: The index.php page is only 100K but the complex queries needed to create the page take 10 seconds to run.  Rather than take a 10-second page load time, you can capture the output buffers and write the index.html version of the index.php page.  Then most clients will see the fast, static page.  A CRON job can periodically delete the index.html page, and the 404 handler can call the index.php page to rebuild the static version.
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Mohamed AbowardaSoftware EngineerAuthor Commented:
Sometimes updating a "number" can delay, like for instance updating campaign budget.
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Mohamed AbowardaSoftware EngineerAuthor Commented:
Another possibility would make sense if it takes a long time to generate a dynamic page.  Example: The index.php page is only 100K but the complex queries needed to create the page take 10 seconds to run.  Rather than take a 10-second page load time, you can capture the output buffers and write the index.html version of the index.php page.  Then most clients will see the fast, static page.  A CRON job can periodically delete the index.html page, and the 404 handler can call the index.php page to rebuild the static version.

In websites like Facebook, usually they have billions of records, why the database updates are instantly and don't take a lot of time to load?
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Guy Hengel [angelIII / a3]Billing EngineerCommented:
it can depend on how much $$$ you can invest on hardware and software for your website ...
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