how does facebook push their changes to the live site

From this video https://www.facebook.com/photo.php?v=778890205865 I understood that Facebook moves their changes to their live site on a daily basis. Some changes are visible, whereas most of them are not visible to the end user. With a userbase of just over 700 million how do they manage to do it without any glitches. 开发者_JS百科Can someone throw light on this? I have read about the Gatekeeper which allows Facebook to run different code to different set of users. Is there any other technology involved? thanks.

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From what I've read, they push code to their internal users first, so that staff are using the 'alpha' code.

It's then pushed to the beta teir (sunday I think) which lets developers test against the beta code and report any issues

The beta tier is then pushed live on Tuesday night to the main website