Hey everyone! FB engineer here who wrote the original code for this and gave the talk during @Scale 2015. There's a recording of our talk here:
<a href="https://youtu.be/ptsCWGZW_P8?t=333" rel="nofollow">https://youtu.be/ptsCWGZW_P8?t=333</a><p>That's a bit more visual and easier to understand. At the end of the day its really just DFS with seen state and selective exploration :)
>The average distance between any two of Facebook’s 1.5 billion users is 4.74 edges. Sorry Kevin. With 1.5 billion users the whole graph can be explored within 5 hops.<p>Nitpicking here, but that does not follow. Starting from a random user and hopping 5 times in all directions you'd expect to explore about half of the graph.
A longer, more involved article from late September has a bit more information: <a href="http://highscalability.com/blog/2015/9/28/how-facebook-tells-your-friends-youre-safe-in-a-disaster-in.html" rel="nofollow">http://highscalability.com/blog/2015/9/28/how-facebook-tells...</a><p>>If a friend is in the same area then a push notification is sent asking if they are OK.<p>One thing that I'd like to understand is how "area" is defined. The limiting factor is, of course, finding out something has happened, but someone at Facebook has to draw a border that encompasses everyone at risk but minimizes those out of harm's way. An earthquake is almost easier - there's (relatively) rapid data on the epicenter and size - but in an event and place like Paris, there have got to be some hard calls made in real time. Who are the people and teams involved there, and how do they make those decisions? I'd love to see that.
> If you are in an area impacted by a disaster Facebook will send you a push notification asking if you are OK.<p>I guess that's great during an earthquake but can be terrible when you're hiding during a terrorist act.
Here's the video from @scale that explains it if you are interested: <a href="https://www.youtube.com/watch?v=ptsCWGZW_P8" rel="nofollow">https://www.youtube.com/watch?v=ptsCWGZW_P8</a>
I wonder what Facebook will say when the governments of the world realise they can use this capability to contact their own citizens - especially if Facebook's continued operation in that country is contingent on its cooperation.<p>'Safety Check' is just the end user experience; the underlying mechanism is a way to propagate messages invasively across a geo-social network. Like any technology, 'good' or 'nasty' depends on how it's used.