
How Facebook's Safety Check Works - xasos
http://highscalability.com/blog/2015/11/14/how-facebooks-safety-check-works.html
======
xxbondsxx
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:
[https://youtu.be/ptsCWGZW_P8?t=333](https://youtu.be/ptsCWGZW_P8?t=333)

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 :)

~~~
marcusgarvey
How does FB decide which disasters qualify for the safety check and which do
not?

~~~
tomschlick
As described in a post by Zuckerberg, before the Paris attack it was only
activated for natural disasters. Paris was the first time it was activated for
human disasters and they will be doing it more in the future.

~~~
omginternets
Respectfully, that doesn't answer the question. What is the current policy for
determining use?

~~~
rmc
The real reason, which they probably can't say, is how much media attention it
gets in the USA.

~~~
omginternets
I'm entertaining the notion that this is a way to justify some new privacy-
invading feature. I'd love to be wrong, and I'm open to a statement from a FB
employee, if such a thing were possible.

~~~
eggie
Or perhaps it motivates the idea that if you do not use facebook you can never
be completely "safe".

------
haraldurt
>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.

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.

~~~
fnbr
Your post is technically correct, but I think that you'd be likelier to
explore more- say, 80% of the graph. I suspect that there are small pockets of
extremely unconnected people which creates a long tail, distorting the mean.

~~~
eggie
Unless the data can somehow be shared how is anyone to know what the structure
is? There is no point in speculating...

------
Amorymeltzer
A longer, more involved article from late September has a bit more
information: [http://highscalability.com/blog/2015/9/28/how-facebook-
tells...](http://highscalability.com/blog/2015/9/28/how-facebook-tells-your-
friends-youre-safe-in-a-disaster-in.html)

>If a friend is in the same area then a push notification is sent asking if
they are OK.

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.

~~~
chipperyman573
They probably are very generous - I'd imagine the entire country was defined
as the "area". The system doesn't work very well if there's false negatives
and people don't see it, and if someone sees it but shouldn't all they have to
do is hit that they're OK.

~~~
smackfu
France is a pretty big country. If you are in Nice, you are a nine hour drive
from Paris. It would seem a bit silly to ask if you were okay, and not someone
in Belgium who is much, much closer.

~~~
cortesoft
It is only about 248 thousand square miles... which is smaller than Texas.

------
ceph_
> The average distance between any two of Facebook’s 1.5 billion users is 4.74
> edges

Pretty crazy to think about.

------
konstruktor
> If you are in an area impacted by a disaster Facebook will send you a push
> notification asking if you are OK.

I guess that's great during an earthquake but can be terrible when you're
hiding during a terrorist act.

~~~
xxbondsxx
If it's any comfort, most people have push notifications turned off for the
main Facebook app (understandably since we send quite a few of notifications).
However anecdotally some of the survivors of the attacks mentioned that they
got phone calls from concerned friends / families while hiding in the
Bataclan, which was understandably stressful.

~~~
konstruktor
If the safety of your products depends on disabling features, you should stay
out of anything remotely safety critical.

~~~
TeMPOraL
What the hell, this can be said about anything. As GP provides an example for,
even an unexpected _phone call_ can be unsafe.

~~~
konstruktor
In my opinion, "other things are just as bad" is as good an argument as "if
you turn it off, it's not dangerous".

I still think it's not smart to send push notifications in an active shooter
situation for a feature that is made to give people who are not involved peace
of mind. If you consider such risks, and not get defensive right away, you
will find that solutions can be pretty simple, e.g. a banner inside the app,
instead of a notification.

~~~
TeMPOraL
I think shooting was aready over before people over at Facebook even knew
something is going on. I imagine they wouldn't even turn it on before the
information hit mainstream news.

------
vjeux
Here's the video from @scale that explains it if you are interested:
[https://www.youtube.com/watch?v=ptsCWGZW_P8](https://www.youtube.com/watch?v=ptsCWGZW_P8)

------
flashman
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.

'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.

~~~
xxbondsxx
Usually this is enforced at the cellphone carrier level. The US has the amber
alert protocol which _also_ has a function for the president to dispatch any
message (and can only be used by the president iirc), and that level of
message can't be turned off via settings.

Even in our best countries FB usage is never 100%, so cellphone / landlines
makes a bit more sense.

~~~
dragonwriter
> The US has the amber alert protocol which also has a function for the
> president to dispatch any message (and can only be used by the president
> iirc), and that level of message can't be turned off via settings.

This is the Wireless Emergency Alert system, which carries Amber Alerts,
Alerts about imminent threats to life/safety, and Presidential Alerts. The
last (and only the last) can't be disabled in settings, because the
legislation requiring support for the capacity required that Presidential
alerts not be disabled. (The system is basically the mobile-device equivalent
of the broadcast Emergency Alert System.)

------
cft
Incidentally that article implies that Facebook contstantly tracks your geo
location even when you do not expressly check in.

~~~
jamesfe
We are not OK with the NSA doing this and (theoretically) not viewing the
data.

We are OK with Facebook doing this and exploiting the data for monetary and
market advantages.

~~~
mason55
Generally people are ok with Facebook doing it because they can opt out.

If you notice, the times people really get upset about companies tracking
their personal information is if it's either not disclosed or it's done in a
shady manner.

Also, capitalist societies tend to treat state actors differently than private
corporations. It's much easier to choose a new corporation to interact with
and the state tends to have a lot more personal/private data about you (which
you're required to share with them).

~~~
jonesb6
Yeah ok but how many end-users are conscious enough of the situation to OPT-
OUT of such an intrusive policy?

On the same vein as accepting every terms and conditions that comes your way,
I think most people are severely uninformed about this. It's like how many
people believe software engineering is "magic", they believe Facebook or the
government will protect them with "magic". Hint: there is no magic.

~~~
mason55
> _Yeah ok but how many end-users are conscious enough of the situation to
> OPT-OUT of such an intrusive policy?_

Well, I meant opt-out as in "not use". Plenty of people don't use Facebook
because of the privacy implications. And as another use mentioned, this is the
reason that people are so upset about Facebook's shadow profiles, because it
removed your ability to opt out of their tracking.

------
ldesegur
url seems to be down. Any url for the cached content?

------
bekimdisha
WhatsApp now uses Facebook Messenger's protocol ... FYI

~~~
tyggy
how do you know?

