

Notification Strategies for Social Networks (w/ math) - snakelemma
http://20bits.com/articles/notification-strategies-for-social-networks/

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jdrock
I'd be curious to see if Facebook and the rest use similar strategies to
reduce the intra-datacenter processing and bandwidth, and if the savings
achieved are worth anything significant.

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jfarmer
You mean, letting the new feature go viral naturally rather than just blasting
it to every person out there?

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jdrock
That's part of it. I was looking at it more from a computation/energy-
efficiency perspective.

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jfarmer
Yeah, I'd even be interested in knowing if Facebook breaks down their server
costs to that level of detail, e.g., the operational cost of Algorithm A
versus Algorithm B.

Since capital expenditures seems to be a big part of their costs they might
just be that detailed.

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bemmu
What does that greedy algorithm work on, given that Facebook doesn't allow you
to store a copy of the friend graph? At the point of notification, all you
know is a list of user IDs that have your application, and possibly how many
friends each one has.

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jfarmer
Well, first, you can bend the rules. How does Facebook know whether you're
storing the graph or not? They won't unless you violate the TOS in some other,
more overt way.

Second, Facebook's rules say you can't store it for more than 24 hours. The
current copy of the graph is available via API calls -- just call getFriends
for each user with a valid session.

Third, if your app is big enough you can approximating it by seeing who sends
notifications to whom, who clicks on newsfeed items, etc.

Fourth, although I used Facebook as an example, this applies to any social
network with a notification mechanism. I just used Facebook because it's
easier to understand -- they limit the number of notifications at the API
level.

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hawk
one reason facebook is so viral is that it largely implements this naturally
by its decomposition. that is, randomly choose seeds within a dense section of
the graph (an island, or in facebook lingo, a network). it will quickly spread
to the whole island, and you can grow virally island by island. this is a
decent approximation under certain assumptions to an optimal centrality
metric.

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andrewljohnson
Very interesting and clear article.

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jfarmer
Thanks!

