Ask HN: How Does WhatsApp Work?

I recently became curious about how WhatsApp - and other large scale messaging services - are actually implemented. The technologies I work with on a day-to-day basis, mostly relational databases, such as MySQL, and document databases, such as DynamoDB, seem completely unsuited for the task.

What technologies and algorithms are used at WhatsApp scale?

Are there any good papers / talks I should look at?