Will users be able to create hierarchies of circles (circles made from multiple circles)?
There are definitely good use cases for this, but we worry about the complexity it would introduce. This might be a great "power-user" feature to build using our APIs (once they're ready, heh).
What were the most difficult specific technical challenges you faced?
Trading off consistency vs. availability is always challenging, and even more so in social applications where your actions affect other users, often in other data centers. For instance, adding/removing someone from a circle impacts (among other things): which posts they can see, the counts of people on your (and their) profile, suggestions (for potentially many people), and so on. Clearly some of these changes need to happen immediately/ASAP, whereas others could be a bit stale and that's ok. Picking the right trade-offs so our systems are fast and robust but users rarely notice any problems was (and continues to be) challenging, and in some cases required some very clever tricks in the backends.
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