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Re:Redis and SQLite: Why wait for good enough? by salsakran on Thursday February 03, 2011 @12:36PM (#2175649)
As far as "Atomic Datasets", he's referring to the ability to send off an atomic operation against a data structure. Yes, you can fake this by normalizing and storing it in a table (or any number of other permutations), but for counters, the ability to fire off an atomic increment is very useful.
Re:Redis and SQLite: Why wait for good enough? by SkyMarshal on Thursday February 03, 2011 @02:14PM (#2176261)
Your 'atomic operation' sounds a lot like a transaction. Or am I misunderstanding?
Re:Redis and SQLite: Why wait for good enough? by salsakran on Thursday February 03, 2011 @02:36PM (#2176386)
Not really.

If you have a bunch of counters, you don't want a hot counter to require a transaction every time you want to increment a page view. I'm sure there's a solution in high end DBs, but in mysql/postgres UPDATE somecounter SET value = value + 1 typically ends up being very painful for any significant write load. With Redis and Mongo, there are "atomic" increment operations that do a +X on a given field of a list or hash that is very fast.

Re:Redis and SQLite: Why wait for good enough? by salsakran on Friday February 04, 2011 @06:29PM (#2181812)
see twitter's Rainbird for what a production scale solution looks like. 100k writes per second, with a huge skew in which rows get hit.