[pbadams]

I agree in the typical case, but they support concurrent add/delete one of their index options. Handling consistency/contention for modifying whatever graph/tree/etc structure they are using is probably nontrivial, and the resulting cache invalidations would also likely affect the QPS.

P.S. Great work on your site, by the way - it's a really inspiring project!

[marginalia_nu]

Seems to me you can do that in a way that ensures low contention between consumers by using a read-biased MRSW lock. It's not free such a construction, but it really shouldn't eat into your read performance all that much. You're adding hundreds of nanoseconds to your query time by acquiring and releasing a lock. Unless you're already serving millions of queries per second per thread, this is piss in the ocean.