There were many potentially other solutions to this issue. The clients could have buffered writes locally, and done batch updates to MongoDB instead of incrementing each time.

Using a local SQLite, I can do "update counts set val=val+1 where id=?;" ~250K times per second on a Macbook Air. It would've been simpler to just do a local aggregate and then a batch update. That avoids Redis durability issues, or a secondary merge process.

FWIW, a local Postgres can do only about 3200 increments per second.