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

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.