[dhpye]

For a notification-heavy app, I'm surprised at the lack of consideration given to Queue/Topic Messaging servers. 5B notifications/month isn't quite 2k/s, which really isn't all that onerous. Routing within messaging apps would seem like a natural fit.

Or perhaps I'm missing something?

[robotadam]

We make heavy use of queue and internal messaging systems. This discussion was solely about the data storage layers, which always causes us more headaches than the messaging/queuing systems.

[moe]

You should check out redis if you haven't yet.

It's ideal for the counter use-case and with bit of smart thinking you can fit a surprising amount of data into a 32G or 64G machine (and then you can always shard).

[dhpye]

Okay, that makes more sense. It just sounds like there's a lot of touching going on. So, what % of your notifications require synchronous db writes?