We recently hit a peak of 850 Million notifications per second, and 5 billion notifications per day. Here's a blog post on how we do it. Written back when we were at "only" 2 billion notifications per week: https://onesignal.com/blog/rust-at-onesignal/
> We recently hit a peak of 850 Million notifications per second
per second?
From your blog "OnePush is fast - We've observed sustained deliveries up to 125,000/second and spikes up to 175,000/second."
I think you may have a typo. The bandwidth would be incredible too, if it were an unlikely 10 bytes per delivery that would be 8.5 GB/Sec.
It's generally under 4 bytes per delivery, depending on the content, and we have several delivery servers. APNS, for example, doesn't support payloads larger than 4 bytes.
iOS, however, is still quite challenging at scale since there's no batching mechanism for APNS, so even bandwidth becomes a bottleneck. We do web push too, which requires a lot of CPU cycles to encrypt each payload for its recipient.