These don't occur on the database server, though... This merely affects the number of rows currently claimed.
> The dequeuer needs to know which tasks to "claim", so this requires some form of locking. Eventually this becomes a bottleneck.
These are just try locks, though-- the row locks are not contended. The big thing you run into is having lots of snapshots around and having to skip a lot of claimed rows for each dequeue.
> What happens if you get 100x traffic? Popularity spikes can do it, so can attacks.
If you get 100x the queueing activity for batch jobs, you're going to have stuff break well before the queue. It's probably not too easy to get 100x the drain rate, even if your queue system can handle it.
This scales well beyond 100M batch tasks per day, which gets you to 1M users with 100 tasks/day each.