The oldest_unacked_message_age vs raw backlog distinction is the right one to lead with, most teams alert on the wrong metric early on. On the ordering-key hot spot problem at 10x, did you consider splitting the hot key onto its own subscription instead of sharding the whole topic? That way a bursty user doesn't stall throughput for everyone else sharing the same ordering key space.
Solid approach for predictable key spaces — isolates the blast radius without touching the whole topic. The catch is when the hot key is unpredictable (bots, sudden high-volume users) — you need tooling to detect and re-route dynamically, which is a different problem.
Relaxing ordering on non-critical event types is usually the safer default at scale. But your suggestion is the right first move before you get there.
Jeongjin Shin
Building small developer and productivity tools.
The oldest_unacked_message_age vs raw backlog distinction is the right one to lead with, most teams alert on the wrong metric early on. On the ordering-key hot spot problem at 10x, did you consider splitting the hot key onto its own subscription instead of sharding the whole topic? That way a bursty user doesn't stall throughput for everyone else sharing the same ordering key space.