“Serialize writers by acquiring a special lock that we hold till after commit.”
Recall.ai discovered through three outages that Postgres LISTEN/NOTIFY acquires a global exclusive lock on the entire database during every transaction commit that includes a NOTIFY. This means all commits serialize through a single bottleneck. The telltale was paradoxical: load spikes paired with plummeting CPU, disk, and network usage, which is the classic fingerprint of lock contention rather than resource exhaustion. They ripped it out and moved notification logic to the application layer, which is where it probably belonged in the first place.