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NOTIFY is transaction-safe: if the NOTIFY-ing transaction rolls back, the NOTIFY will not be delivered. See the discussion here ("NOTIFY interacts with SQL transactions in some important ways..."):

https://www.postgresql.org/docs/9.6/static/sql-notify.html

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Yup, my bad, thanks. I confused the two problems I worked through in working through my use case. The transaction safety one was not the NOTIFY version of the project. That had different issues, which I need to go back and look at my notes for. My memory is a bit hazy, but IIRC we were seeing notifications dropped under high velocities and needed delivery guarantees.

I seem to remember that when it was first implemented there was a fairly small buffer for queued messages. That might have contributed to your problem. The docs say it's 8GB by default now, which is probably sufficient.

The original NOTIFY also didn't have a payload (that was added in PG9) which made it much harder to know what you were notified about.

That one simple addition made pre-9.0 NOTIFY and post-9.0 NOTIFY completely different beasts.

Messages are limited to 8KB, so if the buffer is 8GB, that's a lot of messages that can be queued up.

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