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Comment by hosh | Hacker News Reader

How do you handle things when no listeners are available to be notified?

Presumably there'd be a messages table that you listen/notify on, and you'd replay messages that weren't consumed when a listener rejoins. But yeah, this is the overhead I was referencing.

Yep, but practically speaking, you need those records anyway even if you're using another queue to actually distribute the jobs. At least every system I've ever built of a reasonable size has a job audit table anyway. Plus it's an "Enterprise Feature™" so you can differentiate on it if you like that kind of feature-based pricing

Postgres's LISTEN/NOTIFY doesn't keep those kinds of records. The whole point of using SKIP LOCK is so that you can make updates to rows to keep those kinds of messages with concurrent consumers.

Yes. I'm saying you'll manually need to insert some kind of job audit log into a different table. Cheers

With the way LISTEN/NOTIFY works, Postgres doesn't keep a record of messages that are not sent. So you cannot replay this. Unless you know something about postgresql that I don't know about.

You insert work-to-be-performed into a table, and use NOTIFY only to wake up consumers that there is more work to be had. Consumers that weren't there at the time of NOTIFY can look at the rows in the table at startup.

I see. So the notify is just to say there is work to be performed, but there is no payload that includes the job. The consumer still has to make a query. If there isn’t enough work, the queries should come back empty. This saves from having to poll, but not a true push system.

as far as I can tell NOTIFY is fanout, in the sense that it will send a message to all the LISTENing connections, so it wouldn't make sense in that context anyway. It's not one-to-one, it's about making sure that jobs get picked up in a timely fashion. If you're doing something fancier with event sourcing or equivalent, you can send events via NOTIFY, and have clients decide what to do with those events then.

Quoth the manual: "The NOTIFY command sends a notification event together with an optional “payload” string to each client application that has previously executed LISTEN channel for the specified channel name in the current database. Notifications are visible to all users."

Notify can be triggered with stored procedures to send payloads related to changes to a table. It can be set up to send the id of a row that was inserted or updated, for example. (But WAL replication is usually better for this)

Broadcasting the id to a lot of workers is not useful, only one of them should work on the task. Waking up the workers to do a SELECT FOR UPDATE .. SKIP LOCKED is the trick. At best the NOTIFY payload could include the kind of worker that should wake up.

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