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Thank you, appreciate the kind words! What boxes are you looking to check?

Yes, I'm not a fan of the RabbitMQ dependency either - see here for the reasoning: https://news.ycombinator.com/item?id=39643940.

It would take some work to replace this with listen/notify in Postgres, less work to replace this with an in-memory component, but we can't provide the same guarantees in that case.

2 comments

I come to this only as an interested observer, but my experience with listen/notify is that it outperforms rabbitmq/kafka in small to medium operations and has always pleasantly surprised me. You might find out it's a little easier than you think to slim your dependency stack down.

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."

Boxes-wise, I'd like a management interface at least as good as the one Sidekiq had in Rails for years. Would also need some hard numbers around performance and probably a bit more battle-testing before using this in our current product.