Postgres LISTEN/NOTIFY actually scales

dietr1ch

I recall that in the first release that supported LISTEN/NOTIFY there was a performance issue around it (poor locking IIRC), which today the "bad post" mentioned in here corrects in a errata just after their first paragraph. Since the correction apparently dates from May 8th, I think that a post from July 24th might want to acknowledge that the popular post asserting this feature doesn't (didn't?) scale was not made in bad faith or was even wrong about their claims at the time.

Latty

One way it explicitly doesn't scale (unless something has changed since I last checked and my quick search failed me) is the hard limit on 8000 bytes of data in a notification. If your notification doesn't make sense to exist as a row (where you can just give an ID), then that makes it hard to use. I had a web game and the events were transient descriptions of changes in state that didn't make sense to store in the database, and could be bigger than that, so it didn't work for that use case.

jerf

"Scale" isn't a binary, it's a continuum. "Scales to 60K/s" can be 5 orders of magnitude more than one system needs and 5 orders of magnitude too small for another. Personally I'd knock the general "premature optimization" off the list of "most common developer errors" and put in its place "using techs with the wrong scaling factors". If you use something too small and you exceed its needs, the failure is obvious, but the other way around is a problem too. Bringing in the overhead and management issues of the super scalable techs, as well as their limitations they impose so that they can scale, to a system that would actually be better off with a richer model whose richness prevents it from scaling but would save a lot of effort is also a bad choice. The ceiling of LISTEN/NOTIFY is small enough that you need to pay attention, and I personally like to have at least an order of magnitude of slack left over even after my most pessimistic load numbers are accounted for, but it's still plenty for a lot of projects, and the integration with the rest of the DB, its availability, its not being another service you have to devops, it's definitely not something that should be simply dismissed out of hand as an option. Even the original 2K/s number they cite is a lot of messages for some systems that are more properly measured in seconds per message.

nzoschke

I continue to love DBOS for how it just leverages Postgres (and now SQLite) properly. It's effortless to drop into an existing CRUD stack. Once you start down the "durable workflows" path, you start seeing them everywhere. My latest experiments are treating individual emails as durable workflows, where you, the people you're communicating with, agents and tools like GitHub or Attio all take turns in the flow. https://housecat.com/blog/gmail-durable-workflows-sandbox-vm

mamcx

Is the optimization only possible using dbos? is not clear to me if this mean a way to tune normal PG

jrochkind1

If I understnad right, they are saying it scales (to theri needs) with a custom patch to pg changing their semantics, right? It does seem interesting, and possibly welcome if there were a configuration option or even a way to set individual notifies as serialized or not.

acaloiar

Much of the FUD around using LISTEN/NOTIFY in production comes from people who have never done so. It of course has its limits, and we should remain aware of them. Much like the limits of every piece of tech in our stacks. Choose database queue technology https://news.ycombinator.com/item?id=37636841

tjadfsaj

Sure it "scales" if clients batch commands to amortize the per-request cost. The word "scale" does a lot of load-bearing, maybe it's just not a useful or productive word in practice.

sandeepkd

I think a lot of these kind of posts are standalone assessment of your problems, understanding and solutions. Its debatable to term something as lack of expertise if one is trying to work with default settings of a tool and expecting a certain performance. Everyone is doing a continuous learning with the failures. 1. What I find interesting is that the experiment seems to be using a DB server with 96 cores, 384 GB RAM ( https://github.com/dbos-inc/dbos-postgres-benchmark/blob/mai... ). This is very critical part of any such experiment, it should have been called out. The database is vertically scalable and that too has its limits 2. Who is making connection, and from where has its own impact on performance and overall latency 3. 60k may seem big number, however in real world the things which bring the systems down are the bursts of traffic, not the regular traffic. Personally I would never start with such a big server unless I am a big business. Its > 100K cost for one production DB cluster if I include read replicas and cross region redundancy

d9127acbd6fe281

So in summary, Postgres NOTIFY actually scales if you make your application not write to the database? That's a bit of a tough sell...

dang

Related, presumably: Postgres LISTEN/NOTIFY does not scale - https://news.ycombinator.com/item?id=44490510 - July 2025 (321 comments)

newswangerd

I went through this process when I was designing the sync server for Digital Carrot. In the end, I decided to just go with the simplest solution possible. In my case it's just a barebones Go gRPC service that uses an in memory channel to send notifications between connected clients. The reality is that this simple Go server will scale up to about 1000 simultaneously connected customers on about 2gb of RAM. I don't expect to have more than that many paying customers, and if I do I can always just throw a bigger VM at the problem. Engineers love to over complicate things in the name of infinite scalability, when in reality you can save a lot of time and effort by just understanding the scope of the actual problem you're trying to solve. Fingers crossed that this will become an issue for me some day, but until then most of us just don't need to worry about it!

qphe95

I'm reading the code for this and I'm like yeah it doesn't scale. Like forget about debugging this code, does anybody even know what it is doing?

vhiremath4

I once was the CTO of a company that serviced about 100k requests per day across all our services. We grew to millions and eventually 10's of millions, but, somewhere along the way, an engineer on our team decided he wanted to build a queue off LISTEN/NOTIFY semantics in order to take advantage of strong consistency with the rest of our data model, which seemed reasonable given LISTEN/NOTIFY is not that hard to understand and we did need consistency guarantees for this workflow and this would remove the need for yet another place data got stored and transfered. In practice, this eventually ended up being very awkward because extending the functionality (since we had "built" it) and had to work around internal pg semantics (we should have just moved off much sooner). It also did not scale well. We ended up getting a ton of disk contention on our RDS instance in non-obvious ways, and the vacuum runs on that table was a nightmare. Additionally, it was hard to get other engineers to really debug and take ownership of the system because they automatically viewed a queue (very easy to understand) implemented in a foreign way (off pg internals) as something "scary". It was emotional, not rational, but we are emotional beings, and I do not blame them. These were good engineers with a lot of other things on their plates. Obviously, this is all hand-wavy without discussing the internal schema, indeces, etc. that we had set up, but my main takeaway with core technology from this experience was to always reach for the dumb, expected, simple thing. Even if it adds another moving piece in the infra stack. Unless I need very strong data consistency guarantees, it's always better to use something like SQS, Redis queues, etc. where the understanding is that it is just a queue (or at least the API contract suggests simplicity), and then everything needs to work around it. The fewer mechanistic responsibilities per core data store, the better in my experience.

b-man

related: https://pgdog.dev/blog/scaling-postgres-listen-notify

elendilm

The tested machine appears big and with 96 vcpu + 384 gb yielding 20k writes sounds too low. Jumping to 60k is very good. But still too small a throughput for what the machine is capable of. Batching ensures you run at cpu & memory speeds and only pay significant latency for the flush - which usually linux kernel coalesces well if concurrent.

villgax

Even AI generated art needs taste, looks so bland & off-point.

phamilton

Just sharing a data point and experience. We had a lot of success with LISTEN/NOTIFY when we paired it with a Rust graphql subscription broker. 10s of thousands of subscriptions, but only 3 or 4 LISTEN connections (one for each host). All changes would be pushed out to all hosts, who would each manage the actual user subscriptions and choose what to actually publish. This worked super well. In general, moving from hundreds of Ruby or Node hosts to just a few Rust hosts just allows so many simplifications and things that "don't scale" to actually work quite well.

luciana1u

turns out the database already had pub/sub, it scales fine, and the entire message broker setup was just emotional support infrastructure