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Story
Postgres LISTEN/NOTIFY does not scale
Postgres LISTEN/NOTIFY was a consistent pain point for Oban (background job processing framework for Elixir) for a while. The payload size limitations and connection pooler issues alone would cause subtle breakage.

It was particularly ironic because Elixir has a fantastic distribution and pubsub story thanks to distributed Erlang. That’s much more commonly used in apps now compared to 5 or so years ago when 40-50% of apps didn’t weren’t clustered. Thanks to the rise of platforms like Fly that made it easier, and the decline of Heroku that made it nearly impossible.

I didn’t realize Oban didn’t use Mnesia (Erlang built-in).
Very very few applications use mnsesia. There’s absolutely no way I would recommend it over Postgres.
can you explain why?
I have heard the mnesia is very unreliable, which is a damn shame.

I wonder if that is fixable, or just inherent to its design.

My understanding is that mnesia is sort of a relic. Really hard to work with and lots of edge / failure cases.

I'm not sure if it should be salvaged?

How did you resolve this? Did you consider listening to the WAL?
We have Postgres based pubsub, but encourage people to use a distributed Erlang based notifier instead whenever possible. Another important change was removing insert triggers, partially for the exact reasons mentioned in this post.
> Another important change was removing insert triggers, partially for the exact reasons mentioned in this post.

What did you replace them with instead?

In app notifications, which can be disabled. Our triggers were only used to get subsecond job dispatching though.
Distributed Erlang if application is clustered, redis if it is not.

Source: Dev at one of the companies that hit this issue with Oban

I'd be interested as to how dumb-ol' polling would compare here (the FOR UPDATE SKIP LOCKED method https://leontrolski.github.io/postgres-as-queue.html). One day I will set up some benchmarks as this is the kind of thing people argue about a lot without much evidence either way.

Wasn't aware of this AccessExclusiveLock behaviour - a reminder (and shameless plug 2) of how Postgres locks interact: https://leontrolski.github.io/pglockpy.html

Polling is the way to go, but it's also very tricky to get right. In particular, it's non-trivial to make a reliable queue that's also fast when transactions are held open and vacuum isn't able to clean tuples. E.g. "get the first available tuple" might have to skip over 1000s of dead tuples.

Holding transactions open is an anti-pattern for sure, but it's occasionally useful. E.g. pg_repack keeps a transaction open while it runs, and I believe vacuum also holds an open transaction part of the time too. It's also nice if your database doesn't melt whenever this happens on accident.

An approach that has worked for me is to hash partition the table and have each worker look for work in one partition at a time. There are a number of strategies depending on how you manage workers. This allows you to only consider 1/Nth of the dead tuples, where N is the number of partitions, when looking for work. It does come at the cost of strict ordering, but there are many use cases where strict ordering is not required. The largest scale implementation of this strategy that I have done had 128 partitions with a worker per partition pumping through ~100 million tasks per day.

I also found LISTEN/NOTIFY to not work well at this scale and used a polling based approach with a back off when no work was found.

Quite an interesting problem and a bit challenging to get right at scale.

Can't change the number of partition dynamically.

Additional challenge if jobs comes in funny sizes

If there were a toy or other public implementation of this, I would love to see it.
My colleague did some internal benchmarking and found that LISTEN/NOTIFY performs well under low to moderate load, but doesn't scale well with a large number of listeners. Our findings were pretty consistent with this blog post.

(Shameless plug [1]) I'm working on DBOS, where we implemented durable workflows and queues on top of Postgres. For queues, we use FOR UPDATE SKIP LOCKED for task dispatch, combined with exponential backoff and jitter to reduce contention under high load when many workers are polling the same table.

Would love to hear feedback from you and others building similar systems.

[1] https://github.com/dbos-inc/dbos-transact-py

I use polling with back off up to one minute. So when a workload is done, it immediately polls for more work. If nothing found, wait for 5 seconds, still nothing 10 seconds, ... until one minute and from then on it polls every minute until it finds work again and the back off timer resets to 0 again.
With that experience behind you, would you have feedback for Chancy[1]? It aims to be a batteries-included offering for postgres+python, aiming for hundreds of millions of jobs a day, not massive horizontal worker scaling.

It both polls (configurable per queue) and supports listen/notify simply to inform workers that it can wake up early to trigger polling, and this can be turned off globally with a notifications=false flag.

[1]: https://github.com/tktech/chancy

Have you played with pgmq? It's pretty neat: https://github.com/pgmq/pgmq
Another thing for @leontrolski to add to the benchmarks - which I cannot wait to read.
There's a pretty cool solution built on pgmq called pgflow:

https://www.pgflow.dev/concepts/how-pgflow-works

I'll take the shameless plug. Thank you for putting this together! Very helpful overview of pg locks.
I appreciate this post for two reasons:

* It gives an indication of how much you need to grow before this Postgres functionality starts being a blocker.

* Folks encountering this issue—and its confusing log line—in the future will be able to find this post and quickly understand the issue.

Sounds like ChatGPT appreciated the post
  • acdha
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If you think they’re a bot, flag and move on. No need for a derail about writing style.
This is part of the basis for Supabase offering their realtime service, and broadcast, rather than supporting native LISTEN/NOTIFY. The scaling issues are well known.
Right, plus there's character limitations (column size). This is why I prefer listening to the Postgres WAL for database changes:

https://github.com/cpursley/walex?tab=readme-ov-file#walex (there's a few useful links in here)

I found recently that you can write directly to the WAL with transactional guarantees, without writing to an actual table. This sounds like it would be amazing for queue/outbox purposes, as the normal approaches of actually inserting data in a table cause a lot of resource usage (autovacuum is a major concern for these use cases).

Can’t find the function that does that, and I’ve not seen it used in the wild yet, idk if there’s gotchas

Edit: found it, it’s pg_logical_emit_message

One annoying thing is that there is no counterpart for an operation to wait and read data from WAL. You can poll it using pg_logical_slot_get_binary_changes, but it returns immediately.

It'd be nice to have a method that would block for N seconds waiting for a new entry.

You can also use a streaming replication connection, but it often is not enabled by default.

I think replication is the way to go, it’s kinda what it’s for.

Might be a bit tricky to get debezium to decode the logical event, not sure

Sure, but the replication protocol requires a separate connection. And the annoying part is that it requires a separate `pg_hba.conf` entry to be allowed. So it's not enabled for IAM-based connections on AWS, for example.

pg_logical_slot_get_binary_changes returns the same entries as the replication connection. It just has no support for long-polling.

For node.js users there is postgres.js that can listen to the Postgres WAL and emit node events that can be handled by application code.
Yeah until vendors butcher Postgres replication behaviors and prevent common paths of integrating these capabilities into other tools. Looking at you AWS
My kneejerk reaction to the headline is ‘why would it?’.

It’s unsurprising to me that an AI company appears to have chosen exactly the wrong tool for the job.

Sounds like a deliberate attempt to avoid spinning up Redis, Kafka, or an outbox system early on.. and then underestimated how quickly their scale would make it blow up. Story as old as time.
I find the opposite story more true: additional complexity in the form of caching early, for a scale that never comes. I've worked on one too many sprawling, distributed systems with too little users to justify it.
Kafka head of line blocking sucks.
Yeah I have no idea whether it would. But I'd load test it if it needed to scale.

SQS may have been a good "boring" choice for this?

Interesting. What if you just execute `NOTIFY` in its own connection outside of / after the transaction?
You lose transactional guarantees if you notify outside of the transaction though
Yeah, but pub/sub systems already need to be robust to missed messages. And, sending the notify after the transaction succeeds usually accomplishes everything you really care about (no false positives).
What happens when transaction succeeds but the execution of NOTIFY fails if it's outside of transaction, in it's own separate connection?
For reliability, you can make the recipient poll the table(s) of record for relevant state and use the out-of-band notification channel as a latency-reducer. So, the poller is eventually consistent at some configured polling interval, but opportunistically can respond much sooner when told to check again ahead of the next scheduled poll time.

In my experience, this means you make sure the polling solution is complete and correct, and the notifier gets reduced to a wake-up signal. This signal doesn't even need to carry the actionable change content, if the poller can already pose efficient queries for whatever "new stuff" it needs.

This approach also allows the poller to keep its own persistent cursor state if there is some stateful sequence to how it consumes the DB content. It automatically resynchronizes and the notification channel does not need to be kept in lock-step with the consumption.

fwiw - that's what Oban did for the most part. It sent a signal to a worker that there was a new job to pick up and work on. At scale, even that was an issue.
  • Groxx
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The same thing that happens if the notified process dies suddenly.

If you're not handling that, then whatever you're doing is unreliable either way.

Isn’t it standard practice to have a separate TCP stream for NOTIFY or am I mistaken
You mean for LISTEN?
  • nick_
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My thought as well. You could add notify commands to a temp table during the transaction, then run NOTIFY on each row in that temp table after the transaction commits successfully?
This is roughly the “transactional outbox” pattern—and an elegant use of it, since the only service invoked during the “publish” RPC is also the database, reducing distributed reliability concerns.

…of course, you need dedup/support for duplicate messages on the notify stream if you do this, but that’s table stakes in a lot of messaging scenarios anyway.

  • foota
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Wouldn't you need to then commit to remove the entries from the temp table?
No, so long as the rows in there are transactionally guaranteed to be present or not, a sweeper script can handle removing failed “publishes” (notifys that didn’t delete their row) later.

This does sacrifice ordering and increases the risk of duplicates in the message stream, though.

LISTEN/NOTIFY was always a bit of a puzzler for me. Using it means you can't use things like pgbouncer/pgpool and there are so many other ways to do this, polling included. I guess it could be handy for an application where you know it won't scale and you just want a simple, one-dependency database.
> I guess it could be handy for an application where you know it won't scale and you just want a simple, one-dependency database

That's where we use it at my work. We have host/networking deployment pipelines that used to have up to one minute latency on each step because each was ran on a one-minute cron. A short python script/service that handled the LISTENing + adding NOTIFYs when the next step was ready removed the latency and we'll never do enough for the load on the db to matter

Out of curiosity: Would appreciate if others can share what other things like AccessExclusiveLock should postgres users beware of?

What I already know

- Unique indexes slow inserts since db has to acquire a full table lock

- Case statements in Where break query planner/optimizer and require full table scans

- Read only postgres functions should be marked as `STABLE PARALLEL SAFE`

Can you provide more details? Inserting with unique indexes do not lock the table. Case statements are ok in where clause, use expression indexes to index it
Rls and triggers dont scale either
Yeah, I'm going to remove triggers in next deploy of a POS system since they are adding 10-50ms to each insert.

Becomes a problem if you are inserting 40 items to order_items table.

> Yeah, I'm going to remove triggers in next deploy of a POS system since they are adding 10-50ms to each insert.

Do you expect it to be faster to do the trigger logic in the application? Wouldn't be slower to execute two statements from the application (even if they are in a transaction) than to rely on triggers?

How do you handle trigger logic that compares old/new without having a round trip back to the application?
Do it in a stored procedure not a trigger. Triggers have their place but a stored procedure is almost always better. Triggers can surprise you.
I don't follow how you would do that in a stored procedure outside of a trigger.
I think instead of performing an INSERT you call a stored proc that does the insert and some extra stuff.
Hmm, imho, triggers do scale, they are just slow. But as you add more connections, partitionss, and CPUs, the slowness per operation remains constant.
Have you tried deferring them?
that, and keeping your business logic in the database makes everything more opaque!
> that, and keeping your business logic in the database makes everything more opaque!

Opaque to who? If there's a piece of business logic that says "After this table's record is updated, you MUST update this other table", what advantages are there to putting that logic in the application?

When (not if) some other application updates that record you are going to have a broken database.

Some things are business constraints, and as such they should be moved into the database if at all possible. The application should never enforce constraints such as "either this column or that column is NULL, but at least one must be NULL and both must never be NULL at the same time".

Your database enforces constraints; what advantages are there to code the enforcement into every application that touches the database over simply coding the constraints into the database?

I think the dream is that business requirements are contained to one artifact and everything else responds to that driver. In an ideal world, it would be great to have databases care only about persistence and be able to swap them out based on persistence needs only. But you're right, in the real world the database is much better at enforcing constraints than applications.
Neither do foreign keys the moment you need to shard. Turns out that there's no free lunch when you ask your database to do "secret extra work" that's supposed to be transparent-ish to the user.
Does that only apply when you need to shard within tenants?

If each tenant gets an instance I would call that a “shard” but in that pattern there’s no need for cross-shard references.

Maybe in the analytics stack but that can be async and eventually consistent.

Facebook’s wormhole seems like a better approach here - just tailing the MySQL bin log gets you commit safety for messages without running into this kind of locking behavior.
  • maxdo
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What a discovery , even Postgres itself doesn’t scale easy. There are so many solutions that are dedicated and cost you less.
LISTEN/NOTIFY isn’t just a lock-free trigger. It can jeopardize concurrency under load.

Features that seem harmless at small scale can break everything at large scale.

It's true and folk should also choose the right tool at their scale and monitor it. There are plenty of cases where LISTEN/NOTIFY is the right choice.

However, in 2025 I'd pick Redis or MQTT for this kind of role. I'm typically in multi-lamg environments. Is there something better?

If I understood correctly, the global lock is so that notify events are emitted in order. Would it make sense to have a variant that doesn't make this ordering guarantee if you don't care about it, so that you can "notify" within transactions without locking the whole thing?
possibly, but i think at that point it would make more sense to move the business logic outside of the database (you can wait for a successful commit before triggering an external process via the originating app, or monitor the WAL with an external pub/sub system, or something else more clever than i can think of).
There’s lots of ways to invoke NOTIFY without doing it from with the transaction doing the work.

The post author is too focused on using NOTIFY in only one way.

This post fails to explain WHY they are sending a NOTIFY. Not much use telling us what doesn’t work without telling us the actual business goal.

It’s crazy to send a notify for every transaction, they should be debounced/grouped.

The point of a NOTIFY is to let some other system know something has changed. Don’t do it every transaction.

  • 0xCMP
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Agreed, I am struggling to understand why "it does not scale" is not "we used it wrong and hit the point where it's a problem" here.

Like if it needs to be very consistent I would use an unlogged table (since we're worried about "scale" here) and then `FOR UPDATE SKIP LOCKED` like others have mentioned. Otherwise what exactly is notify doing that can't be done after the first transaction?

Edit: in-fact, how can they send an HTTP call for something and not be able to do a `NOTIFY` after as well?

One possible way I could understand what they wrote is that somewhere in their code, within the same transaction, there are notifies which conditionally trigger and it would be difficult to know which ones to notify again in another transaction after the fact. But they must know enough to make the HTTP call, so why not NOTIFY?

Agreed.

They’re using it wrong and blaming Postgres.

Instead they should use Postgres properly and architect their system to match how Postgres works.

There’s correct ways to notify external systems of events via NOTIFY, they should use them.

  • thom
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Yeah, the way I've always used LISTEN/NOTIFY is just to tell some pool of workers that they should wake up and check some transactional outbox for new work. False positives are basically harmless and therefore don't need to be transactional. If you're sending sophisticated messages with NOTIFY (which is a reasonable thing to think you can do) you're probably headed for pain at some point.
Assuming you skip select transaction, or require logging on it because your regulated industry had bad auditors, then every transaction changes something.
Seriously people just layer shit with NATS for pubsub after persist and make sure there's a proper way to place a 'on restart recoonect' thing.
Amen! NATS is how we do AI streaming! JetStream subject per thread with an ordered consumer on the client.
Got up to the TL;DR paragraph. This was a major red flag given the initial presentation of the discovery of a bottleneck:

''' When a NOTIFY query is issued during a transaction, it acquires a global lock on the entire database (ref) during the commit phase of the transaction, effectively serializing all commits. '''

Am I missing something - this seems like something the original authors of the system should have done due diligence on before implementing a write heavy work load.

I think it's just difficult to predict how heavy is heavy enough to make this a problem. FWIW I had worked at a startup with a much more primitive data storage system where serialized commits were actually totally fine. The startup never outgrew that bottleneck.
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You had one problem with listen notify which was a fair one, but now you have a problem with http latency, network issues, DNS, retries, self-DDoS, etc.
it sounds like the impact of LISTEN/NOTIFY scaling issues was much greater on the overall DB performance than the actual load/scope of the task being performed (based on the end of the article), and they're aware that if they needed something more performant for that offloaded task, they have options (pub/sub via redis or w/e).
Am I supposed to be able to tell from these graphs that one was faster than the other? Because I sure can't.

What were the TPS numbers? What was the workload like? How big is the difference in %?

Transactional databases are not really the best tool for writing tons of (presumably) immutable records. Why are you using it for this? Why not Elastic?
Because transactional databases are perfectly fine for this type of thing when you have 0 to 100k users.
[citaiton needed]
wow thanks for the heads up! no idea this was a thing.
It’s not a thing.
i don’t understand. is the serialized write global lock a thing or no?
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was hoping the solution was: we forked postgres.

cool writeup!

I had a similar thought, as I was clicking through to TFA; “NOTIFY does not scale, but our new Widget can! Just five bucks”
Honestly this article is ridiculous. Most people do not have tens of thousands of concurrent writers. And most applications out there are read heavy, not write. Which means you probably have read replicas distributing loads.

Use LISTEN/NOTIFY. You will get a lot of utility out of it before you’re anywhere close to these problems.

Sounds like one centralized Postgres instance, am I understanding that correctly? Wouldn’t meeting bots be very easy to parallelize across single-tenant instances?
[dead]
It does scale. Just not to recall levels of traffic. Come on guys let's not rewrite everything in cassandra and rust now.
RBDMS are not designed for write-heavy applications, they are designed for read-heavy analysis. Also, an RDBMS is not a message queue or an RPC transport.

I feel like somebody needs to write a book on system architecture for Gen Z that's just filled with memes. A funny cat pic telling people not to use the wrong tool will probably make more of an impact than an old fogey in a comment section wagging his finger.

People have been using RDBMS' for write-heavy workflows for forever. Some people even use stored procs or triggers for getting complicated write operations to work properly.

Databases can do a lot of stuff, and if you're not hurting for DB performance it can be a good idea to just... do it in the database. The advantage is that, if the DB does it, you're much less likely to break things. Putting data constraints in application code can be done, but then you're just waiting for the day those constraints are broken.

There are OLTP and OLAP RDBMSes. Only OLAP ones are designed for read-heavy analyses.
But those rules of thumb aren't true. People use Postgres for job queues and write-heavy applications.

You'd have to at least accompany your memes with empirics. What is write-heavy? A number you might hit if your startup succeeds with thousands of concurrent users on your v1 naive implementation?

Else you just get another repeat of everyone cargo-culting Mongo because they heard that Postgres wasn't web scale for their app with 0 users.