Using PostgreSQL as a Dead Letter Queue for Event-Driven Systems

141 pointsposted 7 hours ago
by tanelpoder
(diljitpr.net)

43 Comments

with

4 minutes ago

Great application of first principles. I think it's totally reasonable also, at even most production loads. (Example: My last workplace had a service that constantly roared at 30k events per second, and our DLQs would at most have orders of hundreds of messages in them). We would get paged if a message's age was older than an hour in the queue.

The idea is that if your DLQ has consistently high volume, there is something wrong with your upstream data, or data handling logic, not the architecture.

TexanFeller

5 hours ago

Ofc I wouldn't us it for extremely high scale event processing, but it's great default for a message/task queue for 90% of business apps. If you're processing under a few 100m events/tasks per day with less than ~10k concurrent processes dequeuing from it it's what I'd default to.

I work on apps that use such a PG based queue system and it provides indispensable features for us we couldn't achieve easily/cleanly with a normal queue system such as being able to dynamically adjust the priority/order of tasks being processed and easily query/report on the content of the queue. We have many other interesting features built into it that are more specific to our needs as well that I'm more hesitant to describe in detail here.

j45

2 hours ago

Very few things dna start at an extremely high scale event processing.

There’s also an order of magnitude higher events when doing event based work in processing.

This seems like a perfectly reasonable starting and gateway points that can have things organized for when the time comes.

Most things don’t scale that big.

rbranson

6 hours ago

Biggest thing to watch out with this approach is that you will inevitably have some failure or bug that will 10x, 100x, or 1000x the rate of dead messages and that will overload your DLQ database. You need a circuit breaker or rate limit on it.

with

10 minutes ago

This is the same risk with any DLQ.

The idea behind a DLQ is it will retry (with some backoff) eventually, and if it fails enough, it will stay there. You need monitoring to observe the messages that can't escape DLQ. Ideally, nothing should ever stay in DLQ, and if it does, it's something that should be fixed.

shayonj

6 hours ago

This! Only thing worse than your main queue backing off is you dropping items from going into the DLQ because it can’t stay up.

rr808

5 hours ago

I worked on an app that sent an internal email with stack trace whenever an unhandled exception occurred. Worked great until the day when there was an OOM in a tight loop on a box in Asia that sent a few hundred emails per second and saturated the company WAN backbone and mailboxes of the whole team. Good times.

pletnes

6 hours ago

If you can’t deliver to the DLQ, then what? Then you’re missing messages either way. Who cares if it’s down this way or the other?

xyzzy_plugh

6 hours ago

Not necessarily. If you can't deliver the message somewhere you don't ACK it, and the sender can choose what to do (retry, backoff, etc.)

Sure, it's unavailability of course, but it's not data loss.

konart

5 hours ago

If you are reading from Kafka (for example) and you can't do anything with a message (broken json as an example) and you can't put it into a DLQ - you have not other option but to skip it or stop on it, no?

Misdicorl

4 hours ago

Your place of last resort with kafka is simply to replay the message back to the same kafka topic since you know it's up. In a simple single consumer setup just throw a retry count on the message and increment it to get monitoring/alerting/etc. Multi consumer? Put an enqueue source tag on it and only process the messages tagged for you. This won't scale to infinity but it scales really really far for really really cheap

singron

5 hours ago

Generally yes, but if you use e.g. the parallel consumer, you can potentially keep processing in that partition to avoid head-of-line blocking. There are some downsides to having a very old unprocessed record since it won't advance the consumer group's offset past that record, and it instead keeps track of the individual offsets it has completed beyond it, so you don't want to be in that state indefinitely, but you hope your DLQ eventually succeeds.

But if your DLQ is overloaded, you probably want to slow down or stop since sending a large fraction of your traffic to DLQ is counter productive. E.g. if you are sending 100% of messages to DLQ due to a bug, you should stop processing, fix the bug, and then resume from your normal queue.

awesome_dude

an hour ago

Sorry, but what's stopping the DLQ being a different topic on that Kafka - I get that the consumer(s) might be dead, preventing them from moving the message to the DLQ topic, but if that's the case then no messages are being consumed at all.

If the problem is that the consumers themselves cannot write to the DLQ, then that feels like either Kafka is dying (no more writes allowed) or the consumers have been misconfigured.

Edit: In fact there seems to be a self inflicted problem being created here - having the DLQ on a different system, whether it be another instance of Kafka, or Postgres, or what have you, is really just creating another point of failure.

RedShift1

6 hours ago

The point is to not take the whole server down with it. Keeps the other applications working.

rbranson

6 hours ago

Sure, but you still need to design around this problem. It’s going to be a happy accident that everything turns out fine if you don’t.

j45

2 hours ago

It will happen eventually in any system.

No need to look down on PG because it makes it more approachable and is more longer a specialized skill.

exabrial

6 hours ago

> FOR UPDATE SKIP LOCKED

Learned something new today. I knew what FOR UPDATE did, but somehow I've never RTFM'd hard enough to know about the SKIP LOCKED directive. Thats pretty cool.

scresswell

2 hours ago

Yes, SKIP LOCKED is great. In practice you nearly always want LIMIT, which the article did not mention. Be careful if your selection spans multiple tables: only the relations you explicitly lock are protected (see SELECT … FOR UPDATE OF t1, t2). ORDER BY matters because it controls fairness and retry behaviour. Also watch ANALYZE: autoanalyze only runs once the dead to live tuple threshold is crossed, and on large or append heavy tables with lots of old rows this can lag, leading to poor plans and bad SKIP LOCKED performance. Finally, think about deletion and lifecycle: deleting on success, scheduled cleanup (consider pg_cron), or partitioning old data all help keep it efficient.

metanonsense

4 hours ago

only learned about SKIP LOCKED because ChatGPT suggested it to solve some concurrency problem I had. Great tool to learn such things.

indigo945

3 hours ago

Great tool that wrote the blog post in the OP also, so it's quite versatile.

kristov

3 hours ago

Why use shedlock and select-for-update-skip-locked? Shedlock stops things running in parallel (sort-of), but the other thing makes parallel processing possible.

shoo

2 hours ago

re: SKIP LOCKED, introduced in postgres 9.5, here's an an archived copy [†] of the excellent 2016 2ndquadrant post discussing it

https://web.archive.org/web/20240309030618/https://www.2ndqu...

corresponding HN discussion thread from 2016 https://news.ycombinator.com/item?id=14676859

[†] it seems that all the old 2ndquadrant.com blog post links have been broken after their acquisition by enterprisedb

upmostly

2 hours ago

We just published a detailed walkthrough of this exact pattern with concrete examples and failure modes:

PostgreSQL FOR UPDATE SKIP LOCKED: The One-Liner Job Queue https://www.dbpro.app/blog/postgresql-skip-locked

It covers the race condition, the atomic claim behaviour, worker crashes, and how priorities and retries are usually layered on top. Very much the same approach described in the old 2ndQuadrant post, but with a modern end-to-end example.

victor106

an hour ago

Love your product. Will you ever provide support of duckdb/motherduck? Wish there is a generic way you provided to add any database type

upmostly

34 minutes ago

Thanks, glad you like it.

DuckDB is on our radar. In practice each database still needs some engine-specific work to feel good, so a fully generic plugin system is harder than it sounds. We are thinking about how to do this in a scalable way.

renewiltord

5 hours ago

Segment uses MySQL as queue not even as DLQ. It works at their scale. So there are many (not all) systems that can tolerate this as queue.

I have simple flow: tasks are order of thousands an hour. I just use postgresql. High visibility, easy requeue, durable store. With appropriate index, it’s perfectly fine. LLM will write skip locked code right first time. Easy local dev. I always reach for Postgres for event bus in low volume system.

gytisgreitai

4 hours ago

Would be interesting to see the numbers this system processes. My bet is that they are not that high.

awesome_dude

an hour ago

I think that using Postgres as the message/event broker is valid, and having a DLQ on that Postgres system is also valid, and usable.

Having SEPARATE DLQ and Event/Message broker systems is not (IMO) valid - because a new point of failure is being introduced into the architecture.

nicoritschel

4 hours ago

lol a FOR UPDATE SKIP LOCKED post hits the HN homepage every few months it feels like

whateveracct

4 hours ago

and another CTO will use this meme as a reason to "just use Postgres" for far longer than they should lmao

reactordev

5 hours ago

Another day, another “Using PostgreSQL for…” thing it wasn’t designed for. This isn’t a good idea. What happens when the queue goes down and all messages are dead lettered? What happens when you end up with competing messages? This is not the way.

direwolf20

4 hours ago

The other system you're using that isn't Postgres can also go down.

Many developers overcomplicate systems. In the pursuit of 100% uptime, if you're not extremely careful, you removed more 9s with complexity than you added with redundancy. And although hyperscalers pride themselves on their uptime (Amazon even achieved three nines last year!) in reality most customers of most businesses are fine if your system is down for ten minutes a month. It's not ideal and you should probably fix that, but it's not catastrophic either.

hinkley

3 hours ago

What I’ve found is that, particularly with internal customers, they’re fine with an hour a month, possibly several, as long as not all of your eggs are in one basket.

The centralization pushes make a situation where if I have a task to do that needs three tools to accomplish, and one of them goes down, they’re all down. So all I can do is go for coffee or an early lunch because I can’t sub in another task into this time slot. They’re all blocked by The System being down, instead of a system being down.

If CI is borked I can work on docs and catch up on emails. If the network is down or NAS is down and everything is on that NAS, then things are dire.

reactordev

an hour ago

>The other system you're using that isn't Postgres can also go down.

Only if DC gets nuked.

Many developers overcomplicate systems and throw a database at the problem.

fcarraldo

5 hours ago

There are a ton of job/queue systems out there that are based on SQL DBs. GoodJob and SupaBase Queues are two examples.

It’s not usable for high scale processing but most applications just need a simple queue with low depth and low complexity. If you’re already managing PSQL and don’t want to add more management to your stack (and managed services aren’t an option), this pattern works just fine. Go back 10-15yrs and it was more common, especially in Ruby shops, as teams willing to adopt Kafka/Cassandra/etc were more rare.

reactordev

an hour ago

And there are a ton that aren’t.

hnguyen14

5 hours ago

How so? There are queues that use SQL (or no-SQL) databases as the persistence layer. Your question is more specific to the implementation, not the database as persistence layer itself. And there are ways to address it.

senbrow

5 hours ago

Criticism without a better solution is only so valuable.

How would you do this instead, and why?

reactordev

an hour ago

Watching a carpenter try to weld is equally only so valuable. I think the explanation is clear.

trympet

2 hours ago

I prefer using MS Exchange mailboxes for my message queue.

odie5533

5 hours ago

You wouldn't ack the message if you're not up to process it.

tonymet

3 hours ago

Postgres is essentially a b-tree with a remote interface. Would you use a b-tree to store a dead letter queue? What is big O of insert & delete? what happens when it grows?

Postgres has a query interface, replication, backup and many other great utilities. And it’s well supported, so it will work for low-demand applications.

Regardless, you’re using the wrong data structure with the wrong performance profile, and at the margins you will spend a lot more money and time than necessary running it . And service will suffer.