[stickfigure]

Until you hit scale, the database you're already using is fine. If that's Postgres, look up SELECT FOR UPDATE SKIP LOCKED. The major convenience here - aside from operational simplicity - is transactional task enqueueing.

For hosted, SQS or Google Cloud Tasks. Google's approach is push-based (as opposed to pull-based) and is far and above easier to use than any other queueing system.

[crabbone]

I'm probably biased, but in the number of cases where I had to work with Kafka, I'd really prefer to simply have an SQL database. In all of those cases I struggled to understand why developers wanted Kafka, what problem was it solving better than the database they already had, and for the life of me, there just wasn't one.

I'm not saying that configuring and deploying databases is easy, but it's probably going to happen anyway. Deploying and configuring Kafka is a huge headache: bad documentation, no testing tools, no way to really understand performance in the light of durability guarantees (which are also obscured by the poor quality documentation). It's just an honestly bad product (from the infra perspective): poor UX, poor design... and worst of all, it's kind of useless from the developer standpoint. Not 100% useless, but whatever it offers can be replaced by other existing tools with a tiny bit of work.

[vrosas]

Cloud Tasks is one of the most undervalued tools in the GCP ecosystem, but mostly because PubSub gets all the attention. I've been using it since it was baked in the AppEngine and love it for 1-to-1 queues or delayed job handling.

[kamikaz1k]

how do you recommend working with Cloud Tasks?

raw dogging gcloud? Terraform? or something more manageable?

I've been curious for one of my smaller projects, but I am worried about adopting more GCPisms.

[stickfigure]

Back when I was all-in on GCP, I had a queue.yaml file which the appengine deployer syncs to cloud tasks (creates/disabled queues, changes the rate limits, concurrency, etc).

Now that I'm mostly on AWS... I still use the same system. I have a thin little project that deploys to GAE and has a queue.yaml file. It sets up the cloud tasks queues. They hit my EB endpoints just like they used to hit my GAE endpoints.

As a bonus, my thin little GAE app also has a cron.yaml that it proxies to my AWS app. Appengine's cron is also better than Amazon's overcomplicated eventbridge system.

It's great.

[vrosas]

Terraform is definitely for the best. Any AI tool should be able to spit it out well enough, but if you do rawdog it in the console or gcloud you might be able to export the terraform with:

    gcloud beta resource-config bulk-export --resource-format=terraform

[boruto]

How could I solve the problem of in-order processing based on a key using skip locked? Basically all records having the key to be processed one after other.

[wmfiv]

Work jobs in the order they were submitted within a partition key. This selects the next partition key that isn't locked. You could make it smarter to select a subset of the jobs checking for partition keys where all of the rows are still unlocked.

  SELECT
  * 
  FROM jobs 
  WHERE partition_key = (
    SELECT partition_key 
    FROM jobs 
    ORDER BY partition_key 
    LIMIT 1
    SKIP LOCKED
  )
  ORDER BY submitted_at
  FOR UPDATE SKIP LOCKED;

[boruto]

Yes, something along the lines could work. But I am not sure if the above query itself would work if rows are appended to the table in parallel.

Also if events for a partition gets processed quick would the last partition get an equal chance?

[monksy]

Famious last words. There are database as a queue antipattern warnings about this.

[srhtftw]

> Famious last words.

These weren't his last words, but Jim Gray had this to say about this so-called "antipattern".

Queues Are Databases (1995)

Message-oriented-middleware (MOM) has become an small industry. MOM offers queued transaction processing as an advance over pure client-server transaction processing. This note makes four points: Queued transaction processing is less general than direct transaction processing. Queued systems are built on top of direct systems. You cannot build a direct system atop a queued system. It is difficult to build direct, conversational, or distributed transactions atop a queued system. Queues are interesting databases with interesting concurrency control. It is best to build these mechanisms into a standard database system so other applications can use these interesting features. Queue systems need DBMS functionality. Queues need security, configuration, performance monitoring, recovery, and reorganization utilities. Database systems already have these features. A full-function MOM system duplicates these database features. Queue managers are simple TP-monitors managing server pools driven by queues. Database systems are encompassing many server pool features as they evolve to TP-lite systems.

https://arxiv.org/abs/cs/0701158

[reval]

Why is that an anti-pattern? Databases have added `SKIP LOCKED` and `SELECT FOR UPDATE` to handle these use cases. What are the downsides?

[maniacalhack0r]

as with everything, it depends on how you're processing the queue.

eg we built a system at my last company to process 150 million objects / hour, and we modeled this using a postgres-backed queue with multiple processes pulling from the queue.

we observed that, whenever there were a lot of locked rows (ie lots of work being done), Postgres would correctly SKIP these rows, but having to iterate over and skip that many locked rows did have a noticeable impact on CPU utilization.

we worked around this by partitioning the queue, indexing on partition, and assigning each worker process a partition to pull from upon startup. this reduced the # of locked rows that postgres would have to skip over because our queries would contain a `WHERE partition=X` clause.

i had some great graphs on how long `SELECT FOR UPDATE ... SKIP LOCKED` takes as the number of locked rows in the queue increases, and how this partiton work around reduced the time to execute the SKIP LOCKED query, but unfortunately they are in the hands of my previous employer :(

[izacus]

How did you get from original post of "low level of load" to overengineering for "150 million objects/hr".

Is the concept of having different solutions for different scales not familiar to you?

[pritambaral]

I did sth similar. Designed and built for 10 million objects / hour. Picked up by workers in batches of 1k. Benchmark peaked above 200 million objects / hour with PG in a small VM. Fast forward two years, the curse of success strikes, and we have a much higher load than designed for.

Redesigned to create batches on the fly and then `SELECT FOR UPDATE batch SKIP LOCKED LIMIT 1` instead of `SELECT FOR UPDATE object SKIP LOCKED LIMIT 1000`. And just like that, 1000x reduction in load. Postgres is awesome.

----

The application is for processing updates to objects. Using a dedicated task queue for this is guaranteed to be worse. The objects are picked straight from their tables, based on the values of a few columns. Using a task queue would require reading these tables anyway, but then writing them out to the queue, and then invalidating / dropping the queue should any of the objects' properties update. FOR UPDATE SKIP LOCKED allows simply reading from the table ... and that's it.

[maniacalhack0r]

smart. although, i guess that pushes the locking from selecting queue entries to making sure that objects are placed into exactly 1 batch. curious if you ran into any bottlenecks there?

[pritambaral]

> ... making sure that objects are placed into exactly 1 batch. curious if you ran into any bottlenecks there?

A single application-layer thread doing batches of batch creation (heh). Not instant, but fast enough. I did have to add 'batchmaker is done' onto the 'no batch left' condition for worker exit.

> ... that pushes the locking from selecting queue entries to ...

To selecting batches. A batch is immutable once created. If work has to be restarted to handle new/updated objects, all batches are wiped and the batchmaker (and workers, anyway) start over.

[ninju]

I believe the article and parent comment were discussing queue solutions for low-volume situations.

[maniacalhack0r]

completely missed this. apologies.

[stickfigure]

40,000 per second is waaaaay beyond where you should use a dedicated queuing solution. Even dedicated queues require tuning to handle that kind of throughput.

(or you can just use SQS or google cloud tasks, which work out of the box)

[pritambaral]

I hit 60k per second in 2020 on a 2-core, 100GB SSD installation of PG on GCP. And "tuning" PG is way easier than any dedicated queueing system I've seen. Does there exist a dedicated queueing system with an equivalent to EXPLAIN (ANALYZE)?

[stickfigure]

If that's true, you managed to do much better than these folks:

https://softwaremill.com/mqperf/

Maybe you should write a letter?

[kstrauser]

I worked at a shop that had to process about 6M RPS for 5 seconds at a time, once a minute or so. That looked a lot like a boatload of Python background threads queueing work in memory then flushing them out into Cassandra. That was a fun little project.

[chupasaurus]

> 150 million objects / hour

Is not a low volume unless this could be done in batches of hundreds.

[maniacalhack0r]

completely missed this. apologies.

[makeitdouble]

I suppose you are referring to this:

https://mikehadlow.blogspot.com/2012/04/database-as-queue-an...

The main complaint seems to be that it's not optimal...but then, the frame of the discussion was "Until you hit scale", so IMHO convenience and simpler infra trumps having the absolute most efficient tool at that stage.

[golergka]

Can you elaborate? I guess it has to do with connection pooling?