Comment by tonyhb | Hacker News Reader

Comment by tonyhb | Hacker News Reader

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Anything that guarantees exactly once is selling snake oil. Side effects happen inside any transaction, and only when it commits (checkpoints) are the side effects safe.

Want to send an email, but the app crashes before committing? Now you're at-least-once.

You can compress the window that causes at-least-once semantics, but it's always there. For this reason, this blog post oversells the capabilities of these types of systems as a whole. DBOS (and Inngest, see the disclaimer below) try to get as close to exactly once as possible, but the risk always exists, which is why you should always try to use idempotency in external API requests if they support it. Defense in layers.

Disclaimer: I built the original `step.run` APIs at https://www.inngest.com, which offers similar things on any platform... without being tied to DB transactions.

As the post says, the exactly-once guarantee is ONLY for steps performing database operations. For those, you actually can get an exactly-once guarantee by running the database operations in the same Postgres transaction as your durable checkpoint. That's a pretty cool benefit of building workflows on Postgres! Of course, if there are side effects outside the database, those happen at-least-once.

You can totally leverage postgres transactions to give someone... postgres transactions!

I just figured that the exactly once semantics were so worth discussing that any external side effects (which is what orchestration is for) aren't included in that, which is a big caveat.

> Anything that guarantees exactly once is selling snake oil.

That's a pretty spicy take. I'll agree that exactly-once is hard, but it's not impossible. Obviously there are caveats, but the beauty of DBOS using Postgres as the method of coordination instead of the an external server (like Temporal or Inngest) is that the exactly-once guarantees of Postgres can carry over to the application. Especially so if you're using that same Postgres to store your application data.

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