c4710n
Is `Repo.insert_or_update` safe for race condition?
I have already used upsert feature via Repo.insert/2 for a long time.
Today, when I read other’s code, I found something like this:
case MyRepo.get(Subscription, id) do
nil -> %Subscription{id: id}
subscription -> subscription
end
|> Subscription.changeset(changes)
|> MyRepo.insert_or_update()
I’m wondering:
- is this usage safe for race condition?
- Should I use
Ecto.Mulit.insert_or_updateinstead? - If it’s not safe, why
MyRepo.insert_or_updateexists?
An example for race condition:
- A try to get subscription with id
reminder, getnil, then create a new subscription struct.- B try to get subscription with id
reminder, and getnil, then create a new subscription struct.- A try to insert, done.
- B try to insert, rejected.
Marked As Solved
trisolaran
- No, this is a classic example of a race condition, which you identified in your example (A and B trying to insert a record with the same id)
Multiis kind of irrelevant with respect to race conditions, I assume what you’re asking is “would wrapping this code in a transaction help?”. The answer is: no, it won’t help in the general case. It would only help if the transaction isolation level of your DB is set to “serializable”, which is typically not the case for performance reasons.insert_or_updateis not there to avoid race conditions, but as a convenience function that allows you to persist a changeset regardless of whether the underlying data is persisted in the DB or not (otherwise you’d have to chooseinsertorupdate)
You should check the result of MyRepo.insert_or_update(): if the result is {:error, changeset} and the changeset contains a primary key constraint violation, then you know that you hit the race condition and you can react appropriately.
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thousandsofthem
https://hexdocs.pm/ecto/constraints-and-upserts.html#upserts
I think you need this. Single sql query, no concurrency issues
03juan
Surely B’s insert would be rejected only if there’s a uniqueness constraint on the fields being updated? Otherwise you’ll get two rows with identical data but different ids.
If there’s a constraint then you can catch the error after the insert_or_update, find the existing id, and then re-run the function, which will update.
trisolaran
Yes pretty much: ecto/lib/ecto/repo/schema.ex at 960725b9e0719c79d8296c4852072780e8d5cc3e · elixir-ecto/ecto · GitHub
Also check @thousandsofthem’s suggestion. That’s a good point and might make your code simpler.
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