Add a remark about upsert’s race conditions

[dtinth]

Describe this PR

I have been bitten hard by this issue:

• upsert across HTTP requests has a race condition prisma#3242

This is because I have assumed that Prisma’s upsert behavior would be equivalent to MongoDB’s behavior.

• MongoDB’s upsert behavior is similar to using a ON DUPLICATE KEY or ON CONFLICT clause, so the database makes sure that 2 simulataneous upsert’s on the same document with same unique key are treated sequentially — one upsert results in an insert, and the other results in an update.
• Prisma’s upsert behavior first does a SELECT followed by INSERT/UPDATE. If 2 simultaneous upserts are taking place, it could lead to a race condition where both upserts does not see an existing record and both attempts to insert, resulting in a P2002 error.

Changes

Document this behavior. This is the remark I wish I saw when I decided to use this method.

What issue does this fix?

prisma/prisma#3242

Any other relevant information

N/A

[vercel]

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[garrensmith]

@dtinth thanks for this documentation. I've added more in-depth documentation here #3952

I'm going to close this issue since we going to use the other issue that will lead to more comprehensive documentation on this topic.