Hi,
On 2020-08-11 21:31:52 +0300, Pavel Biryukov wrote:
> Entity Framework is an ORM for .Net (and .Net Core). It has different providers
> for different databases (NpgSql for Postgres). It uses Optimistic concurrency.
> The common use case is to use xmin as "concurrency token".
>
> In code we make "var e = new Entity();", "dbContext.Add(e)" and
> "dbContext.SaveChanges()" (smth like that), and EF Core constructs sql for us,
> classical ORM;
>
> When new row is inserted, EF makes an insert with "RETURNING xmin" to keep it
> as concurrency token for further updates (update is made like "where id = [id]
> AND xmin=[xmin]" to be sure the row hasn't been updated by other clients).
That's not really a safe use of xmin, e.g. it could have wrapped around
leading you to not notice a concurrent modification. Nor does it
properly deal with multiple statements within a transaction. Perhaps
those are low enough risk for you, but I don't xmin is a decent building
block for this kind of thing.
Greetings,
Andres Freund
