Under one isolation level, a transaction reads 50 twice while another session has already saved 40. Under the other, its second read says 40. Neither is a bug, and the right choice depends on what your code does with the number. Here are the two levels running side by side, and what a write does under both.
An isolation level decides what a transaction can see of other transactions' work. MySQL has a default and three scopes to change it in, and the three statements look almost identical. This guide checks the current level and changes it at each scope, then proves which level a transaction really ran at.
You run an UPDATE, and it's saved. You didn't ask for that, and that's autocommit doing its job. It's on by default, and turning it off changes what every later statement does in that session. Here's each setting at work, and the surprise waiting when you close the client.
Move 10 units from one product to another, and that's two UPDATE statements. If only the first one runs, 10 units have vanished. A transaction groups statements so they all take effect together, or none of them do. This guide runs one to the end and one back, and shows what another session sees while it's open.
I used a TransactionScope once, and afterwards a plain query on the same pooled connection reported its isolation level as SERIALIZABLE. The scope's default level is Serializable, the driver sets it for the whole session, and nothing puts it back. Passing the level you want avoids all of it.
A request without my header got a 400 Bad Request with an empty explanation, and my endpoint never ran. The log had the reason: Required parameter "string source" was not provided from header. One ? decides whether a header is required.
I raised a value with ExecuteUpdateAsync, and read the product again, and got the old value back. The database had the new one. FirstAsync had gone to MySQL, read the new value, and then handed me the object it was already tracking, unchanged.
int.Parse on a number from configuration stopped the build with error CA1305: The behavior of 'int.Parse(string)' could vary based on the current user's locale settings. The analyzer has a point, and the fix is one argument, once you decide whose culture the text is in.
Asking for IsolationLevel.Snapshot failed with NotSupportedException: Snapshot isolation level is not supported. Coming from SQL Server, that sounds like MySQL can't do it. It can: MySQL's default level already reads from a snapshot, under a different name.
SELECT VERSION() is about the simplest query there is, and db.Database.SqlQuery<string>(...).FirstAsync() failed with Unknown column 's.Value' in 'field list'. My SQL has no s and no Value. EF Core added both, and it expects one thing from you.
