MySQL并发和相同事务的麻烦

Question

OK here's the basic idea of what is happening:

begin transaction
some_data=select something from some_table where some_condition;
if some_data does not exists or some_data is outdated
  new_data = insert a_new_entry to some_table
  commit transaction
  return new_data
else
  return some_data
end

When multiple processes execute the code above simultaneously(like the client issues a lot of identical requests at a same time), a lot of 'new_data' will be inserted while actually only one is needed.

I think it's a quite typical scenario of concurrency, but still I can't figure out a decent way to avoid it. Things I can think about maybe like having a single worker process to do the select_or_insert job, or maybe set the isolation level to Serializable(unacceptable). But neither is quite satisfactory to me.

PS: The database is mysql, table engine is innodb, and isolation level is repeatable read

Answer 1

In your initial SELECT , use SELECT ... FOR UPDATE .

This ensures that the record is locked against other clients reading it until the transaction has been committed (or rolled-back), so they wait on the SELECT command and do not continue through to the rest of the logic until the first client has completed its UPDATE .

Note that you will need to ROLLBACK the transaction in the else condition, or else the lock will continue blocking until the connection is dropped.