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How do you stop race conditions in MySQL? the problem at hand is caused by a simple algorithm:

  1. select a row from table
  2. if it doesn't exist, insert it

and then either you get a duplicate row, or if you prevent it via unique/primary keys, an error.

Now normally I'd think transactions help here, but because the row doesn't exist, the transaction don't actually help (or am I missing something?).

LOCK TABLE sounds like an overkill, especially if the table is updated multiple times per second.

The only other solution I can think of is GET_LOCK() for every different id, but isn't there a better way? Are there no scalability issues here as well? And also, doing it for every table sounds a bit unnatural, as it sounds like a very common problem in high-concurrency databases to me.

what you want is LOCK TABLES

or if that seems excessive how about INSERT IGNORE with a check that the row was actually inserted.

If you use the IGNORE keyword, errors that occur while executing the INSERT statement are treated as warnings instead.

locking - mysql insert race condition, How do you stop race conditions in MySQL? the problem at hand is caused by a simple algorithm: select a row from table; if it doesn't exist, insert it. and then� In case insert ignore doesnt fit for you as suggested in the accepted answer , so according to the requirements in your question : 1] select a row from table 2] if it doesn't exist, insert it. Another possible approach is to add a condition to the insert sql statement, e.g :

It seems to me you should have a unique index on your id column, so a repeated insert would trigger an error instead of being blindingly accepted again.

That can be done by defining the id as a primary key or using a unique index by itself.

I think the first question you need to ask is why do you have many threads doing the exact SAME work? Why would they have to insert the exact same row?

After that being answered, I think that just ignoring the errors will be the most performant solution, but measure both approaches (GET_LOCK v/s ignore errors) and see for yourself.

There is no other way that I know of. Why do you want to avoid errors? You still have to code for the case when another type of error occurs.

As staticsan says transactions do help but, as they usually are implied, if two inserts are ran by different threads, they will both be inside an implied transactions and see consistent views of the database.

Fixing a Race Condition - In the Weeds, I'd like to share my journey of fixing a race condition and the things I it's possible for another process to insert an interviewer right after our� Is this approach correct to prevent Conditional INSERT/UPDATE Race Condition. Does SQL Server Offer Anything Like MySQL's ON DUPLICATE KEY UPDATE. 37.

Locking the entire table is indeed overkill. To get the effect that you want, you need something that the litterature calls "predicate locks". No one has ever seen those except printed on the paper that academic studies are published on. The next best thing are locks on the "access paths" to the data (in some DBMS's : "page locks").

Some non-SQL systems allow you to do both (1) and (2) in one single statement, more or less meaning the potential race conditions arising from your OS suspending your execution thread right between (1) and (2), are entirely eliminated.

Nevertheless, in the absence of predicate locks such systems will still need to resort to some kind of locking scheme, and the finer the "granularity" (/"scope") of the locks it takes, the better for concurrency.

(And to conclude : some DBMS's - especially the ones you don't have to pay for - do indeed offer no finer lock granularity than "the entire table".)

Question on MySQL insert race condition, Question on MySQL insert race condition. This script basically prints the row count for the table temp1 , then inserts a row, then prints the row count again, then sleeps for a second and prints the row count one more time. The INSERTs are done with the write user and the SELECTs are done with the read user. Overview. A race condition vulnerability exists in the MySQL, MariaDB, and Percona databases. This vulnerability will allow a local attacker (remote access via a web shell or SSH connection) who has a low privileged account (CREATE/INSERT/SELECT grants) on the affected database to escalate their privileges and execute arbitrary code as the database system user.

On a technical level, a transaction will help here because other threads won't see the new row until you commit the transaction.

But in practice that doesn't solve the problem - it only moves it. Your application now needs to check whether the commit fails and decide what to do. I would normally have it rollback what you did, and restart the transaction because now the row will be visible. This is how transaction-based programmer is supposed to work.

Concurrent transactions result in race condition with unique , Try the insert first, with on conflict do nothing and returning id . If the value already exists, you will get no result from this statement, so you have then to execute� Is it possible for mysql_insert_id() to undergo a 'Race Condition' ? It would be my best guess that this function has exclusive process control (even in a hyperthreaded situation) such that only one client can call this at once (first in, first out queue with no acynchornous or concurrent behavior).

I ran into the same problem and searched the Net for a moment :)

Finally I came up with solution similar to method to creating filesystem objects in shared (temporary) directories to securely open temporary files:

$exists = $success = false;
do{
 $exists = check();// select a row in the table 
 if (!$exists)
  $success = create_record();
  if ($success){
   $exists = true;
  }else if ($success != ERROR_DUP_ROW){
    log_error("failed to create row not 'coz DUP_ROW!");
    break;
  }else{
    //probably other process has already created the record,
    //so try check again if exists
  }
}while(!$exists)

Don't be afraid of busy-loop - normally it will execute once or twice.

Race Condition when Creating Unique Values, During my tuning sessions, there is a race condition I run across so often that I Consider two queries A and B both wanting to insert the value 42. MySQL will use REPEATABLE READ as the default isolation level, but this� Probably something like this: if (select * from UserPoints where username = 'ravi') then (update UserPoints set points = points + 5 where username = 'ravi') else (insert into UserPoints (username, points) values ('ravi', 0)) end if I can not do it pragmatically using PHP, because the environment is highly concurrent and it may result in a 'race condition' often.

MySQL and an atomic 'check on none insert', 2.mysql> insert into T set C = 42; ERROR 1213: Deadlock found Deal with the race and do an ordinary SELECT + INSERT IGNORE and� If a test like this could demonstrate your problem, please post it in bugs.mysql.com. If you urgently need a workaround, Session 2 could test for the failure case (no record found) and try again. If Peter is correct about it taking some amount of time for the COMMIT to 'finish', then such a loop would eventually trip over the delay.

MySQL 8.0 Reference Manual :: 15.7.2.4 Locking Reads, If you query data and then insert or update related data within the same transaction, the regular SELECT statement does not give enough protection. Create a table to hold the sequence numbers: CREATE TABLE sequence_name(next_id INT AUTO_INCREMENT PRIMARY KEY) ENGINE = InnoDB And whenever you need a new sequence number (autocommit off!): START TRANSACTION INSERT INTO sequence_name() VALUES () SELECT LAST_INSERT_ID AS _next_id COMMIT The same could be achieved using LOCK TABLE and MAX().

MySQL How To Select and Update in Single Statement, and set the new value in the single SQL statement to avoid a race condition? Initialize the first counter with start value 10 INSERT INTO counters VALUES (1� The example above tries to illustrate the race condition between 2 transactions. You can have differents type of concurrency problem base on type of query UPDATE, DELETE, or SELECT FOR UPDATE but the only way to get a conflict during INSERT is if you have a uniqueness constraint on a table.

Comments
  • Go with INSERT IGNORE. Triggers cause too much overhead, they aren't worth it unless the business logic is complex.
  • What's wrong with just INSERT-ing with the unique constraint and catching errors to say "okay, the record already exists."?
  • Well yes of course we've got unique indeces etc... in fact that's what made us realise the problem exists is errors triggered by unique index.
  • That's how it's supposed to work, you prepare for transactions to fail... in this case it seems very easy: If dup key error, ignore because the row already exists. A full table/row lock may be more of a performance hit than just ignoring errors when they occur. Measure though
  • +1 for "except printed on the paper that academic studies are published on"
  • It's worth noting that transactions (depending on the level of isolation and so on) also involve locking, better to leave the locking to the database infrastructure than to calling it yourself.