I’ve come across something strage using Amazon RDS (MySQL 5.6.23).
I had issues when concurrent transactions were locking and then calculating a value from multiple tables. The following is a quick demo of what was happening when using transactions with isolation level REPEATABLE READ.
+------+-----------------------------------------------+-----------------------------------------------+
| Time | first transaction | second transaction |
+------+-----------------------------------------------+-----------------------------------------------+
| 1 | START TRANSACTION; | START TRANSACTION; |
| | | |
| 2 | SELECT * FROM stock WHERE sku=SKU0 FOR UPDATE | SELECT * FROM stock WHERE sku=SKU0 FOR UPDATE |
| | | |
| 3 | [GOT THE LOCK] | [WAIT] |
| | | |
| 4 | SELECT s.qty - COUNT(r.fk_stock) AS qty | |
| | FROM stock s | |
| | LEFT JOIN reservation r | [WAIT] |
| | WHERE s.sku = SKU0 | |
| | GROUP BY s.simple_sku; | |
| | | |
| 5 | qty === 10 | |
| | | |
| 6 | INSERT INTO reservation .... | [WAIT] |
| | | |
| 7 | SELECT s.qty - COUNT(r.fk_stock) AS qty | |
| | FROM stock s | |
| | LEFT JOIN reservation r | [WAIT] |
| | WHERE s.sku = SKU0 | |
| | GROUP BY s.simple_sku; | |
| | | |
| 8 | qty === 9 | |
| | | |
| 9 | UPDATE stock SET qty = 9 | |
| | | |
| 10 | COMMIT; | |
| | | |
| 11 | | [GOT THE LOCK] |
| | | |
| 12 | | SELECT s.qty - COUNT(r.fk_stock) AS qty |
| | | FROM stock s |
| | | LEFT JOIN reservation r |
| | | WHERE s.sku = SKU0 |
| | | GROUP BY s.simple_sku; |
| | | |
| 13 | | qty === 10 <=== it should be 9 |
| | | |
| 14 | | INSERT INTO reservation .... |
| | | |
| 15 | | SELECT s.qty - COUNT(r.fk_stock) AS qty |
| | | FROM stock s |
| | | LEFT JOIN reservation r |
| | | WHERE s.sku = SKU0 |
| | | GROUP BY s.simple_sku; |
| | | |
| 16 | | qty === 9 <=== it should be 8 |
| | | |
| 17 | | UPDATE stock SET qty = 9 |
| | | |
| | | COMMIT; |
| | | |
| | | |
+------+-----------------------------------------------+-----------------------------------------------+
As soon as I set isolation level to READ COMMITTED the problem is gone.
On my local mysql (MySQL 5.6.19) installation, I can do the same operation and I see the right value for Not true when using the same dataset that I have on RDS. See the answer belowqty using REPEATABLE READ isolation level.
For what I found the REPEATABLE READ isolation level creates a read view at the beginning of first read and keeps reading values from that “view”. The READ COMMITTED instead will create a read view for every read in a transaction, reading always the most fresher values.
This may explain …, but I would expect to have the same behaviour also on my local mysql.
This explain what’a happening on RDS.
Has someone experienced something like this before?