| parent | RECORD | S,REC_NOT_GAP | 2 | GRANTED | | parent | RECORD | S,REC_NOT_GAP | 3 | GRANTED | | parent | RECORD | X,REC_NOT_GAP | 1, 1 | GRANTED | If we look at the trigger in the where section This makes reads consistent and therefore makes the replication between servers consistent. When we run the same query twice, we get the same result, regardless other session modifications on that table. What does a gap lock mean? A gap lock is a lock on a gap between index records, or a lock on the gap before the first or after the last index record. We want to read the records that are not touched by the second session. | parent | RECORD | S | supremum pseudo-record | GRANTED | | id | parent_name | child_id | child_name | Mysql> select * from parent where id1 for share Session one locks two rows in the parent table, one of which is a gap lock. | parent | RECORD | X,GAP | 2, 2 | GRANTED | | parent | RECORD | X,REC_NOT_GAP | 1 | GRANTED | | child | RECORD | X,REC_NOT_GAP | 1 | GRANTED | | parent | RECORD | X,REC_NOT_GAP | 1 | WAITING | | object_name | lock_type | lock_mode | lock_data | lock_status | Mysql> SELECT object_name, lock_type, lock_mode, lock_data, lock_status FROM performance_schema.data_locks
Mysql deadlock update#
Mysql> update parent set parent_name='parent2' where id=1 We’re gonna add a column to the parent table. Please check this article in order to prepare your setup. So now that we found a valid case for using triggers, let’s see what we need to be careful when using them.
Mysql deadlock code#
And in the same time we don’t want to work on the monolith to add more code which would support this migration, so the obvious choice would be to use database triggers. Of course in order to do this we need to synchronise the data. We want to keep the monolith working but in the same time prepare the new services so that we can do a canary release at some point. So, we’re not going with a bing bang approach but with incremental changes. A deadlock in MySQL happens when two or more transactions mutually hold and request for locks, creating a cycle of dependencies. One case for this would be refactoring your monolith to microservices and part of that is this database migration. Or you could break one table in multiple tables, in order to separate domain concepts. Let’s say that part of the migration you create new separate schema which you plan to use it for the new version. But if you plan to do a database migration you may find them handy. Who uses triggers you may wonder? That’s a valid question.
Mysql deadlock driver#
A wait timeout is analogous to a driver giving up and turning back because of delays.We are in 2019.
It's possible (even, likely) that the lock would have become available and have been acquired if the transaction had waited longer, but the timeout exists to avoid applications waiting on the database indefinitely. LATEST DETECTED DEADLOCK - 11:05:21 13c0 (1) TRANSACTION: TRANSACTION 17064867, ACTIVE 17 sec starting index read mysql tables in use 1, locked 1 LOCK WAIT 8 lock struct(s), heap size 3112, 7 row lock(s), undo log entries 2 MySQL thread id 7778, OS thread handle 0xb9c, query id 23386973 10.0.0. In the case of InnoDB locks) elapses while a transaction awaits a lock, perhaps because a slow transaction is holding the lock and has not finished executing or perhaps because a number of transactions are queuing for the lock. A deadlock is analogous to a policeman solving gridlock (the situation at a road junction when no vehicle is able to move forward) by ordering a random participant to reverse.Ī wait timeout occurs when the configured timeout period (e.g. Thus the database picks a transaction to abort/rollback application code should detect this eventuality and handle accordingly, usually by attempting the transaction again. A deadlock occurs whenever a circular dependency arises among the locks that transactions must acquire in order to proceed: for example, imagine that transaction 1 holds lock A but needs to acquire lock B to proceed and transaction 2 holds lock B but needs to acquire lock A to proceed-the transactions are immediately deadlocked (no timeout required) and neither can proceed until one releases its locks.
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