SQL Server allows you to control which transaction is more likely to be rolled back during a deadlock situation via the DEADLOCK_PRIORITY session variable. OracleĪccording to the Oracle documentation, the transaction that detected the deadlock is the one whose statement will be rolled back. As a rule of thumb, the database might choose to roll back the transaction with a lower rollback cost. While the database chooses to rollback one of the two transactions being stuck, it's not always possible to predict which one will be rolled back. Hence, a rollback leaves the database in a consistent state. Unlike the JVM, a database transaction is designed as an atomic unit of work. When a cycle is detected, the database engine picks one transaction and aborts it, causing its locks to be released, so that the other transaction can make progress. The database engine runs a separate process that scans the current conflict graph for lock-wait cycles (which are caused by deadlocks). Preserving the lock order becomes the responsibility of the data access layer, and the database can only assist in recovering from a deadlock situation. However, a database system cannot enforce a given lock acquisition order since it's impossible to foresee what other locks a certain transaction will want to acquire further. Instead, Java defines an interrupt method, which acts as a hint as a thread that gets interrupted can simply ignore the interruption and continue its execution.įor this reason, a Java application cannot recover from a deadlock situation, and it is the responsibility of the application developer to order the lock acquisition requests in such a way that deadlocks can never occur. If this happens in a Java application, the JVM cannot just force a Thread to stop its execution and release its locks.Įven if the Thread class exposes a stop method, that method has been deprecated since Java 1.1 because it can cause objects to be left in an inconsistent state after a thread is stopped. Deadlocks can occur in any concurrency environment, not just in a database system.įor instance, a multithreading program can deadlock if two or more threads are waiting on locks that were previously acquired so that no thread can make any progress. If you're using a Concurrency Control algorithm that relies on locks, then there is always the risk of running in a deadlock situation. One real world example of a deadlock is when you lock your house keys in your car, and your car keys in your house.Ī deadlock happens when two concurrent transactions cannot make progress because each one waits for the other to release a lock, as illustrated in the following diagram.īecause both transactions are in the lock acquisition phase, neither one releases a lock prior to acquiring the next one. the database) will kill queries that don't finish in time. In practice this usually results in timeout errors since it is not desired to have any query hanging forever, and the underlying system (e.g. Proc2: Lock A - starts waiting until proc1 releases A Proc1: Lock B - starts waiting until proc2 releases B At the same moment, someone else does the same thing in opposite order - locks B first, then locks A. One example: Let's say I have table A and table B, I need to do some update in A and then B and I decide to lock both of them at the moment of usage (this is really stupid behaviour, but it serves it's purpose now). When you configure your system for deadlock and lockwait timeouts and an application could be chosen as a victim when the transaction times out, you should program your application to handle them.In general, deadlock means that two or more entities are blocking some sources, and none of them is able to finish, because they are blocking sources in a cyclic way. Programming applications to handle deadlocks.If deadlocks occur frequently in your multi-user system with a particular application, you might need to do some debugging. You configure the amount of time a transaction waits before Derby does any deadlock checking with the property. Configuring deadlock detection and lock wait timeouts.When a transaction waits more than a specific amount of time to obtain a lock (called the deadlock timeout), Derby can detect whether the transaction is involved in a deadlock.Įven if a transaction is not involved in a deadlock, it might have to wait a considerable amount of time to obtain a lock because of a long-running transaction or transactions holding locks on the tables it needs. Using both row-level locking and the TRANSACTION_READ_COMMITTED isolation level makes it likely that you will avoid deadlocks (both settings are Derby defaults).
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