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Transaction blocking can cause deadlock, the situation where a set of transactions arrive at a state where none of them can proceed.
A deadlock can arise for two reasons:
Transaction A is blocked on transaction B, and transaction B is blocked on transaction A. More time will not solve the problem, and one of the transactions must be canceled, allowing the other to proceed. The same situation can arise with more than two transactions blocked in a cycle.
To eliminate a transactional deadlock, the database server selects a connection from those involved in the deadlock, rolls back the changes for the transaction that is active on that connection and returns an error. The database server selects the connection to roll back by using an internal heuristic that prefers the connection with the smallest blocking wait time left as determined by the blocking_timeout option. If all connections are set to wait forever, then the connection that caused the server to detect a deadlock is selected as the victim connection.
When a transaction becomes blocked, its worker is not relinquished. For example, if the database server is configured with three workers and transactions A, B, and C are blocked on transaction D which is not currently executing a request, then a deadlock situation has arisen since there are no available workers. This situation is called thread deadlock.
Suppose that the database server has n workers. Thread deadlock occurs when n-1 workers are blocked, and the last worker is about to block. The database server's kernel cannot permit this last worker to block, since doing so would result in all workers being blocked, and the database server would hang. Instead, the database server ends the task that is about to block the last worker, rolls back the changes for the transaction active on that connection, and returns an error (SQLCODE -307, SQLSTATE 40W06).
Database servers with tens or hundreds of connections may experience thread deadlock in cases where there are many long-running requests either because of the size of the database or because of blocking. In this case, increasing the database server's multiprogramming level may be an appropriate solution. The design of your application may also cause thread deadlock because of excessive or unintentional contention. In these cases, scaling the application to larger data sets can make the problem worse, and increasing the database server's multiprogramming level may not solve the problem.
The number of database threads that the server uses depends on the individual database's setting.