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set to help control locking behavior. One of the locking behaviors to consider is whether all rows fetched using a cursor are locked until the next COMMIT or ROLLBACK, or whether previously read rows are released when the next row is fetched. Consult your database vendor documentation for more details. The main problem with locking mechanisms is that locks cause contention, meaning that the placement of locks to prevent loss of data from concurrent updates has the side effect of causing concurrent sessions to compete for the right to apply updates. At the least, lock contention slows user processes as sessions wait for locks. At the worst, competing lock requests can stall sessions inde nitely, as you will see in the next section.
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A deadlock is a situation where two or more database sessions have locked some data and then each requests a lock on data that another session has locked. Figure 9-2 illustrates this situation. This example again uses two users from our ctitious company, cleverly named A and B. User A is a customer representative in the customer service department and is attempting to correct a payment that was credited to the wrong customer account. He needs to subtract (debit) the payment from Customer 1 and add (credit) it to Customer 2. User B is a database specialist in the IT department, and she has written an SQL statement to update some of the customer phone numbers with one area code to a new area code in response to a recent area code split by the phone company. The statement has a WHERE clause that limits the update to only those customers having a phone number with certain pre xes in area code 510 and updates those phone numbers to the new area code. User B submits her SQL UPDATE statement while User A is working on his payment credit problem. Customers 1 and 2 both have
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Correct a payment posting error: Debit Customer 1 and Credit Customer 2 Split telephone area code 510
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1. Select and Update Customer 1 (locks the Customer 1 row) Database User A 3. Select and Update Customer 2 (must wait due to User B lock) Customer Schema 2. Update Customer 2 (locks the Customer 2 row) Database User B 4. Update Customer 1 (must wait due to User A lock)
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phone numbers that need to be updated. The sequence of events (all happening within seconds of each other), as illustrated in Figure 9-2, takes place as follows: 1. User A selects the data from Customer 1 and applies an update to debit the balance due. No commit is issued yet because this is only part of the transaction that must take place. The row for Customer 1 now has a lock on it due to the update. 2. The statement submitted by User B updates the phone number for Customer 2. The entire SQL statement must run as a single transaction, so there is no commit at this point, and thus User B holds a lock on the row for Customer 2. 3. User A selects the balance for Customer 2 and then submits an update to credit the balance due (same amount as debited from Customer 1). The request must wait because User B holds a lock on the row to be updated. 4. The statement submitted by User B now attempts to update the phone number for Customer 1. The update must wait because User A holds a lock on the row to be updated. These two database sessions are now in deadlock. User A cannot continue due to a lock held by User B, and vice versa. In theory, these two database sessions will be stalled forever. Fortunately, modern DBMSs contain provisions to handle this situation. One method is to prevent deadlocks. Few DBMSs have this capability due to the considerable overhead this approach requires and the virtual impossibility of predicting what an interactive database user will do next. However, the theory is to inspect each lock request for the potential to cause contention and not permit the lock to take place if a deadlock is possible. The more common approach is deadlock detection, which then aborts one of the requests that caused the deadlock. This can be done either by timing lock waits and giving up after a preset time interval or by periodically inspecting all locks to nd two sessions that have each other locked out. In either case, one of the requests must be terminated and the transaction s changes rolled back in order to allow the other request to proceed.
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