barcode generator java source code free Databases: A Beginner s Guide in Software

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Databases: A Beginner s Guide
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Column One or more columns within a row in the table are locked This method sounds terrific in theory, but it s not very practical because of the resources required to place and release locks at this level of granularity Very sparse support for it exists in modern commercial database systems
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Locks are always placed when data is updated or deleted Most RDBMSs also support the use of a FOR UPDATE OF clause on a SELECT statement to allow locks to be placed when the database user declares an intent to update something Some locks may be considered read-exclusive, which prevents other sessions from even reading the locked data Many RDBMSs have session parameters that can be 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 s 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 indefinitely, as you will see in the next section
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Deadlocks
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A deadlock is a situation in which two or more database sessions have locked some data and then each has requested a lock on data that another session has locked Figure 11-2 illustrates this situation
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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) 3 Select and update customer 2 (must wait due to user B lock) Customer Schema
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2 Update customer 2 (locks the customer 2 row) 4 Update customer 1 (must wait due to user A lock)
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Figure 11-2 The deadlock
11: Deploying Databases
This example again uses two users from our fictitious company, cleverly named A and B User A is a 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 those customers having a phone number with certain prefixes 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 phone numbers that need to be updated The sequence of events (all happening within seconds of each other), as illustrated in Figure 11-2, 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
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