Detecting and Resolving Lock Contention in Software

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Detecting and Resolving Lock Contention
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There are views that will tell you what is going on with locking in the database, but this is one case where even very experienced DBAs will often prefer to use the graphical tools To reach the Database Control lock manager, take the Performance tab from the database home page, then the Instance Locks link in the Additional Monitoring Links section Figure 8-6 shows the Instance Locks window, with Blocking Locks selected There may be any number of locks within the database, but it is usually only the locks that are causing sessions to hang that are of interest These are known as blocking locks In Figure 8-6, there are two problems Session number 116, logged on as user SCOTT, is holding an exclusive lock on one or more rows of the table HREMPLOYEES This session is not hanging it is operating normally But session number 129, logged on as user MPHO, is blocked it is waiting for an exclusive lock on one or more of the rows locked by session 116 Session 129 is hanging at this moment and will continue to hang until session 116 releases its lock(s) by terminating its transaction, with a COMMIT or a ROLLBACK The second problem is worse: JON is blocking two sessions, those of ISAAC and ROOP
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8: DML and Concurrency
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Figure 8-6 Showing locks with Database Control
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Lock contention is a natural consequence of many users accessing the same data concurrently The problem can be exacerbated by badly designed software, but in principle lock contention is part of normal database activity It is therefore not possible for the DBA to resolve it completely he can only identify that it is a problem, and suggest to system and application designers that they bear in mind the impact of lock contention when designing data structures and programs If locks are becoming an issue, as in Figure 8-6, they must be investigated Database Control can provide the necessary information Clicking the values in the SQL ID column will let you see what statements being executed caused the lock contention In the figure, SCOTT and MPHO have both executed one statement JON, ISAAC, and ROOP have executed another The ROWID column can be used to find the exact row for which the sessions are contending You cannot drill down to the row from this window, but the rowid can be used in a SELECT statement to retrieve the row in another (unblocked) session When the code and the rows that cause the contention are known, a solution can be discussed with the system designers and developers In an emergency, however, it is possible for the DBA to solve the problem by terminating the session, or sessions, that are holding too many locks for too long When a session is terminated forcibly, any locks it holds will be released as its active transaction is rolled back The blocked sessions will then become free and can continue To terminate a session, either use Database Control, or the ALTER SYSTEM KILL SESSION command In the preceding example, if you decided that the SCOTT session is holding its lock for an absurd period of time, you would select the radio button for the session and click the KILL SESSION button SCOTT s transaction will be rolled back, and MPHO s session will then be able to take the lock(s) it requires and continue working In the case of the second problem in the figure, killing JON s session would free up ISAAC, who would then be blocking ROOP
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Deadlocks
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It is possible to encounter a situation where two sessions block each other in such a fashion that both will hang, each waiting for the other to release its lock This is a deadlock Deadlocks are not the DBA s problem; they are caused by bad program design and resolved automatically by the database itself Information regarding deadlocks is written out to the alert log, with full details in a trace file part of your daily monitoring will pick up the occurrence of deadlocks and inform your developers that they are happening If a deadlock occurs, both sessions will hang but only for a brief moment One of the sessions will detect the deadlock within seconds, and it will roll back the statement that caused the problem This will free up the other session, returning the message ORA-00060 Deadlock detected This message must be trapped by your programmers in their exceptions clauses, which should take appropriate action It must be emphasized that deadlocks are a program design fault They occur because the code attempts to do something that is logically impossible Well-written code will always request locks in a sequence that cannot cause deadlocks to occur, or will test whether incompatible locks already exist before requesting them Exercise 8-6: Detect and Resolve Lock Contention In this exercise, you will first use SQL*Plus to cause a problem, and detect and solve it with Database Control 1 Using SQL*Plus, connect to your database in two sessions as user WEBSTORE 2 In your first session, lock all the rows in the PRODUCTS table:
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select * from products for update;
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3 In your second session, attempt to update a row The session will hang:
update products set stock_count=stock_count-1;
4 Connect to your database as user SYSTEM with Database Control 5 Navigate to the Instance Locks window, by taking the Performance tab from the database home page, and then the Database Locks link in the Additional Monitoring Links section 6 Observe that the second SYSTEM session is shown as waiting for an EXCLUSIVE lock Select the radio button for the first, blocking, session and click KILL SESSION 7 In the confirmation window, click SHOW SQL This will show a command something like
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