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And Franchise Colleges HSQ - DATABASES & SQL 06 (a) Supplement RDBMS Issues 2 By MANSHA NAWAZ Section 06 (a) RDBMS 2 1 RDBMS Issues 2: Database Recovery & Concurrency Database Recovery This lecture is concerned with advanced server based RDBMS needs. Recovery is Needed • When: – Hardware errors occur – Software errors occur – External circumstances occur (e.g., flood, fire, etc.) • Redundancy – is key to recovery, but this redundancy is hidden from the user and is different to data redundancy as seen previously Section 06 (a) RDBMS 2 2 Transaction A transaction is a unit of work, – that is either done completely – or not done at all Transaction – is the unit of recovery and – the unit of concurrency • (see later on concurrency) Section 06 (a) RDBMS 2 3 Transaction Process Assume an SQL statement is atomic, then we are concerned with situations requiring more than one SQL statements (e.g., crediting a bank account and debiting another) BEGIN TRANSACTION Do lots of things ….. ABORT TRANSACTION (ROLLBACK) END TRANSACTION (COMMIT) Section 06 (a) RDBMS 2 4 Transaction Process Q: How do we know how to rollback – what were the previous operations since the Begin Transaction statement? A: keep a LOG – A System File • keeps details of transactions begun • operations performed (before and after states) • transactions aborted • transactions committed • and any other information of importance to recovery (see later) Section 06 (a) RDBMS 2 5 Transaction Process • Log enables an undo, as it keeps details of new and old values of attributes (i.e., update Attribute A from 3 to 5) – Old values are restored on an undo • Failure occurs during undo, then undo – undoing many times = undo once • Failure between operation & writing log – Write Ahead Log protocol Section 06 (a) RDBMS 2 6 Possible Recovery Situations • Particular types of software and hardware situations where recovery may be needed: – – – – Planned situations by programmer (in software) Unplanned by programmer (in software) Systems failures (e.g., CPU) Media Failures (e.g., disk head crash) Section 06 (a) RDBMS 2 7 Possible Recovery Situations • Planned by programmer – force an abort of a transaction by writing the code explicitly IF Balance < 0 THEN Message (“Can’t transfer – aborting transaction’); Abort; END; Section 06 (a) RDBMS 2 8 Possible Recovery Situations • Unplanned by programmer – Temp:= count / 0 – Divide by zero causes a software fault and results in an abort of the current transaction. Section 06 (a) RDBMS 2 9 Possible Recovery Situations • System failures (e.g., CPU failure) – When a COMMIT is performed, the changes are considered to be permanent by the user – However, internally the changes may not have been transmitted from primary storage (buffers) to secondary storage (disk) at the time of systems failure, and CPU failure affects the buffers (but not the disk). Section 06 (a) RDBMS 2 10 Possible Recovery Situations Q: How can we find out which transactions are affected? A: LOG? – insufficient on its own • whilst a transaction commit may have been logged on the log, via write ahead protocol, either the actual update may not have taken place • and/or the page associated with the last operation(s) of the transaction may not have been written to disk. Section 06 (a) RDBMS 2 11 Checkpoints • System controlled points • (snapshot) – flush all data out of the buffers to guarantee changes – issue a checkpoint record to the log • what transactions are active • address of the most recent log record for each transaction – on abort, the most recent checkpoint ensures that we know which transactions are of interest to us Section 06 (a) RDBMS 2 12 Using Checkpoints Tc Tf T1 T2 T3 T4 T5 Checkpoint -------------LOG----------> Section 06 (a) RDBMS 2 13 Using Checkpoints Process for determining Undo and Redo list of transactions at Tf •Redo – completely undone, then done again – using log details of past and present states •Redo several times = redo once •At Checkpoint Tc: T2 & T3 are active, so put on Undo List – Redo list is initially empty •Work through log from checkpoint to Tf •Find BEGIN transaction – put on Undo list •Find END transaction – change from Undo List to Redo List Section 06 (a) RDBMS 2 14 Using Checkpoints • For our transactions diagram: – Undo {initial} = {T2, T3} – Redo {initial} = {} – Undo {Tf} = {T3, T5} – Redo {Tf} = {T2, T4} Section 06 (a) RDBMS 2 15 Media Failures: • These affect the database • periodic dumping at appropriate times (e.g., end of the week) • page updating/incremental dumping (e.g., end of day) • cold start - use archived dump information and the log if intact Section 06 (a) RDBMS 2 16 Summary: Database Recovery • What database recovery is about – Transaction – Log & Write Ahead Log Protocol • Types of failures: – Software failures, planned and unplanned – Systems failures (e.g., CPU) where only recent transactions may be affected • Checkpoints – Media Failures, where database may be affected Section 06 (a) RDBMS 2 17 Database Concurrency This is about ensuring that all concurrent transactions leave the database in the same state as if the transactions were executed one after the other – Multi-user problem ONLY – Look at the major types of problems and some common solutions – Note: work in terms of reads and writes • 1 SQL statement may involve both reads and writes • e.g., UPDATE Section 06 (a) RDBMS 2 18 The Lost Update Problem Transaction 1 Read R Write R = R + 1 Time Period 1 2 3 4 5 6 Transaction 2 Read R Write R = R + 1 With R = 5, R’ = 6 rather than 7 i.e., lost one of the updates, as it was written over! Section 06 (a) RDBMS 2 19 Uncommitted Dependency Problem Transaction 1 Read R Time Period Transaction 2 1 2 3 4 5 6 Write R = some_value Abort First transaction has a read value of R which no longer exists Section 06 (a) RDBMS 2 20 Inconsistent Analysis Problem Transaction 1 Read R Read P Time Period 1 2 3 4 5 6 7 Transaction 2 Read R Write R = R + 10 Read P Write P = P + 10 Commit First transaction has an inconsistent picture of R and P Section 06 (a) RDBMS 2 21 Solutions • Locking (most common): – Lock an object so that other transactions cannot interfere with it – Unlock it when finished – Lock + Unlock = 2 Phase Locking (Protocol) – Most common lock types • Exclusive / write / X Lock • Shared / read / S Lock X Lock S Lock Section 06 (a) X Lock N N RDBMS 2 S Lock N Y 22 Solutions • Transaction can promote its S lock into an X lock, as long as there are no other S locks on it • N --- Transaction goes into a wait state • Locking can occur when a statement is used, or at the beginning of a transaction (greedy!) • Unlocking occurs typically at COMMIT or ROLLBACK Section 06 (a) RDBMS 2 23 Lost Update Problem Transaction 1 Read R Write R = R + 1 • • Time Period 1 2 3 4 5 Transaction 2 Read R Write R = R + 1 Solved the Lost Update, but introduced Deadlock or Deadly Embrace Solutions: – Never allow transactions together that could result in deadlock – Choose a victim and abort, with error message Section 06 (a) RDBMS 2 24 Uncommitted Dependency Problem Transaction 1 Read R Section 06 (a) Time Period Transaction 2 1 2 3 4 5 6 Write R = some_value RDBMS 2 Abort 25 Inconsistent Analysis Problem Transaction 1 Read R Read P Section 06 (a) Time Period 1 2 3 4 5 6 7 8 9 10 11 RDBMS 2 Transaction 2 Read R Write R = R + 10? Write R = R + 10 Read P Write P = P + 10 Commit 26 Granularity of Locking • What to lock …. – Whole Database – Whole Table – Whole Record – Whole Attribute • Trade-off between simplicity of method vs. concurrency allowed Section 06 (a) RDBMS 2 27 Summary: Database Concurrency • Three common concurrency problems • Standard Method of dealing with them – 2 Phase Locking • Show how they overcome concurrency problems • Deadlock or Deadly Embrace • Granularity of Locking Section 06 (a) RDBMS 2 28 End of Lecture Section 06 (a) RDBMS 2 29