Final Exam Review

1. Final Exam Review Last Lecture R&G - All Chapters Covered The end crowns all, And that old common arbitrator, Time, Will one day end it. William Shakespeare. Troilus and Cressida.

2. Topics Covered • Transactions • Concurrency Control • Crash Recovery

3. ACID properties of Transaction Executions • Atomicity: All actions in the Xact happen, or none happen. • Consistency: If each Xact is consistent, and the DB starts consistent, it ends up consistent. • Isolation: Execution of one Xact is isolated from that of other Xacts. • D urability: If a Xact commits, its effects persist.

4. Concurrency Control • Transaction: basic unit of operation • made up of reads and writes • Goal: ACID Transactions • A & D are provided by Crash Recovery • C & I are provided by Concurrency Control • Bottom line: reads and writes for various transactions MUST be ordered such that the final state of the database is the same as some serial ordering of the transactions

5. Scheduling Transactions • Serial schedule: Schedule that does not interleave the actions of different transactions. • Equivalent schedules:For any database state, the effect (on the set of objects in the database) of executing the first schedule is identical to the effect of executing the second schedule. • Serializable schedule: A schedule that is equivalent to some serial execution of the transactions. (Note: If each transaction preserves consistency, every serializable schedule preserves consistency. )

6. Conflict Serializable Schedules • Two schedules are conflict equivalent if: • Involve the same actions of the same transactions • Every pair of conflicting actions is ordered the same way • Schedule S is conflict serializable if S is conflict equivalent to some serial schedule

7. Example • A schedule that is not conflict serializable: • The cycle in the graph reveals the problem. The output of T1 depends on T2, and vice-versa. T1: R(A), W(A), R(B), W(B) T2: R(A), W(A), R(B), W(B) A T1 T2 Dependency graph B

8. Dependency Graph • Dependency graph: One node per Xact; edge from Ti to Tj if an operation of Ti conflicts with an operation of Tj and Ti’s operation appears earlier in the schedule than the conflicting operation of Tj. • Theorem: Schedule is conflict serializable if and only if its dependency graph is acyclic

9. An Aside: View Serializability • Schedules S1 and S2 are view equivalent if: • If Ti reads initial value of A in S1, then Ti also reads initial value of A in S2 • If Ti reads value of A written by Tj in S1, then Ti also reads value of A written by Tj in S2 • If Ti writes final value of A in S1, then Ti also writes final value of A in S2 • View serializability is “weaker” than conflict serializability! • Every conflict serializable schedule is view serializable, but not vice versa! • I.e. admits more legal schedules T1: R(A) W(A) T2: W(A) T3: W(A) T1: R(A),W(A) T2: W(A) T3: W(A)

10. Approaches to Concurrency Control • 2PL - all objects have Shared and eXclusive locks • once one lock is released, no more locks may be acquired • Strict 2PL: don’t release locks until commit time • Conservative 2PL: acquire all locks at start, release all at end • Locking issues • must either prevent or detect deadlock • may want multiple granularity locks (table, page, record) using IS, IX, SIX, S, X locks (check compatibility matrix!) • locking in B-trees usually not 2PL • phantom problem: locking all records of a given criteria (e.g., age > 20)

11. Database Tables Pages Tuples Multiple-Granularity Locks • Hard to decide what granularity to lock (tuples vs. pages vs. tables). • Shouldn’t have to make same decision for all transactions! • Data “containers” are nested: contains

12. IX SIX S X IS Ö Ö Ö Ö IS Database IX Ö Ö SIX Ö Tables S Ö Ö Pages X Tuples Solution: New Lock Modes, Protocol • Allow Xacts to lock at each level, but with a special protocol using new “intention” locks: • Still need S and X locks, but before locking an item, Xact must have proper intension locks on all its ancestors in the granularity hierarchy. • IS – Intent to get S lock(s) at finer granularity. • IX – Intent to get X lock(s) at finer granularity. • SIX mode: Like S & IX at the same time. Why useful?

13. Multiple Granularity Lock Protocol • Each Xact starts from the root of the hierarchy. • To get S or IS lock on a node, must hold IS or IX on parent node. • What if Xact holds SIX on parent? S on parent? • To get X or IX or SIX on a node, must hold IX or SIX on parent node. • Must release locks in bottom-up order. Protocol is correct in that it is equivalent to directly setting locks at the leaf levels of the hierarchy.

14. Tables Tuples IX SIX S X IS Ö Ö Ö Ö IS IX Ö Ö SIX Ö S Ö Ö X Examples – 2 level hierarchy • T1 scans R, and updates a few tuples: • T1 gets an SIX lock on R, then get X lock on tuples that are updated. • T2 uses an index to read only part of R: • T2 gets an IS lock on R, and repeatedly gets an S lock on tuples of R. • T3 reads all of R: • T3 gets an S lock on R. • OR, T3 could behave like T2; can use lock escalation to decide which. • Lock escalation • Dynamically asks for coarser-grained locks when too many low level locks acquired

15. Other Approaches to CC • Optimistic CC • Timestamp CC • Multiversion CC NOT COVERED in CLASS – NOT ON FINAL

16. Question from old final - True/False • “Our lock manager uses S and X locks, which guarantees that all schedules are conflict-serializable” • “Because we use 2PL, deadlocks can never occur” • “Because we use Strict 2PL, there will be no cascading aborts”

17. Question from old final - True/False • “Our lock manager uses S and X locks, which guarantees that all schedules are conflict-serializable” • “Because we use 2PL, deadlocks can never occur” • “Because we use Strict 2PL, there will be no cascading aborts”

18. More questions .. • Read Uncommitted (UR): • Always read data without setting any locks • Always set exclusive locks before writing and hold them until end-of-transaction. • Or: No S-Locks, Strict 2-Phase X-Locks. Which of the following conflicts can occur between a UR Xact, and a Strict 2PL Xact: • Write-Write • Read-Write • Write-Read

19. More questions .. • Read Committed (RC): • Obtain locks in the same way as Strict 2PL, and release exclusive locks at the end-of-transaction just like Strict 2PL. • Or: RC transactions use Short-Term S-Locks, and Strict 2-Phase X-Locks. Which of the following conflicts can occur between a RC Xact, and a Strict 2PL Xact: • Write-Write • Read-Write • Write-Read

20. More questions .. • All SQL lock modes – even the relaxed modes – use Strict 2-Phase Write Locks. Consider what would happen if a transaction released a shared lock immediately after a read, and released an exclusive lock immediately after a write (Short-Term R-Locks and Short-Term X-Locks). Which of the following could happen in those cases? • Write-Write conflicts. • Inconsistent data in the database • Deadlocks

21. Crash Recovery • ACID - need way to ensure A & D • We studied approach of Aries system • Buffer management Steal, no Force • Every Write to a page is first logged in WAS • log record is in stable storage before data page on disk • log record has Xact#, before value, after value • Checkpoints record which pages dirty, which XActs running

22. Buffer Mgmt Plays a Key Role • Force policy – make sure that every update is on disk before commit. • Provides durability without REDO logging. • But, can cause poor performance. • No Steal policy – don’t allow buffer-pool frames with uncommited updates to overwrite committed data on disk. • Useful for ensuring atomicity without UNDO logging. • But can cause poor performance. Of course, there are some nasty details for getting Force/NoSteal to work…

23. Preferred Policy: Steal/No-Force • This combination is most complicated but allows for highest performance. • NO FORCE(complicates enforcing Durability) • What if system crashes before a modified page written by a committed transaction makes it to disk? • Write as little as possible, in a convenient place, at commit time, to support REDOing modifications. • STEAL (complicates enforcing Atomicity) • What if the Xact that performed udpates aborts? • What if system crashes before Xact is finished? • Must remember the old value of P (to support UNDOing the write to page P).

24. No UNDO UNDO REDO REDO No UNDO UNDO No REDO No REDO Buffer Management summary No Steal No Steal Steal Steal No Force Fastest No Force Force Slowest Force Performance Implications Logging/Recovery Implications

25. Basic Idea: Logging • Record REDO and UNDO information, for every update, in a log. • Sequential writes to log (put it on a separate disk). • Minimal info (diff) written to log, so multiple updates fit in a single log page. • Log: An ordered list of REDO/UNDO actions • Log record contains: <XID, pageID, offset, length, old data, new data> • and additional control info (which we’ll see soon).

26. Write-Ahead Logging (WAL) • The Write-Ahead Logging Protocol: • Must force the log record for an update before the corresponding data page gets to disk. • Must force all log records for a Xact beforecommit. (alt. transaction is not committed until all of its log records including its “commit” record are on the stable log.) • #1 (with UNDO info) helps guarantee Atomicity. • #2 (with REDO info) helps guarantee Durability. • This allows us to implement Steal/No-Force • Exactly how is logging (and recovery!) done? • We’ll look at the ARIES algorithms from IBM.

27. Transaction Commit • write Commit record to log • flush log tail to stable storage • remove Xact from Xact table • write End record to log

28. Transaction Abort • write Abort record to log • go back through log, undoing each write (and add CLR to log) • when done, write End record to log

29. Crash Recovery - 3 phases • Analysis: starting from checkpoint, go forward in the log to see: • what pages were dirty • what transactions were active at time of crash • Redo: start from oldest transaction that wrote to a dirty page, and redo all writes to dirty pages. • Undo: start at the end of the log (time of crash), work backward undoing all writes made by transactions that were active at time of crash • What happens when you encounter a CLR ?