Transaction Management OverviewR & G Chapter 16There are three side effects of acid. Enhanced long term memory, decreased short term memory, and I forget the third.- Timothy LearyConcurrency Control & Recovery• Concurrency Control– Provide correct and highly available data access in the presence of concurrent access by many users• Recovery– Ensures database is fault tolerant, and not corrupted by software, system or media failure– 24x7 access to mission critical data• A boon to application authors!– Existence of CC&R allows applications be be written without explicit concern for concurrency and fault toleranceRoadmap• Overview (Today)• Concurrency Control (1-2 lectures)• Recovery (1-2 lectures)Query Optimizationand ExecutionRelational OperatorsFiles and Access MethodsBuffer ManagementDisk Space ManagementDBThese layers must consider concurrencycontrol and recovery(Transaction, Lock, Recovery Managers)Structure of a DBMSTransactions and Concurrent Execution • Transaction (“xact”)- DBMS’s abstract view of a user program (or activity): – A sequence of reads and writes of database objects.– Unit of work that must commit or abort as an atomic unit• Transaction Manager controls the execution of transactions.• User’s program logic is invisible to DBMS!– Arbitrary computation possible on data fetched from the DB– The DBMS only sees data read/written from/to the DB.• Challenge: provide atomic transactions to concurrent users!– Given only the read/write interface.Concurrency: Why bother?• The latency argument• The throughput argument• Both are critical!ACID properties of Transaction Executions• A tomicity: All actions in the Xact happen, or none happen.• C onsistency: If each Xact is consistent, and the DB starts consistent, it ends up consistent.• I solation: Execution of one Xact is isolated from that of other Xacts.• D urability: If a Xact commits, its effects persist.Atomicity and Durability• A transaction ends in one of two ways:–commitafter completing all its actions• “commit” is a contract with the caller of the DB–abort(or be aborted by the DBMS) after executing some actions. • Or system crash while the xact is in progress; treat as abort. • Two important properties for a transaction:–Atomicity: Either execute all its actions, or none of them–Durability: The effects of a committed xact must survive failures.• DBMS ensures the above by loggingall actions:–Undothe actions of aborted/failed transactions.–Redo actions of committed transactions not yet propagated to disk when system crashes.A.C.I.D.Transaction Consistency• Transactions preserve DB consistency– Given a consistent DB state, produce another consistent DB state• DB Consistency expressed as a set of declarative Integrity Constraints– CREATE TABLE/ASSERTION statements• E.g. Each CS186 student can only register in one project group. Each group must have 2 students.– Application-level• E.g. Bank account total of each customer must stay the same during a “transfer” from savings to checking account• Transactions that violate ICs are aborted– That’s all the DBMS can automatically check!A.C.I.D.Isolation (Concurrency)• DBMS interleaves actions of many xacts concurrently– Actions = reads/writes of DB objects• DBMS ensures xacts do not “step onto” one another.• Each xact executes as if it were running by itself.– Concurrent accesses have no effect on a Transaction’s behavior– Net effect must beidentical to executing all transactions for some serial order.– Users & programmers think about transactions in isolation• Without considering effects of other concurrent transactions!A.C.I.D.Example• Consider two transactions (Xacts):T1: BEGIN A=A+100, B=B-100 ENDT2: BEGIN A=1.06*A, B=1.06*B END• 1st xact transfers $100 from B’s account to A’s• 2nd credits both accounts with 6% interest.• Assume at first A and B each have $1000. What are the legal outcomes of running T1 and T2?• T1 ; T2 (A=1166,B=954)• T2 ; T1 (A=1160,B=960)• In either case, A+B = $2000 *1.06 = $2120 • There is no guarantee that T1 will execute before T2 or vice-versa, if both are submitted together. Example (Contd.)• Consider a possible interleaved schedule:T1: A=A+100, B=B-100 T2: A=1.06*A, B=1.06*B This is OK (same as T1;T2). But what about:T1: A=A+100, B=B-100 T2: A=1.06*A, B=1.06*B• Result: A=1166, B=960; A+B = 2126, bank loses $6 !• The DBMS’s view of the second schedule:T1: R(A), W(A), R(B), W(B)T2: R(A), W(A), R(B), W(B)Scheduling Transactions: Definitions•Serial schedule:no concurrency– Does not interleave the actions of different transactions.•Equivalent schedules: same result on any DB state– 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: equivalent to a serial schedule– A schedule that is equivalent to some serial execution of the transactions.(Note: If each transaction preserves consistency, every serializable schedule preserves consistency. )Anomalies with Interleaved Execution• Reading Uncommitted Data (WR Conflicts, “dirty reads”):• Unrepeatable Reads (RW Conflicts):T1: R(A), W(A), R(B), W(B), AbortT2: R(A), W(A), CT1: R(A), R(A), W(A), CT2: R(A), W(A), CAnomalies (Continued)• Overwriting Uncommitted Data (WW Conflicts):T1: W(A), W(B), CT2: W(A), W(B), CLock-Based Concurrency Control• A simple mechanism to allow concurrency but avoid the anomalies just described…•Two-phase Locking (2PL) Protocol:– Always obtain a S (shared) lock on object before reading– Always obtain an X (exclusive) lock on object before writing.– If an Xact holds an X lock on an object, no other Xact can get a lock (S or X) on that object.– DBMS internally enforces the above locking protocol– Two phases: acquiring locks, and releasing them• No lock is ever acquired after one has been released• “Growing phase” followed by “shrinking phase”.• Lock Manager tracks lock requests, grants locks on database objects when they become available.Strict 2PL• 2PL allows only serializable schedules but is subjected to cascading aborts.• Example: rollback of T1 requires rollback of T2!• To avoid Cascading aborts, use Strict 2PL• Strict Two-phase Locking (Strict 2PL) Protocol:– Same as 2PL, except:–
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