Transaction Management Overview

1. Transaction Management Overview Chapter 16

2. Transactions • A transactionis the DBMS’s abstract view of a user program: a sequence of reads and writes. • Concurrent execution of user programs is essential for good DBMS performance. • Increasing system throughput (# of completed transactions in any given time) by overlapping I/O and CPU operations • Increasing response time (time for completing a transaction) by avoiding short transactions getting stuck behind long ones • A user’s program may carry out many (in-memory) operations on the data retrieved from the database, but the DBMS is only concerned about what data is read/written from/to the database.

3. The ACID Properties (in a Nutshell) • Atomicity: Either all actions of the transactions are executed or none at all. • Consistency: Any transaction that starts executing in a consistent database state must leave it in a consistent state upon completion. • Isolation: A transaction is protected from effects of concurrently running transactions. • Durability: Effects of committed transactions must persist and overcome any system failure (system crash/media failure).

4. Consistency and Isolation • Users submit transactions, and can think of each transaction as executing by itself. • Concurrency is achieved by the DBMS, which interleaves actions (reads/writes of DB objects) of various transactions. The net effect is the same as serially executing the transactions one after the other. • Each transaction must leave the database in a consistent state if the DB is consistent when the transaction begins. • DBMS will enforce some ICs, depending on the ICs declared in CREATE TABLE statements. • Beyond this, the DBMS does not really understand the semantics of the data. (e.g., it does not understand how the interest on a bank account is computed). • Issue:Coping with effects of interleaving transactions (Concurrency control).

5. Atomicity and Durability • A transaction mightcommitafter completing all its actions, or it could terminate unsuccessfully: • It could abort(or be aborted by the DBMS) after executing some actions. • The system may crash while transactions are in progress. • There could be a media failure preventing reads/writes. • How does the DBMS achieve atomicity and durabilityof all transactions? • Atomicity: the DBMS logs all actions so that it can undothe actions of aborted transactions. • Durability: committed actions are written to disk or (in case of a crash) the system must redoactions of committed Xacts which were not yet written to disk. • Issue:Coping with effects of crashes(Recovery).

6. Scheduling Transactions • Schedule: Interleaving of the actions of different 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. )

7. Concurrency Control: Example • Consider two transactions: T1: BEGIN A=A+100, B=B-100 END T2: BEGIN A=1.06*A, B=1.06*B END • Intuitively, the first transaction is transferring $100 from B’s account to A’s account. The second is crediting both accounts with a 6% interest payment. • There is no guarantee that T1 will execute before T2 or vice-versa, if both are submitted together. However, the net effect must be equivalent to these two transactions running serially in some order.

8. Example (Contd.) • Consider a possible interleaving (schedule / history): T1: A=A+100, B=B-100, C1 T2: A=1.06*A, B=1.06*B, C2 • This is serializable. But the following is not: T1: A=A+100, B=B-100, C1 T2: A=1.06*A, B=1.06*B, C2 • The DBMS’s view of the second schedule: T1: R(A), W(A), R(B), W(B), C1 T2: R(A), W(A), R(B), W(B), C2

9. Anomalies with Interleaved Execution • Reading Uncommitted Data (WR Conflicts, “dirty reads”): • Unrepeatable Reads (RW Conflicts): T1: R(A), W(A), R(B), W(B), Abort T2: R(A), W(A), C T1: R(A), R(A), W(A), C T2: R(A), W(A), C

10. Anomalies (Continued) • Overwriting Uncommitted Data (WW Conflicts, ‘’lost updates’’): T1: W(A), W(B), C T2: W(A), W(B), C

11. Lock-Based Concurrency Control • A DBMS ensures that only serializable schedules are allowed by using a suitable CC protocol. • Strict Two-phase Locking (Strict 2PL) Protocol: • Each transaction must obtain a S (shared) lock on object before reading, and an X (exclusive) lock on object before writing. • All locks held by a transaction are released when the transaction completes • If a transaction holds an X lock on an object, no other transaction can get a lock (S or X) on that object. • Strict 2PL does however allow deadlocks, i.e. cycles of transactions waiting for locks to be released. A DBMS must either prevent or detect (and resolve) deadlocks.

12. Aborting a Transaction • If a transaction Ti is aborted, all its actions have to be undone. Not only that, if Tj reads an object last written by Ti, Tj must be aborted as well! • Most systems avoid such cascading abortsby releasing a transaction’s locks only at commit time. • If Ti writes an object, Tj can read this only after Ti commits. • In order to undo the actions of an aborted transaction, the DBMS maintains a log in which every write is recorded. The log is also used to recover from system crashes: all active transactions at the time of the crash are aborted when the system comes back up.

13. Support for Transactions in SQL • SQL provides users with support to specify transaction-level behavior. A transaction is automatically started with each user SQL statement (except with CONNECT) and terminated with either a COMMIT or a ROLLBACK command. • SQL99 supports transactions with savepoints and chained transactions. • One defines a savepoint and may later selectively roll back to it: e.g., SAVEPOINT point1 ... ROLLBACK TO SAVEPOINT point1. • One may commit/rollback and immediately initiate another transaction: e.g. SELECT * FROM Sailors COMMIT AND CHAIN SELECT * FROM Students ROLLBACK • A DBMS can lock DB objects at different granularities: row-level granularity vs. table-level granularity. • Row-level granularity is not immune to the phantom phenomenon, i.e. a transaction sees twice a different collection of objects (tuples). The vanishing/new tuples are called phantoms.

14. Support for Transactions in SQL (Contd.) • SQL provides support for controlling the access mode to DB objects, the diagnostics size, and the isolation level of transactions. • Diagnostics size: # of error conditions recordable by the DBMS • Access mode: READ ONLY / WRITE ONLY / READ WRITE • Isolation level: controls what other transactions see of a given transaction. Level Dirty read Unrepeatable read Phantom READ UNCOMMITTED Possible Possible Possible READ COMMITTED No Possible Possible REPEATABLE READ No No Possible SERIALIZABLE No No No • Example: SET TRANSACTION ISOLATION LEVEL SERIALIZABLE READ WRITE

15. Crash Recovery: Stealing / Forcing Pages • The recovery manager ensures atomicity (by undoing actions of uncommitted transactions) and durability (by guaranteeing that committed transactions survive crashes/failures) of transactions. • The transaction manager ensures serializability and consistency of transactions by using an appropriate locking protocol. • Alternatives in managing buffers: • Steal approach: the changes made to an object O by a transaction T may be written to disk before T commits. This arises when the buffer manager decides to replace the frame containing O by a page (belonging to a different transaction) from disk. • Force approach: Force all writes of a committed transaction to disk. • Most systems use a steal, no-force approach which is a realistic one: • If any dirty frame is chosen for replacement, the page it contains is written to disk (steal). • Dirty pages in buffer are not forced to disk at commit times of associated transactions (no-force).

16. Crash Recovery: the Log • The following actions are recorded in the log: • Ti writes an object: the old value and the new value are recorded. • Log record must go to diskbeforethe changed page (WAL)! • Ti commits/aborts: a log record indicating this action. • Log records are chained together by transaction id, so it’s easy to undo a specific transaction. • The log is often duplexed and archived on stable storage. • All log related activities (and in fact, all concurrency control related activities such as lock/unlock, dealing with deadlocks etc.) are handled transparently by the DBMS.

17. Crash Recovery: the ARIES Approach • A steal, no-force approach in 3 phases: • Analysis: Scan the log forward (from the most recent checkpoint) to identify • all transactions that were active, and • all dirty pages in the buffer pool at the time of the crash. • Redo: Redoes all updates to dirty pages in the buffer pool, as needed, to ensure that all logged updates are in fact carried out and written to disk. • Undo: Undoes writes of all transactions that were active at the crash time (by restoring the before value of the update, which is in the log record for the update), working backwards in the log.

18. Summary • Concurrency control and recovery are among the most important functions provided by a DBMS. • Users need not worry about concurrency. • System automatically inserts lock/unlock requests and schedules actions of different Xacts in such a way as to ensure that the resulting execution is equivalent to executing the Xacts one after the other in some order. • Write-ahead logging (WAL) is used to undo the actions of aborted transactions and to restore the system to a consistent state after a crash. • Consistent state: Only the effects of commited Xacts seen.