Gestione delle transazioni: controllo di affidabilità: T03

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1 Gestione delle transazioni: controllo di affidabilità: T03 Paolo Atzeni, Stefano Ceri 6/05/2003 Definition of a Transaction An elementary unit of work performed by an application, with specific features for what concerns correctness, robustness and isolation Each transaction is encapsulated within two commands begin transaction (start transaction in SQL) end positive (commit): commit work negative (abort): rollback work Transactional System: a system capable of providing the definition & execution of transactions on behalf of multiple, concurrent applications 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 2

2 Una transazione start transaction; update ContoCorrente set Saldo = Saldo + 10 where NumConto = 12202; update ContoCorrente set Saldo = Saldo 10 where NumConto = 42177; commit work; 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 3 Una transazione con varie decisioni start transaction; update ContoCorrente set Saldo = Saldo + 10 where NumConto = 12202; update ContoCorrente set Saldo = Saldo 10 where NumConto = 42177; select Saldo into A from ContoCorrente where NumConto = 42177; if (A>=0) then commit work else rollback work; 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 4

3 Transazioni in JDBC Scelta della modalità delle transazioni: un metodo definito nell'interfaccia Connection: setautocommit(boolean autocommit) con.setautocommit(true) (default) "autocommit": ogni operazione è una transazione con.setautocommit(false) gestione delle transazioni da programma con.commit() con.rollback() non c'è start transaction 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 5 ACID Properties of Transactions ACID is an acronym for: Atomicity Consistency Isolation Durability 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 6

4 Atomicity A transaction is an atomic unit of work It cannot leave the database in an intermediate state: a fault or error prior to commit causes the UNDO of the work made earlier A fault or error after the commit may require the REDO of the work made earlier, if its effect on the database state is not guaranteed Possible behaviors Commit = normality (99.9%) Abort (or rollback) requested by the application = suicide requested by the system (constraint violation, concurrency reasons, uncertainty or uncompletion in case of failure) = murder 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 7 Consistency Consistency amounts to requiring that the transaction does not violate any integrity constraint Integrity constraint verification can be: Immediate: during the transaction (the operation causing the violation is rejected, but control is kept within the transaction) Deferred: at the end of the transaction (if some integrity constraint is violated, the entire transaction is rejected) 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 8

5 Isolation Isolation requires that any transaction executes independently from the execution of all other concurrent transactions [isolation requires that the concurrent execution of a collection of transaction yields to the same result as an arbitrary sequential execution of the same transactions] Durability (Persistence) Durability requires that the effect of a transaction that has successfully committed be not lost (the effect will last forever ) 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 9 Transactions and system modules Atomicity and durability are guaranteed by the Reliability Control System Isolation is guaranteed by the Concurrency Control System Consistency is managed during the normal query execution by the DBMS System (verification activities are generated by the DDL Compilers and executed during query processing) 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 10

6 Gestore degli accessi e delle interrogazioni Gestore delle transazioni Gestore di Interrogazioni e aggiornamenti Gestore delle transazioni Gestore dei metodi d accesso Gestore del buffer Gestore della concorrenza Gestore della affidabilità Gestore della memoria secondaria Memoria secondaria 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 11 Reliability control Responsible for executing the transactional commands: start transaction (B) commit work (C) rollback work (A, for abort) and the primitives for recovery after malfunctions: warm restart and cold restart Ensures atomicity and durability Uses as main data structure the log: A permanent archive which registers the various actions carried out by the DBMS Two metaphors: Arianna s thread, Hansel and Gretel s crumbs of bread 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 12

7 Architettura del controllore dell'affidabilità Gestore dei metodi d accesso fix, unfix Gestore delle transazioni begin, commit, abort Gestore della affidabilità fix, unfix, force (pagine BD e log) Gestore del buffer read, write Gestore della memoria secondaria BD Log 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 13 Stable memory A memory that is failure-resistant It is an abstraction, in that no memory can have zero probability of failure (but replication and robust writing protocols can bring such a probability close to zero) A failure of stable memory is assumed as catastrophic and impossible, at least in this context Organized in different ways depending on the criticality of the application: a tape unit a pair of devices of different kind (e.g.: a tape and a disk) two mirrored disk units 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 14

8 Log organization The log is a sequential file managed by the reliability control system, written in the stable memory The actions carried out by the various transactions are recorded in the log in chronological order (written sequentially to the top block) There are two types of log record Transaction records begin, B(T) insert, I(T,O,AS) delete, D(T,O,BS) update, U(T,O,BS,AS) commit, C(T), or abort, A(T) System records dump (rare) checkpoint (more frequent) 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 15 Struttura del log dump B(T1) B(T2) CK C(T2) B(T3) U(T2, ) U(T2, ) U(T1, ) U(T1, ) U(T3, ) U(T3, ) Crash 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 16

9 Log, checkpoint e dump: a che cosa servono? Il log serve "a ricostruire" le operazioni Checkpoint e dump servono ad evitare che la ricostruzione debba partire dall'inizio dei tempi si usano con riferimento a tipi di guasti diversi 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 17 Undo and redo Undo of an action on an object O: update, delete: copy the value BS into the object O insert: delete the object O Redo of an action on an object O: insert, update: copy the value AS into the object O delete: re-insert the object O Idempotence of undo and redo: an arbitrary number of undos and redos of the same action is equivalent to the carrying out of such actions only once: undo(undo(a)) = undo(a) redo(redo(a)) = redo(a) 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 18

10 Checkpoint Operazione che serve a "fare il punto" della situazione, semplificando le successive operazioni di ripristino: ha lo scopo di registrare quali transaszioni sono attive in un certo istante (e dualmente, di confermare che le altre o non sono iniziate o sono finite) Paagone (estremo): la "chiusura dei conti" di fine anno di una amministrazione: dal 25 novembre (ad esempio) non si accettano nuove richieste di "operazioni" e si concludono tutte quelle avviate prima di accettarne di nuove 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 19 Checkpoint (2) Varie modalità, vediamo la più semplice: si sospende l'accettazione di richieste di ogni tipo (scrittura, inserimenti,, commit, abort) si trasferiscono in memoria di massa (tramite force) tutte le pagine sporche relative a transazioni andate in commit si registrano sul log in modo sincrono (force) gli identificatori delle transazioni in corso si riprende l'accettazione delle operazioni Così siamo sicuri che per tutte le transazioni che hanno effettuato il commit i dati sono in memoria di massa le transazioni "a metà strada" sono elencate nel checkpoint 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 20

11 Dump A dump is a complete copy of the database, which is normally created when the system is not operative The copy is stored in the stable memory, typically on tape, and is called backup A dump record DUMP in the log signals the presence of a backup made at a given time and identifies the file or device where the dump took place 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 21 Transactional rules The reliability control system must follow two rules: WAL rule (write-ahead log): before-state parts of the log records must be written in the log before carrying out the corresponding operation on the database Commit-Precedence rule: after-state parts of the log records must be written in the log before carrying out the commit 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 22

12 Transaction outcome The atomic outcome of a transaction is established at the time when it writes the commit record in the log synchronously, using the force primitive Before this event, a failure is followed by the undo of the actions, so reconstructing the original state of the database After this event, a failure is followed by the redo of the actions carried out to reconstruct the final state of the transaction abort records can be simply written asynchronously into the top block of the log 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 23 Scrittura nel l og e nella base di dati B(T) U(T,X,BS,AS) U(T,Y,BS,AS) C Scritture nel log t w(x) w(y) Scritture nel database (a) B(T) U(T,X,BS,AS) U(T,Y,BS,AS) C t w(y) w(x) (b) B(T) U(T,X,BS,AS) U(T,Y,BS,AS) C t w(x) w(y) (c) 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 24

13 Modalita differita Il DB non contiene valori AS provenienti da transazioni uncommitted In caso di abort, non occorre fare niente Rende superflua la procedura di Undo. Richiede Redo dump CK Crash T1 T2 T3 T4 T5 Niente Redo Redo Niente Niente 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 25 Modalita immediata Il DB contiene valori AS provenienti da transazioni uncommitted Richiede Undo delle operazioni di transazioni uncommited al momento del guasto Non richiede Redo dump CK Crash T1 T2 T3 T4 T5 Niente Niente Niente Undo Undo 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 26

14 Essite una terza modalita : modalità mista La scrittura puo avvenire in modalita sia immediata che differita Consente l ottimizzazione delle operazioni di flush Richiede sia Undo che Redo dump CK Crash T1 T2 T3 T4 T5 Niente Redo Redo Undo Undo 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 27 Optimizations Several log records are normally written into the same log page Several log records of the same transactions can be forced at commit time Several transactions can commit together by forcing their commit record with the same operation (group commit) Systems can exploit parallelism in writing logs 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 28

15 Failures in data management System failures: software bugs, for example of the operating system, or interruptions of the functioning of the devices (due, for example, to loss of power) With loss of the contents of the main memory (and thus all the buffers) With no loss of the contents of secondary memory Device failures: failures of secondary memory devices (for example, disk head crashes) with loss of secondary memory contents with no loss of stable storage (i.e.: of the log) Restart protocols Warm restart used with system crashes Cold restart used with device crashes 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 29 Modello "fail-stop" Normal Restart completed Fail Fail Restart Stop Boot 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 30

16 Restart process Objective: classify transactions as: Completed (whose actions were recorded in stable storage) Committed but possibly not completed (whose actions must be redone) Not committed (whose actions have to be undone) 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 31 Warm restart Four successive phases: Trace back the log until the most recent checkpoint record. Construct the UNDO set (transactions to be undone) and the REDO set (transactions to be redone). Trace back the log until the first action of the oldest transaction in the two sets, UNDO and REDO, is found, and undo all the actions of the transactions in the UNDO set Trace forward the log and redo all the actions of the transactions in the REDO set The protocol guarantees: atomicity: all the transactions in progress at the time of failure leave the database either in the initial state or in the final one durability: all pages of transactions in progress are written to secondary memory 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 32

17 Esempio di warm restart B(T1) B(T2) U(T2, O1, B1, A1) I(T1, O2, A2) UNDO = {T2,T3,T4} B(T3) CK Crash C(T1) B(T4) T1 C U(T3,O2,B3,A3) T2 U(T4,O3,B4,A4) T3 A CK(T2,T3,T4) C(T4) T4 B(T5) T5 C U(T3,O3,B5,A5) U(T5,O4,B6,A6) D(T3,O5,B7) A(T3) C(T5) I(t2,O6,A8) 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni Ricerca dell ultimo checkpoint B(T1) B(T2) U(T2, O1, B1, A1) I(T1, O2, A2) UNDO = {T2,T3,T4} B(T3) CK Crash C(T1) B(T4) T1 C U(T3,O2,B3,A3) T2 U(T4,O3,B4,A4) T3 A CK(T2,T3,T4) C(T4) T4 B(T5) T5 C U(T3,O3,B5,A5) U(T5,O4,B6,A6) D(T3,O5,B7) A(T3) C(T5) I(t2,O6,A8) 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 34

18 2. Costruzione degli insiemi UNDO e REDO B(T1) 0. UNDO = {T2,T3,T4}. REDO = {} B(T2) 8. U(T2, O1, B1, A1) 1. C(T4) {T2, T3}. REDO = {T4} I(T1, O2, A2) B(T3) 2. B(T5) {T2,T3,T5}. REDO = {T4} Setup C(T1) 3. C(T5) {T2,T3}. REDO = {T4, T5} B(T4) 7. U(T3,O2,B3,A3) 9. U(T4,O3,B4,A4) CK(T2,T3,T4) 1. C(T4) 2. B(T5) 6. U(T3,O3,B5,A5) 10. U(T5,O4,B6,A6) 5. D(T3,O5,B7) A(T3) 3. C(T5) 4. I(T2,O6,A8) 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni Fase UNDO B(T1) 0. UNDO = {T2,T3,T4}. REDO = {} B(T2) 8. U(T2, O1, B1, A1) 1. C(T4) {T2, T3}. REDO = {T4} I(T1, O2, A2) B(T3) 2. B(T5) {T2,T3,T5}. REDO = {T4} Setup C(T1) 3. C(T5) {T2,T3}. REDO = {T4, T5} B(T4) 7. U(T3,O2,B3,A3) 4. D(O6) 9. U(T4,O3,B4,A4) CK(T2,T3,T4) 5. O5 =B7 1. C(T4) 6. O3 = B5 Undo 2. B(T5) 6. U(T3,O3,B5,A5) 7. O2 =B3 10. U(T5,O4,B6,A6) 5. D(T3,O5,B7) 8. O1=B1 A(T3) 3. C(T5) 4. I(T2,O6,A8) 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 36

19 4. Fase REDO B(T1) 0. UNDO = {T2,T3,T4}. REDO = {} B(T2) 8. U(T2, O1, B1, A1) 1. C(T4) {T2, T3}. REDO = {T4} I(T1, O2, A2) B(T3) 2. B(T5) {T2,T3,T5}. REDO = {T4} Setup C(T1) 3. C(T5) {T2,T3}. REDO = {T4, T5} B(T4) 7. U(T3,O2,B3,A3) 4. D(O6) 9. U(T4,O3,B4,A4) CK(T2,T3,T4) 5. O5 =B7 1. C(T4) 6. O3 = B5 Undo 2. B(T5) 6. U(T3,O3,B5,A5) 7. O2 =B3 10. U(T5,O4,B6,A6) 5. D(T3,O5,B7) 8. O1=B1 A(T3) 9. O3 = A4 3. C(T5) Redo 4. I(T2,O6,A8) 10. O4 = A6 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 37 Cold restart Divided into three phases. During the first phase, the dump is accessed and the damaged parts are selectively copied from the database. The most recent dump record in the log is then accessed The log is traced forward. The actions on the database and the commit or abort actions are applied as appropriate to the damaged parts of the database. The situation preceding the failure is thus restored A warm restart is carried out 6/05/2003 T03: P. Atzeni, S. Ceri Gestione delle transazioni 38