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PART 5

PART 5. TRANSACTION MANAGEMENT. Chapter 16 Concurrency Control. Introduction to Chapter 16. Concurrency control of processes OS 中多个 进程 对 共享资源 的使用： 互斥使用 进程并发控制（互斥与同步）机制： 软件方法，硬件方法， 信号量 ，管程，临界域 信号量方法： wait(S) critical section signal(S)

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PART 5

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  1. PART 5 TRANSACTION MANAGEMENT

  2. Chapter 16 Concurrency Control

  3. Introduction to Chapter 16 • Concurrency control of processes • OS中多个进程对共享资源的使用：互斥使用 • 进程并发控制（互斥与同步）机制： 软件方法，硬件方法，信号量，管程，临界域 • 信号量方法： wait(S) critical section signal(S) • 锁/lock S：二元信号量，实现对共享资源的互斥使用，{0，1} 表示资源使用情况，锁操作控制进程的执行和等待 • deadlock handling • deadlock prevention, deadlock avoidance, deadlock detection and recovery Database System Concepts - Chapter 16 Concurrency Control -

  4. Introduction to Chapter 16 (cont.) • Transaction concurrency control • /*根据可串行化基本原理, 采用以下机制控制多个进程对共享数据的互斥访问 , 实现对事务的并发调度 • lock-based protocols (§16.1) • timestamp-based protocols (§16.2) • validation-based protocols (验证，§16.3) • multi-version schemes (多版本，§16.4) • deadlock handling (§16.6) • deadlock prevention • deadlock detection and recovery Database System Concepts - Chapter 16 Concurrency Control -

  5. §16.1 Lock-based Protocol • Principles /*用二元信号量“lock”控制并发事务Ti对共享数据Q的访问 • Ti存取Q时，先向DBMS申请对Q加锁 • 如果此时Q未被其它事务加锁，则Ti加锁成功，继续执行对Q的访问 • 如果此时Q已经被其它事务加锁，则Ti等待，直至Q上的锁被释放， Ti获得锁后继续执行 • Ti执行完对Q的操作后，解锁Q • DBMS根据lock-compatibility matrix将lock授予请求的事务 • 事务执行过程: 申请锁——加锁——读写——解锁 Database System Concepts - Chapter 16 Concurrency Control -

  6. 16.1-1 Locks • DBMS use the lock (as the binary semaphore) to control transaction’s access to shared data items • a transaction is allowed to access a data item only if it is currently holding a lock at that item • With respect to a shared data item Q, two modes in which Q is locked, and corresponding two modes of locks on Q • shared-mode lock: S(Q), only read(Q) are permitted • exclusive-mode lock: X(Q), read(Q) and write(Q) are permitted • Lock primitives • lock-S(Q), unlock-S(Q) • lock-X(Q), unlock-X(Q); Database System Concepts - Chapter 16 Concurrency Control -

  7. 16.1-1 Locks (cont.) • Concurrency-control manager/scheduler in DBMS grants lock to transactions who want to access Q according to lock-compatibility matrix • assuming that Ti requests a lock of mode A on data item Q on which another transaction Tj currently holds a lock of mode B, if Ti can be granted a lock of mode A, then mode A is compatible with modeB Tj B Ti A Fig.16.1 Lock-compatibility matrix Database System Concepts - Chapter 16 Concurrency Control -

  8. 16.1-1 Locks (cont.) • In accordance with Fig.16.1, several shared locks can be held simultaneously (by different transactions) on shared data item Q; • When Ti requests a lock of mode Ato operate on data item Q, e.g. S(Q) related to read(Q), while Q is already locked in a incompatible mode B byTj, e.g. X(Q) related to write(Q), then • Ti must wait until the lock in mode B held by Tj is released, the lock is then granted to Ti Database System Concepts - Chapter 16 Concurrency Control -

  9. 16.1-1 Locks (cont.) • Procedures of Ti’s executing to access a data item Q (1) other operations in Ti (2) request a lock S(Q) or X(Q), by the primitive lock-S(Q) or lock-X(Q) (3) wait for concurrency-control manager’s granting of the lock requested; (4) whenever obtaining the lock, make access on Q; (5) after finishing data access on Q, release lock on Q, by unlock-S(Q) or unlock-X(Q); (6) other operations; Database System Concepts - Chapter 16 Concurrency Control -

  10. An Example • T1: transfer $50 from account-B to account-A, Fig.16.2 • T2: display the total amount of A and B, Fig.16.3 • T1 and T2unlock data item A and B immediately after its final access of that data • Initially, A=100, B=200 • For the concurrent schedule S1 (given in Fig.16.4) on T1 and T2 , an inconsistent result display(A+B) = 250 exist, why? • T1 unlocked data item B too early, as a result of which T2 saw a inconsistent state Database System Concepts - Chapter 16 Concurrency Control -

  11. A T4 T3 B An Example (cont.) • T3: transfer $50 from account-B to account-A, Fig.16.5 • T4: display the total amount of A and B, Fig.16.6 • T3 and T4unlock data item A and B at the end of transaction • Initially, A=100, B=200 • The concurrent schedule S2 (given in Fig.16.7) on T3 and T4 leads to deadlock Database System Concepts - Chapter 16 Concurrency Control -

  12. Locking protocol • As revealed by this example, in lock-based schemes, the orders of granting locks to and revoking locks from transactions must be limited to avoid inconsistent DB states and starvation of transactions • Locking protocol • a set of rules that transactions should follow, indicating when a transaction may lock and unlock each of the data items • Locking protocols restrict the number of possible schedules, and lead to a subsetof all possible conflict serializable schedules Database System Concepts - Chapter 16 Concurrency Control -

  13. 16.1-2 Two Phase Locking Protocol/2PL • In accordance with 2PL, each transaction Ti issues lock and unlock requests in two phases • growing phase: Ti may obtain locks, but may not release any locks • shrinking phase:Ti may release locks, but may not obtain any locks • when Ti begins, it enters growing phase • when Ti releases its first lock by unlock, it enters shrinking phase, and is not allowed to apply for locks, i.e. not allowed to enter growing phase Database System Concepts - Chapter 16 Concurrency Control -

  14. 16.1-2 Two Phase Locking Protocol (cont.) • E.g. T3 in Fig.16.5 and T4 in Fig.16.6 obey 2PL, but T1 in Fig.16.2 and T2 in Fig.16.3 not T2: lock-S(A); read (A); unlock(A); lock-S(B); read (B); unlock(B); display(A+B) Fig.16.3 Database System Concepts - Chapter 16 Concurrency Control -

  15. Ti Tj lock-X(R) waiting … obtaining the lock read(R) lock-X(Q) read(Q) lock-S(R) read(R) unlock(R) write(Q)

  16. 16.1-2 Two Phase Locking Protocol (cont.) • Lock point • define1. the point in the transaction/schedule where the transactions has obtain its final lock, but has not yet unlock any locks • define2. the point just before the transaction releases its first lock • E.g. lock-point in T5 in Fig.16.8 Database System Concepts - Chapter 16 Concurrency Control -

  17. T5 Lock-X(A) read (A) Lock-S(B) read (B) Unlock (B) write (A) Unlock (A) growing phase lock point Shrinking phase 16.1-3 Two Phase Locking Protocol (cont.) Fig.16.8 Partial schedule under two-phase locking Database System Concepts - Chapter 16 Concurrency Control -

  18. !! Properties of 2PL • 2PL guarantees conflict serializability, i.e. generates conflict serializable concurrent schedules • 2PL does not ensure freedom from deadlock • 2PL does not avoid cascading rollback Database System Concepts - Chapter 16 Concurrency Control -

  19. 16.1-3 Two Phase Locking Protocol (cont.) • Given a concurrent schedule S in line with 2PL, S is equivalent to a serial schedule S’, such that • the ordering of lock points of each transactions in S is the same as the ordering of transactions in S’ • E.g. S3 in Fig.16.0.1 is conflict serializable, and equivalent to <T5; T6’ > • the lock point of T5 is before the lock point of T6’ Database System Concepts - Chapter 16 Concurrency Control -

  20. T5 Lock-X(A) read (A) Lock-S(B) read (B) Unlock (B) write (A) Unlock (A) T6’ Lock-X(B) read (B) write (B) Unlock (B) LP5 Lock-X(B) waiting LP6’ Fig. 16.0.1 A concurrent schedule S3 under 2PL protocol constraints

  21. How to construct the schedule in line with 2PL • /*给定若干事务, 构造满足2PL的并发调度S！！ • 为各事务添加加锁和解锁操作，各事务应满足2PL要求，即事务内部分为生长和收缩2个阶段 • 根据各事务的锁点位置和锁的互斥性 (相容关系)，安排各事务的并发执行 • S中各事务的growing phase和shrinking phase 应尽可能错开，不允许访问同一数据的2个事务（冲突事务）同时处于growing phase • E.g. A concurrent schedule S4 in Fig.16.0.2 Database System Concepts - Chapter 16 Concurrency Control -

  22. T6’ Lock-X(B) read (B) write (B) Unlock (B) T5 Lock-X(A) read (A) Lock-S(B) read (B) Unlock (B) write (A) Unlock (A) G G S S • S4 is equivalent to <T6’;T5> Fig. 16.0.2 A concurrent schedule S4 under 2PL protocol constraints

  23. T7 read (A) read (B) write (A) T7’ Lock-X(A) read (A) Lock-S(B) read (B) Unlock (B) write (A) Unlock (A) T8 read (B) write (B) read (C) T8’ Lock-X(B) read (B) Lock-S(C) write (B) read (C) Unlock (B) Unlock (C) Fig.16.0.3-1 A concurrent schedule S5 How to construct the schedule in line with 2PL (cont.) • E.g. Construct a concurrent schedule that is equivalent to < T7;T8 > • step1. 分别为T7和T8加锁操作, 构成T7’和T8’ Database System Concepts - Chapter 16 Concurrency Control -

  24. step2. 根据锁的相容性构造调度，假设T7’的锁点先于T8’的锁点 T7’ Lock-X(A) read (A) Lock-S(B) read (B) Unlock (B) write (A) Unlock (A) T8’ Lock-X(B) read (B) Lock-S(C) write (B) read (C) Unlock (B) Unlock (C) Fig.16.0.3-2 A concurrent schedule S5

  25. §16.1-3 Enhanced Protocol • There are two types of enhanced 2PL protocols, that is, strict 2PL and rigorous 2PL • Strict (严格) 2PL protocol • all X-lockstaken by a transaction are held until that transaction commits, that is, all X-locks can only be unlocked at the end of the transaction • leading to cascadeless and serializable schedules Database System Concepts - Chapter 16 Concurrency Control -

  26. T1 Lock-X(A) read (A) Lock-S(B) read (B) Unlock (B) write (A) Unlock (A) growing phase lock point shrinking phase §16.1-3 Enhanced Protocol (cont.) • E.g. schedule S6 in Fig.16.0.4 Fig.16.0.4 A schedule S6 under strict 2PL Database System Concepts - Chapter 16 Concurrency Control -

  27. §16.1-3 Enhanced Protocol (cont.) • Rigorous (强制) 2PL protocol • all X-locks and S-locks taken by a transaction are held until that transaction commits • leading to transactions are serialized in the order in which they commit • Schedule S7 in Fig.16.0.5 • Most DBMS implement either strict or rigorous 2PL protocol. Database System Concepts - Chapter 16 Concurrency Control -

  28. T1 Lock-X(A) read (A) Lock-S(B) read (B) write (A) Unlock (A) Unlock (B) growing phase lock point shrinking phase §16.1-3 Enhanced Protocol (cont.) Fig.16.0.5 A schedule S7 under rigorous 2PL Database System Concepts - Chapter 16 Concurrency Control -

  29. §16.2 Timestamp-based Protocol • 16.2-1 Timestamps • Each transaction Ti is allocated a timestamp TS(Ti)when it enters the system, i.e. it is created • for an“old” transaction Ti and a “new/young” transaction Tj , TS(Ti) <TS(Tj). /*事务启动时被赋予时间戳，DBMS利用时间戳对事务进行全局排序：较早启动的事务具有较小的时间戳 • note: timestamps can be implemented by system clock or logical counter Database System Concepts - Chapter 16 Concurrency Control -

  30. 16.2-1 Timestamps (cont.) • Each data Q in database is associated with two timestamp values • W-timestamp(Q) is the largest time-stamp of any transaction that executed write(Q) successfully, i.e. W-timestamp(Q) = max {TS(Ti) | Tisuccessfully executes write on Q } /* 成功执行“写Q”的“最年轻”事务Ti的时间戳， 或 /*最后/新一次写Q的事务Ti的时间戳 • refer to Fig.16.0.6-1 and Exercise 16.11 in the textbook in P678 • in Fig.16.0.6-1 (b), the write issued by Ti1 will be rejected, Ti1is roll backed and reassigned a new timestamp that is larger than TS (Ti2) and TS (Ti3) Database System Concepts - Chapter 16 Concurrency Control -

  31. be rejected Ti1: … .. write(Q) … Ti1: … write(Q) … W-timestamp(Q) = TS (Ti3) W-timestamp(Q) = TS (Ti3) … write(Q) … … … Ti2: … write(Q) … … Ti2: Ti3: … … write(Q) … Ti3: … … write(Q) … t t W W W W W W Q Q (a) (b) Fig.16.0.6-1 Definitions of W-timestamp(Q)

  32. 16.2-1 Timestamps (cont.) • R-timestamp(Q) is the largest time-stamp of any transaction that executed read(Q) successfully, i.e., R-timestamp(Q) = max {TS(Ti) | Ti successfully executes read on Q } /* 成功执行“读Q”的“最年轻”事务Ti的时间戳， • refer to Fig.16.0.6-2 • in Fig.16.0.6-2 (b), the read issued by Ti1 will not be rejected and Ti1 conducts read successfully • because the read issued by Ti1 is not conflict with those issued by Ti2 and Ti3 Database System Concepts - Chapter 16 Concurrency Control -

  33. … read(Q) … .. Ti1: Ti2: … read(Q) …. Ti3: … read(Q) … … t R R R Q Ti1: … read(Q) … … R-timestamp(Q) = TS (Ti3) … read(Q) … … Ti2: Ti3: … … read(Q) … t W W W Q R-timestamp(Q) = TS (Ti1) Fig.16.0.6-2 Definitions of timestamps on data item Q

  34. 16.2-2 Timestamp-ordering Protocol • For a set of concurrent transactions, the orders of conflicting read and write operations are arranged in accordance with timestamp orders • /* 2事务发生冲突时，较早启动的事务(具有较小的时间戳)中的read and write具有较高的执行优先级 • /*冲突操作按时间戳顺序依次串行执行, 即先启动事务中的冲突操作应该先执行!! • For a transaction Ti , when it issues a read(Q) or write(Q), the protocol operates as one of the following five cases Database System Concepts - Chapter 16 Concurrency Control -

  35. 16.2-2 Timestamp-ordering Protocol (cont.) — Case One • When Ti issues a read(Q), if TS(Ti ) < W-timestamp(Q), then • the read (Q) is rejected • Ti is rolled back （撤销Ti对DB的修改）and assigned a new timestamp • refer to Fig.16.0.7 • Why? • /*假设W-timestamp(Q) = TS(Tj), 即最近一次对Q的写操作是由Tj完成的，则TS(Ti ) < TS(Tj)= W-timestamp(Q) • write(Q) in Tj与read(Q) in Ti相互冲突，后启动/年轻的Tj先于Ti的读操作之前执行了写操作，不满足执行顺序要求 Database System Concepts - Chapter 16 Concurrency Control -

  36. Ti: … …. … read(Q) …. …. write(Q) … …. … Tj: t W TS(Ti) TS(Tj) R Q Tj is younger than Ti newTS(Ti) TS(Ti ) <W-timestamp(Q) = TS(Tj) Fig. 16.0.7 Read-Writeconflicton data item Q

  37. 16.2-2 Timestamp-ordering Protocol (cont.) — Case Two • When Ti issues a read(Q), if TS(Ti )  W-timestamp(Q), then • the read (Q) is successfully executed • R-timestamp(Q) is set to R-timestamp(Q) = max{R-timestamp(Q), TS(Ti)} • refer to Fig. 16.0.8 • Why? • /*假设TS(Ti ) > W-timestamp(Q) = TS(Tj), 则最近一次对Q的写操作是由Tj完成的; • /*假设TS(Ti ) = W-timestamp(Q), 最近一次对Q的写操作是由Ti自己完成的; • /* write(Q) in Tj先于read(Q) in Ti执行，即先启动的事务Tj先执行操作，满足执行顺序要求，无冲突 Database System Concepts - Chapter 16 Concurrency Control -

  38. Tj: … … write(Q) … … … write(Q) … read(Q) …. Ti: t W TS(Tj) TS(Ti) R Q Tj is older than Ti TS(Tj ) <TS(Ti) (a) TS(Ti ) >W-timestamp(Q) = TS(Tj), or (b) TS(Ti ) = W-timestamp(Q) Fig. 16.0.8 Non-conflictRead-Writeon data item Q

  39. 16.2-2 Timestamp-ordering Protocol (cont.) — Case Three • When Ti issues a write(Q), if TS(Ti ) < R-timestamp(Q), then • the write(Q) is rejected • Ti is rolled back （撤销Ti对DB的修改）and assigned a new timestamp • refer to Fig. 16.0.9 • Why? • /*假设R-timestamp(Q)= TS(Tj), 即最近一次对Q的读操作read(Q)是由Tj完成的，则TS(Ti ) <TS(Tj) • read(Q) in Tj与write(Q) in Ti相互冲突，并且后启动的Tj先于Ti的写操作write(Q)之前执行了读操作read(Q) ，不满足执行顺序要求 Database System Concepts - Chapter 16 Concurrency Control -

  40. … … … write(Q) … Ti: Tj: … read(Q) … … R TS(Ti) TS(Tj) W Q Tj is younger than Ti t newTS(Ti) TS(Ti) <TS(Tj) TS(Ti ) <R-timestamp(Q) = TS(Tj) Fig. 16.0.9 Write-Readconflicton data item Q

  41. 16.2-2 Timestamp-ordering Protocol (cont.) — Case Four • When Ti issues a write(Q), if TS(Ti ) < W-timestamp(Q), then • the write(Q) is rejected • Ti is rolled back （撤销Ti对DB的修改）and assigned a new timestamp • refer to Fig. 16.0.10 • Why? • /*假设W-timestamp(Q)= TS(Tj), 即最近一次对Q的写操作write(Q)是由Tj完成的，则TS(Ti ) <TS(Tj) • write(Q) in Tj与write(Q) in Ti相互冲突，并且后启动的Tj先于Ti的写操作write(Q)之前执行了写操作write(Q) ，不满足执行顺序要求 Database System Concepts - Chapter 16 Concurrency Control -

  42. …. write(Q) … ………… Ti: Tj: … … write(Q) …. …. t W TS(Ti) TS(Tj) W Q TS(Ti) <TS(Tj) Tj is younger than Ti newTS(Ti) TS(Ti ) <W-timestamp(Q) = TS(Tj) Fig.16.0.10 Conflict Write-Writeon data item Q

  43. 16.2-2 Timestamp-ordering Protocol (cont.) — Case Five • When Ti issues a write(Q), if TS(Ti )  W-timestamp(Q) and TS(Ti )  R-timestamp(Q), then • the write(Q) is executed, and • W-timestamp(Q) is set to TS(Ti), i.e. W-timestamp(Q) := TS(Ti) • refer to Fig.16.0.11 • Why? • 已经完成的对Q的读read(Q)或写write(Q)都是由先于Ti启动的事务（>）或Ti自身（=）完成的 • write(Q) in Ti与以前完成的 in Tj或in Tj无冲突，满足执行顺序要求 Database System Concepts - Chapter 16 Concurrency Control -

  44. … …. read(Q)… write(Q) … Tj: read(Q)… write(Q)…… … write(Q) …. Ti: t R W TS(Tj) TS(Ti) W Q Tj is older than Ti TS(Tj ) <TS(Ti) TS(Ti ) >W-timestamp(Q) = TS(Tj), orTS(Ti ) =W-timestamp(Q) ; AND TS(Ti ) >R-timestamp(Q) = TS(Tj), or TS(Ti ) = R-timestamp(Q) Fig.16.0.11 Non-conflict Write-Read or Write-Write on data item Q

  45. 16.2-3 Properties of Timestamp-based Protocol • It guarantees conflict serializability • It ensures freedom from deadlock • Why? no transaction ever waits • the transaction that can not successfully conduct write or read are rolled back and reassigned a new timestamp to restart • In contrast with the properties of 2PL ( ) and strict 2PL( ) • Timestamp-based protocol may lead to starvation of long transaction • The schedule in line with this protocol may not be cascade-free, and may not even be recoverable Database System Concepts - Chapter 16 Concurrency Control -

  46. T2 read(A) A:= A-50 write(A) read(B) B:= B+50 write(B) display(A+B) T1 read(B) read(A) display(A+B) 16.2-4 Examples about Timestamp-based Protocol • E.g.1 Assuming TS(T1) < TS(T2), is the following schedule S under the timestamp protocol ? Why? • S is not under the timestamp protocol • because the read(A) in T1 is executed after the write(A) in T2 Database System Concepts - Chapter 16 Concurrency Control -

  47. 16.2-4 Examples about Timestamp-based Protocol (cont.) • E.g.2A timestamp-based partial schedule for transactions T1, T2, T3, T4, and T5, which are with initial timestamps 1, 2, 3, 4, 5 respectively, and operate on data items X, Y, and Z • refer toFig.16.0.12 • write(Z) in T3 is rejected • T3 is reallocated a new timestamp, e.g. TS(T3 )=30, to restart (denoted as T3’ ) • T5 that is dependent on T3 may also needed to rollback • E.g.3Analysisof timestamp-based schedule S3 for T14 and T15 in Fig.16.13 in the textbook • Fig. 16.0.13 Database System Concepts - Chapter 16 Concurrency Control -

  48. timestamps: 1 2 3 4 5 30 transaction: T3’ T1 T2 T3 T4 T5 … … … … … read(X) read(Y) read(Y) write(Y) write(Z) read(Z) read(X) read(X) write(Z) rollback write(Y) write(Z) write(Y) restart write(Z) t write(Z) Fig.16.0.12 A timestamp-based schedule under timestamp constraints

  49. T14 T15 TS(T14) TS(T15) R-TS(A) W-TS(A) R-TS(B) W-TS(B) 1 0 0 0 0 .. .. 2 1 > 0→ 1 0 read(B) read(B) B:=B-50 write(B) 2 > 1→ 2 0 2 = 2 > 0 0→ 2 read(A) 1 > 0→1 0 read(A) A:=A+50 write(A) dis(A+B) 0 2 > 1→2 dis(A+B) 2 = 2 > 0 0→2 Fig.16.0.13 Analysisof S3 in Fig.16.13

  50. 16.2-4 Examples about Timestamp-based Protocol (cont.) • E.g.4 Analysisof timestamp-based schedule for T16 and T17, in which T16 is rolled back • Fig.16.0.14 Database System Concepts - Chapter 16 Concurrency Control -

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