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Chapter 4 Concrruncy controling techniques (1).ppt
1.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Chapter 4 Concurrency Control Techniques
2.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 2 Outline Databases Concurrency Control 1. Purpose of Concurrency Control 2. Two-Phase locking 3. Time Stamp Locking Techniques 4. Multi-version Locking Techniques 5. Validation (Optimistic) Concurrency 6. Lock Granularity
3.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 3 Database Concurrency Control 1 Purpose of Concurrency Control To enforce Isolation (through mutual exclusion) among conflicting transactions. To preserve database consistency through consistency preserving execution of transactions. To resolve read-write and write-write conflicts. Example: In concurrent execution environment if T1 conflicts with T2 over a data item A, then the existing concurrency control decides if T1 or T2 should get the A and if the other transaction is rolled-back or waits.
4.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 5- 4 Database Concurrency Control Two-Phase Locking Techniques Locking is an operation which secures (a) permission to Read (b) permission to Write a data item for a transaction. Example: Lock (X). Data item X is locked in behalf of the requesting transaction. Unlocking is an operation which removes these permissions from the data item. Example: Unlock (X): Data item X is made available to all other transactions. Lock and Unlock are Atomic operations.
5.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 5 Database Concurrency Control Two-Phase Locking Techniques: Essential components Two locks modes: (a) shared (read) (b) exclusive (write). Shared mode: shared lock (X) More than one transaction can apply share lock on X for reading its value but no write lock can be applied on X by any other transaction. Exclusive mode: Write lock (X) Only one write lock on X can exist at any time and no shared lock can be applied by any other transaction on X. Conflict matrix Read Write Read Write N N N Y
6.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 6 Database Concurrency Control Two-Phase Locking Techniques: Essential components Lock Manager: Managing locks on data items. Lock table: Lock manager uses it to store the identify of transaction locking a data item, the data item, lock mode and pointer to the next data item locked. One simple way to implement a lock table is through linked list. T1 Transaction ID Data item id lock mode Ptr to next data item Next X1 Read
7.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 7 Database Concurrency Control Two-Phase Locking Techniques: Essential components Database requires that all transactions should be well-formed. A transaction is well-formed if: It must lock the data item before it reads or writes to it. It must not lock an already locked data items and it must not try to unlock a free data item.
8.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 8 Database Concurrency Control Two-Phase Locking Techniques: Essential components The following code performs the lock operation: B:if LOCK (X) = 0 (*item is unlocked*) then LOCK (X) 1 (*lock the item*) else begin wait (until lock (X) = 0) and the lock manager wakes up the transaction); goto B end;
9.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 9 Database Concurrency Control Two-Phase Locking Techniques: Essential components The following code performs the unlock operation: LOCK (X) 0 (*unlock the item*) if any transactions are waiting then wake up one of the waiting the transactions;
10.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 10 Database Concurrency Control Two-Phase Locking Techniques: Essential components The following code performs the read operation: B: if LOCK (X) = “unlocked” then begin LOCK (X) “read-locked”; no_of_reads (X) 1; end else if LOCK (X) “read-locked” then no_of_reads (X) no_of_reads (X) +1 else begin wait (until LOCK (X) = “unlocked” and the lock manager wakes up the transaction); go to B end;
11.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 11 Database Concurrency Control Two-Phase Locking Techniques: Essential components The following code performs the write lock operation: B: if LOCK (X) = “unlocked” then begin LOCK (X) “read-locked”; no_of_reads (X) 1; end else if LOCK (X) “read-locked” then no_of_reads (X) no_of_reads (X) +1 else begin wait (until LOCK (X) = “unlocked” and the lock manager wakes up the transaction); go to B end;
12.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 12 Database Concurrency Control Two-Phase Locking Techniques: Essential components The following code performs the unlock operation: if LOCK (X) = “write-locked” then begin LOCK (X) “unlocked”; wakes up one of the transactions, if any end else if LOCK (X) “read-locked” then begin no_of_reads (X) no_of_reads (X) -1 if no_of_reads (X) = 0 then begin LOCK (X) = “unlocked”; wake up one of the transactions, if any end end;
13.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 13 Database Concurrency Control Two-Phase Locking Techniques: Essential components Lock conversion Lock upgrade: existing read lock to write lock if Ti has a read-lock (X) and Tj has no read-lock (X) (i j) then convert read-lock (X) to write-lock (X) else force Ti to wait until Tj unlocks X Lock downgrade: existing write lock to read lock Ti has a write-lock (X) (*no transaction can have any lock on X*) convert write-lock (X) to read-lock (X)
14.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 14 Database Concurrency Control Two-Phase Locking Techniques: The algorithm Two Phases: (a) Locking (Growing) (b) Unlocking (Shrinking). Locking (Growing) Phase: A transaction applies locks (read or write) on desired data items one at a time. Unlocking (Shrinking) Phase: A transaction unlocks its locked data items one at a time. Requirement: For a transaction these two phases must be mutually exclusively, that is, during locking phase unlocking phase must not start and during unlocking phase locking phase must not begin.
15.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 15 Database Concurrency Control Two-Phase Locking Techniques: The algorithm T1 T2 Result read_lock (Y); read_lock (X); Initial values: X=20; Y=30 read_item (Y); read_item (X); Result of serial execution unlock (Y); unlock (X); T1 followed by T2 write_lock (X); Write_lock (Y); X=50, Y=80. read_item (X); read_item (Y); Result of serial execution X:=X+Y; Y:=X+Y; T2 followed by T1 write_item (X); write_item (Y); X=70, Y=50 unlock (X); unlock (Y);
16.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 16 Database Concurrency Control Two-Phase Locking Techniques: The algorithm T1 T2 Result read_lock (Y); X=50; Y=50 read_item (Y); Nonserializable because it. unlock (Y); violated two-phase policy. read_lock (X); read_item (X); unlock (X); write_lock (Y); read_item (Y); Y:=X+Y; write_item (Y); unlock (Y); write_lock (X); read_item (X); X:=X+Y; write_item (X); unlock (X); Time
17.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 17 Database Concurrency Control Two-Phase Locking Techniques: The algorithm T’1 T’2 read_lock (Y); read_lock (X); T1 and T2 follow two-phase read_item (Y); read_item (X); policy but they are subject to write_lock (X); Write_lock (Y); deadlock, which must be unlock (Y); unlock (X); dealt with. read_item (X); read_item (Y); X:=X+Y; Y:=X+Y; write_item (X); write_item (Y); unlock (X); unlock (Y);
18.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 18 Database Concurrency Control Two-Phase Locking Techniques: The algorithm Two-phase policy generates two locking algorithms (a) Basic (b) Conservative Conservative: Prevents deadlock by locking all desired data items before transaction begins execution. Basic: Transaction locks data items incrementally. This may cause deadlock which is dealt with. Strict: A more stricter version of Basic algorithm where unlocking is performed after a transaction terminates (commits or aborts and rolled-back). This is the most commonly used two-phase locking algorithm.
19.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 19 Database Concurrency Control Dealing with Deadlock and Starvation Deadlock T’1 T’2 read_lock (Y); T1 and T2 did follow two-phase read_item (Y); policy but they are deadlock read_lock (X); read_item (Y); write_lock (X); (waits for X) write_lock (Y); (waits for Y) Deadlock (T’1 and T’2)
20.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 20 Database Concurrency Control Dealing with Deadlock and Starvation Deadlock prevention A transaction locks all data items it refers to before it begins execution. This way of locking prevents deadlock since a transaction never waits for a data item. The conservative two-phase locking uses this approach.
21.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 21 Database Concurrency Control Dealing with Deadlock and Starvation Deadlock detection and resolution In this approach, deadlocks are allowed to happen. The scheduler maintains a wait-for-graph for detecting cycle. If a cycle exists, then one transaction involved in the cycle is selected (victim) and rolled-back. A wait-for-graph is created using the lock table. As soon as a transaction is blocked, it is added to the graph. When a chain like: Ti waits for Tj waits for Tk waits for Ti or Tj occurs, then this creates a cycle. One of the transaction o
22.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 22 Database Concurrency Control Dealing with Deadlock and Starvation Deadlock avoidance There are many variations of two-phase locking algorithm. Some avoid deadlock by not letting the cycle to complete. That is as soon as the algorithm discovers that blocking a transaction is likely to create a cycle, it rolls back the transaction. Wound-Wait and Wait-Die algorithms use timestamps to avoid deadlocks by rolling-back victim.
23.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 23 Database Concurrency Control Dealing with Deadlock and Starvation Starvation Starvation occurs when a particular transaction consistently waits or restarted and never gets a chance to proceed further. In a deadlock resolution it is possible that the same transaction may consistently be selected as victim and rolled-back. This limitation is inherent in all priority based scheduling mechanisms. In Wound-Wait scheme a younger transaction may always be wounded (aborted) by a long running older transaction which may create starvation.
24.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 24 Database Concurrency Control Timestamp based concurrency control algorithm Timestamp A monotonically increasing variable (integer) indicating the age of an operation or a transaction. A larger timestamp value indicates a more recent event or operation. Timestamp based algorithm uses timestamp to serialize the execution of concurrent transactions.
25.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Timestamps Integer or Long Format: The number of units (such as seconds or milliseconds) since a specified reference point (epoch). Example: 1637688675 (Unix timestamp, representing the number of seconds since January 1, 1970, 00:00:00 UTC). Date and Time Format: Example: 2023-11-24T12:34:56Z (ISO 8601 format, representing the date and time in UTC). Slide 18- 25
26.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Timestamp Cont’d… write_TS(X) > TS(T1) means that the last committed transaction that wrote to the data item X did so after the current transaction T1 started. In other words, the data item X has been modified by a transaction with a timestamp greater than the timestamp of the current transaction. read_TS(X) > TS(T1) means that the last committed transaction that read the data item X did so after the current transaction T1 started. Slide 18- 26
27.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 27 Database Concurrency Control Timestamp based concurrency control algorithm Basic Timestamp Ordering 1. Transaction T issues a write_item(X) operation: If read_TS(X) > TS(T) or if write_TS(X) > TS(T), then an younger transaction has already read the data item so abort and roll-back T and reject the operation. If the condition in part (a) does not exist, then execute write_item(X) of T and set write_TS(X) to TS(T). 2. Transaction T issues a read_item(X) operation: If write_TS(X) > TS(T), then an younger transaction has already written to the data item so abort and roll-back T and reject the operation. If write_TS(X) TS(T), then execute read_item(X) of T and set read_TS(X) to the larger of TS(T) and the current read_TS(X).
28.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 28 Database Concurrency Control Timestamp based concurrency control algorithm Strict Timestamp Ordering 1. Transaction T issues a write_item(X) operation: If TS(T) > read_TS(X), then delay T until the transaction T’ that wrote or read X has terminated (committed or aborted). 2. Transaction T issues a read_item(X) operation: If TS(T) > write_TS(X), then delay T until the transaction T’ that wrote or read X has terminated (committed or aborted).
29.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 29 Database Concurrency Control Timestamp based concurrency control algorithm Thomas’s Write Rule If read_TS(X) > TS(T) then abort and roll-back T and reject the operation. If write_TS(X) > TS(T), then just ignore the write operation and continue execution. This is because the most recent writes counts in case of two consecutive writes. If the conditions given in 1 and 2 above do not occur, then execute write_item(X) of T and set write_TS(X) to TS(T).
30.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 30 Database Concurrency Control Multiversion concurrency control techniques This approach maintains a number of versions of a data item and allocates the right version to a read operation of a transaction. Thus unlike other mechanisms a read operation in this mechanism is never rejected. Side effect: Significantly more storage (RAM and disk) is required to maintain multiple versions. To check unlimited growth of versions, a garbage collection is run when some criteria is satisfied.
31.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 31 Database Concurrency Control Multiversion technique based on timestamp ordering This approach maintains a number of versions of a data item and allocates the right version to a read operation of a transaction. Thus unlike other mechanisms a read operation in this mechanism is never rejected. Side effects: Significantly more storage (RAM and disk) is required to maintain multiple versions. To check unlimited growth of versions, a garbage collection is run when some criteria is satisfied.
32.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 32 Database Concurrency Control Multiversion technique based on timestamp ordering Assume X1, X2, …, Xn are the version of a data item X created by a write operation of transactions. With each Xi a read_TS (read timestamp) and a write_TS (write timestamp) are associated. read_TS(Xi): The read timestamp of Xi is the largest of all the timestamps of transactions that have successfully read version Xi. write_TS(Xi): The write timestamp of Xi that wrote the value of version Xi. A new version of Xi is created only by a write operation.
33.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 33 Database Concurrency Control Multiversion technique based on timestamp ordering To ensure serializability, the following two rules are used. If transaction T issues write_item (X) and version i of X has the highest write_TS(Xi) of all versions of X that is also less than or equal to TS(T), and read _TS(Xi) > TS(T), then abort and roll-back T; otherwise create a new version Xi and read_TS(X) = write_TS(Xj) = TS(T). If transaction T issues read_item (X), find the version i of X that has the highest write_TS(Xi) of all versions of X that is also less than or equal to TS(T), then return the value of Xi to T, and set the value of read _TS(Xi) to the largest of TS(T) and the current read_TS(Xi). Rule 2 guarantees that a read will never be rejected.
34.
Copyright © 2007
Ramez Elmasri and Shamkant B. Navathe Slide 18- 34 Database Concurrency Control Multiversion Two-Phase Locking Using Certify Locks Concept Allow a transaction T’ to read a data item X while it is write locked by a conflicting transaction T. This is accomplished by maintaining two versions of each data item X where one version must always have been written by some committed transaction. This means a write operation always creates a new version of X.
35.
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Ramez Elmasri and Shamkant B. Navathe Cont’d… Certify Lock (CL): A Certify Lock is a lock associated with a particular version of a data item. It certifies that the transaction holds a lock on the correct version of the data item for its read operations. Slide 18- 35
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Ramez Elmasri and Shamkant B. Navathe MV2PL Protocol: Growing Phase (Lock Acquisition): Transactions follow the traditional 2PL protocol during the growing phase, acquiring locks as needed. Write locks are obtained for write operations, and Certify Locks are obtained for read operations. Certification Phase: During this phase, the transaction validates that it holds Certify Locks on the correct versions of the data items it wants to read. Slide 18- 36
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Ramez Elmasri and Shamkant B. Navathe Cont’d… Validation: For each read operation, the transaction checks that the Certify Lock it holds is still valid (i.e., the version of the data item has not changed since the lock was obtained). If the Certify Lock is still valid, the read operation proceeds. Otherwise, the transaction may need to be rolled back. Slide 18- 37
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Ramez Elmasri and Shamkant B. Navathe Cont’d… Release Phase: If the Certification Phase is successful, the transaction enters the Release Phase. Locks are released during this phase, allowing other transactions to acquire them. Slide 18- 38
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Ramez Elmasri and Shamkant B. Navathe Steps of Multiversion Two-Phase Locking (MV2PL) 1. Initial State: There is a committed version of a data item X (let's call it X). No transactions have acquired any locks on X. 2. Transaction T Creates a Second Version (X'): Transaction T starts and obtains a write lock on the existing committed version of the data item X. T then creates a new version of the data item X, referred to as X', by making modifications. Slide 18- 39
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Ramez Elmasri and Shamkant B. Navathe Cont’d.. 3. Other Transactions Continue to Read X: While T is creating the new version X', other transactions that started before T's write lock are still reading the original committed version X. These transactions are not affected by T's ongoing modifications. 4.Transaction T Obtains a Certify Lock on X': When T is ready to commit, it obtains a Certify Lock on the newly created version X'. Slide 18- 40
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Ramez Elmasri and Shamkant B. Navathe Cont’d.. The Certify Lock certifies that T has the correct and consistent version of the data item for its read operations. 5. Committed Version X Becomes X': Upon successful certification and before T commits, the new version X' becomes the committed version. This is essentially the point where T's modifications are accepted and become visible to other transactions. Slide 18- 41
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Ramez Elmasri and Shamkant B. Navathe Cont’d.. 6. T Releases Its Certify Lock on X' (Which Is Now X): After the commitment of T, it releases the Certify Lock on the version X' (now the committed version). At this point, the Certify Lock is essentially released on the committed version X, and other transactions can acquire read locks on this version. Slide 18- 42
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Ramez Elmasri and Shamkant B. Navathe Slide 18- 43 Database Concurrency Control Multiversion Two-Phase Locking Using Certify Locks Note: In multiversion 2PL read and write operations from conflicting transactions can be processed concurrently. This improves concurrency but it may delay transaction commit because of obtaining certify locks on all its writes. It avoids cascading abort but like strict two phase locking scheme conflicting transactions may get deadlocked.
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Ramez Elmasri and Shamkant B. Navathe Slide 18- 44 Database Concurrency Control Validation (Optimistic) Concurrency Control Schemes In this technique only at the time of commit serializability is checked and transactions are aborted in case of non- serializable schedules. Three phases: 1. Read phase 2. Validation phase 3. Write phase 1. Read phase: A transaction can read values of committed data items. However, updates are applied only to local copies (versions) of the data items (in database cache).
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Ramez Elmasri and Shamkant B. Navathe Slide 18- 45 Database Concurrency Control Validation (Optimistic) Concurrency Control Schemes 2. Validation phase: Serializability is checked before transactions write their updates to the database. This phase for Ti checks that, for each transaction Tj that is either committed or is in its validation phase, one of the following conditions holds: Tj completes its write phase before Ti starts its read phase. Ti starts its write phase after Tj completes its write phase, and the read_set of Ti has no items in common with the write_set of Tj Both the read_set and write_set of Ti have no items in common with the write_set of Tj, and Tj completes its read phase. When validating Ti, the first condition is checked first for each transaction Tj, since (1) is the simplest condition to check. If (1) is false then (2) is checked and if (2) is false then (3 ) is checked. If none of these conditions holds, the validation fails and Ti is aborted.
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Ramez Elmasri and Shamkant B. Navathe Slide 18- 46 Database Concurrency Control Validation (Optimistic) Concurrency Control Schemes 3. Write phase: On a successful validation transactions’ updates are applied to the database; otherwise, transactions are restarted.
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Ramez Elmasri and Shamkant B. Navathe Slide 18- 47 Database Concurrency Control Granularity of data items and Multiple Granularity Locking A lockable unit of data defines its granularity. Granularity can be coarse (entire database) or it can be fine (a tuple or an attribute of a relation). Data item granularity significantly affects concurrency control performance. Thus, the degree of concurrency is low for coarse granularity and high for fine granularity. Example of data item granularity: 1. A field of a database record (an attribute of a tuple) 2. A database record (a tuple or a relation) 3. A disk block 4. An entire file 5. The entire database
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Ramez Elmasri and Shamkant B. Navathe Slide 18- 48 Database Concurrency Control Granularity of data items and Multiple Granularity Locking The following diagram illustrates a hierarchy of granularity from coarse (database) to fine (record). DB f1 f2 p11 p12 ... p1n r111 ... r11j r111 ... r11j r111 ... r11j r111 ... r11j r111 ... r11j r111 ... r11j p11 p12 ... p1n
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Ramez Elmasri and Shamkant B. Navathe Slide 18- 49 Database Concurrency Control Granularity of data items and Multiple Granularity Locking To manage such hierarchy, in addition to read and write, three additional locking modes, called intention lock modes are defined: Intention-shared (IS): indicates that a shared lock(s) will be requested on some descendent nodes(s). Intention-exclusive (IX): indicates that an exclusive lock(s) will be requested on some descendent node(s). Shared-intention-exclusive (SIX): indicates that the current node is locked in shared mode but an exclusive lock(s) will be requested on some descendent nodes(s).
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Ramez Elmasri and Shamkant B. Navathe Slide 18- 50 Database Concurrency Control Granularity of data items and Multiple Granularity Locking These locks are applied using the following compatibility matrix: IS IX S SIX X yes yes yes yes no yes yes no no no yes no yes no no yes no no no no no no no no no IS IX S SIX X Intention-shared (IS Intention-exclusive (IX) Shared-intention-exclusive (SIX)
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Ramez Elmasri and Shamkant B. Navathe Slide 18- 51 Database Concurrency Control Granularity of data items and Multiple Granularity Locking The set of rules which must be followed for producing serializable schedule are 1. The lock compatibility must adhered to. 2. The root of the tree must be locked first, in any mode.. 3. A node N can be locked by a transaction T in S or IX mode only if the parent node is already locked by T in either IS or IX mode. 4. A node N can be locked by T in X, IX, or SIX mode only if the parent of N is already locked by T in either IX or SIX mode. 5. T can lock a node only if it has not unlocked any node (to enforce 2PL policy). 6. T can unlock a node, N, only if none of the children of N are currently locked by T.
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Ramez Elmasri and Shamkant B. Navathe Slide 18- 52 Database Concurrency Control Granularity of data items and Multiple Granularity Locking: An example of a serializable execution: T1 T2 T3 IX(db) IX(f1) IX(db) IS(db) IS(f1) IS(p11) IX(p11) X(r111) IX(f1) X(p12) S(r11j) IX(f2) IX(p21) IX(r211) Unlock (r211) Unlock (p21) Unlock (f2) S(f2)
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Ramez Elmasri and Shamkant B. Navathe Slide 18- 53 Database Concurrency Control Granularity of data items and Multiple Granularity Locking: An example of a serializable execution (continued): T1 T2 T3 unlock(p12) unlock(f1) unlock(db) unlock(r111) unlock(p11) unlock(f1) unlock(db) unlock (r111j) unlock (p11) unlock (f1) unlock(f2) unlock(db)
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Ramez Elmasri and Shamkant B. Navathe Application Area 1. Lock-Based Concurrency Control: • Application Area: Financial Systems • Explanation: In financial systems where transactions involve updates to account balances, lock-based concurrency control ensures that multiple transactions cannot concurrently modify the same account balance. This prevents issues such as lost updates and ensures accurate financial records. Slide 18- 54
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Ramez Elmasri and Shamkant B. Navathe Two-Phase Locking (2PL): • Application Area: Reservation Systems (e.g., airline or hotel booking) • Explanation: In reservation systems, multiple users may attempt to reserve the same resource simultaneously. Two-Phase Locking ensures that conflicting operations, such as booking the same seat on a flight, are serialized. Once a reservation is made, it is locked until the transaction is committed or rolled back. Slide 18- 55
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Ramez Elmasri and Shamkant B. Navathe 3. Multi-Version Concurrency Control (MVCC): • Application Area: Content Management Systems • Explanation: Content management systems often involve concurrent read and write operations. MVCC allows multiple users to read the content concurrently, even while others are making updates. Each user sees a consistent snapshot of the content as of the start of their transaction. Slide 18- 56
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Ramez Elmasri and Shamkant B. Navathe 4. Optimistic Concurrency Control: • Application Area: E-commerce Systems • Explanation: In e-commerce systems, optimistic concurrency control is suitable for scenarios where conflicts are rare. For example, when users are browsing products, optimistic concurrency allows them to add items to their carts without locking the products. Conflicts are checked at the time of checkout. Slide 18- 57
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Ramez Elmasri and Shamkant B. Navathe Timestamp-Based Concurrency Control: Application Area: Manufacturing Systems (e.g., inventory management) Explanation: Timestamp-based concurrency control is useful in manufacturing systems where multiple processes may update inventory levels concurrently. Each transaction is assigned a timestamp, and conflicts are resolved based on these timestamps to ensure accurate and timely inventory updates. Slide 18- 58
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Ramez Elmasri and Shamkant B. Navathe Slide 18- 59 Summary Databases Concurrency Control 1. Purpose of Concurrency Control 2. Two-Phase locking 3. Limitations of CCMs 4. Index Locking 5. Lock Compatibility Matrix 6. Lock Granularity
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