Chapter 16 : Concurrency Control

Chapter 16 : Concurrency Control

اسلاید 1: Chapter 16 : Concurrency Control

اسلاید 2: Chapter 16: Concurrency ControlLock-Based ProtocolsTimestamp-Based ProtocolsValidation-Based ProtocolsMultiple GranularityMultiversion SchemesDeadlock HandlingInsert and Delete OperationsConcurrency in Index Structures

اسلاید 3: Lock-Based ProtocolsA lock is a mechanism to control concurrent access to a data itemData items can be locked in two modes : 1. exclusive (X) mode. Data item can be both read as well as written. X-lock is requested using lock-X instruction. 2. shared (S) mode. Data item can only be read. S-lock is requested using lock-S instruction.Lock requests are made to concurrency-control manager. Transaction can proceed only after request is granted.

اسلاید 4: Lock-Based Protocols (Cont.)Lock-compatibility matrixA transaction may be granted a lock on an item if the requested lock is compatible with locks already held on the item by other transactionsAny number of transactions can hold shared locks on an item, but if any transaction holds an exclusive on the item no other transaction may hold any lock on the item.If a lock cannot be granted, the requesting transaction is made to wait till all incompatible locks held by other transactions have been released. The lock is then granted.

اسلاید 5: Lock-Based Protocols (Cont.)Example of a transaction performing locking: T2: lock-S(A); read (A); unlock(A); lock-S(B); read (B); unlock(B); display(A+B)Locking as above is not sufficient to guarantee serializability — if A and B get updated in-between the read of A and B, the displayed sum would be wrong.A locking protocol is a set of rules followed by all transactions while requesting and releasing locks. Locking protocols restrict the set of possible schedules.

اسلاید 6: Pitfalls of Lock-Based ProtocolsConsider the partial schedule Neither T3 nor T4 can make progress — executing lock-S(B) causes T4 to wait for T3 to release its lock on B, while executing lock-X(A) causes T3 to wait for T4 to release its lock on A.Such a situation is called a deadlock. To handle a deadlock one of T3 or T4 must be rolled back and its locks released.

اسلاید 7: Pitfalls of Lock-Based Protocols (Cont.)The potential for deadlock exists in most locking protocols. Deadlocks are a necessary evil.Starvation is also possible if concurrency control manager is badly designed. For example:A transaction may be waiting for an X-lock on an item, while a sequence of other transactions request and are granted an S-lock on the same item. The same transaction is repeatedly rolled back due to deadlocks.Concurrency control manager can be designed to prevent starvation.

اسلاید 8: The Two-Phase Locking ProtocolThis is a protocol which ensures conflict-serializable schedules.Phase 1: Growing Phasetransaction may obtain locks transaction may not release locksPhase 2: Shrinking Phasetransaction may release lockstransaction may not obtain locksThe protocol assures serializability. It can be proved that the transactions can be serialized in the order of their lock points (i.e. the point where a transaction acquired its final lock).

اسلاید 9: The Two-Phase Locking Protocol (Cont.)Two-phase locking does not ensure freedom from deadlocksCascading roll-back is possible under two-phase locking. To avoid this, follow a modified protocol called strict two-phase locking. Here a transaction must hold all its exclusive locks till it commits/aborts.Rigorous two-phase locking is even stricter: here all locks are held till commit/abort. In this protocol transactions can be serialized in the order in which they commit.

اسلاید 10: The Two-Phase Locking Protocol (Cont.)There can be conflict serializable schedules that cannot be obtained if two-phase locking is used. However, in the absence of extra information (e.g., ordering of access to data), two-phase locking is needed for conflict serializability in the following sense: Given a transaction Ti that does not follow two-phase locking, we can find a transaction Tj that uses two-phase locking, and a schedule for Ti and Tj that is not conflict serializable.

اسلاید 11: Lock ConversionsTwo-phase locking with lock conversions: – First Phase: can acquire a lock-S on itemcan acquire a lock-X on itemcan convert a lock-S to a lock-X (upgrade) – Second Phase:can release a lock-Scan release a lock-Xcan convert a lock-X to a lock-S (downgrade)This protocol assures serializability. But still relies on the programmer to insert the various locking instructions.

اسلاید 12: Automatic Acquisition of LocksA transaction Ti issues the standard read/write instruction, without explicit locking calls.The operation read(D) is processed as: if Ti has a lock on D then read(D) else begin if necessary wait until no other transaction has a lock-X on D grant Ti a lock-S on D; read(D) end

اسلاید 13: Automatic Acquisition of Locks (Cont.)write(D) is processed as: if Ti has a lock-X on D then write(D) else begin if necessary wait until no other trans. has any lock on D, if Ti has a lock-S on D then upgrade lock on D to lock-X else grant Ti a lock-X on D write(D) end;All locks are released after commit or abort

اسلاید 14: Implementation of LockingA lock manager can be implemented as a separate process to which transactions send lock and unlock requestsThe lock manager replies to a lock request by sending a lock grant messages (or a message asking the transaction to roll back, in case of a deadlock)The requesting transaction waits until its request is answeredThe lock manager maintains a data-structure called a lock table to record granted locks and pending requestsThe lock table is usually implemented as an in-memory hash table indexed on the name of the data item being locked

اسلاید 15: Lock TableBlack rectangles indicate granted locks, white ones indicate waiting requestsLock table also records the type of lock granted or requestedNew request is added to the end of the queue of requests for the data item, and granted if it is compatible with all earlier locksUnlock requests result in the request being deleted, and later requests are checked to see if they can now be grantedIf transaction aborts, all waiting or granted requests of the transaction are deleted lock manager may keep a list of locks held by each transaction, to implement this efficiently

اسلاید 16: Graph-Based ProtocolsGraph-based protocols are an alternative to two-phase lockingImpose a partial ordering  on the set D = {d1, d2 ,..., dh} of all data items.If di  dj then any transaction accessing both di and dj must access di before accessing dj.Implies that the set D may now be viewed as a directed acyclic graph, called a database graph.The tree-protocol is a simple kind of graph protocol.

اسلاید 17: Tree ProtocolOnly exclusive locks are allowed.The first lock by Ti may be on any data item. Subsequently, a data Q can be locked by Ti only if the parent of Q is currently locked by Ti.Data items may be unlocked at any time.

اسلاید 18: Graph-Based Protocols (Cont.)The tree protocol ensures conflict serializability as well as freedom from deadlock.Unlocking may occur earlier in the tree-locking protocol than in the two-phase locking protocol.shorter waiting times, and increase in concurrencyprotocol is deadlock-free, no rollbacks are requiredDrawbacksProtocol does not guarantee recoverability or cascade freedomNeed to introduce commit dependencies to ensure recoverability Transactions may have to lock data items that they do not access.increased locking overhead, and additional waiting timepotential decrease in concurrencySchedules not possible under two-phase locking are possible under tree protocol, and vice versa.

اسلاید 19: Timestamp-Based ProtocolsEach transaction is issued a timestamp when it enters the system. If an old transaction Ti has time-stamp TS(Ti), a new transaction Tj is assigned time-stamp TS(Tj) such that TS(Ti) <TS(Tj). The protocol manages concurrent execution such that the time-stamps determine the serializability order.In order to assure such behavior, the protocol maintains for each data Q two timestamp values:W-timestamp(Q) is the largest time-stamp of any transaction that executed write(Q) successfully.R-timestamp(Q) is the largest time-stamp of any transaction that executed read(Q) successfully.

اسلاید 20: Timestamp-Based Protocols (Cont.)The timestamp ordering protocol ensures that any conflicting read and write operations are executed in timestamp order.Suppose a transaction Ti issues a read(Q)If TS(Ti)  W-timestamp(Q), then Ti needs to read a value of Q that was already overwritten.Hence, the read operation is rejected, and Ti is rolled back.If TS(Ti) W-timestamp(Q), then the read operation is executed, and R-timestamp(Q) is set to the maximum of R-timestamp(Q) and TS(Ti).

اسلاید 21: Timestamp-Based Protocols (Cont.)Suppose that transaction Ti issues write(Q).If TS(Ti) < R-timestamp(Q), then the value of Q that Ti is producing was needed previously, and the system assumed that that value would never be produced. Hence, the write operation is rejected, and Ti is rolled back.If TS(Ti) < W-timestamp(Q), then Ti is attempting to write an obsolete value of Q. Hence, this write operation is rejected, and Ti is rolled back.Otherwise, the write operation is executed, and W-timestamp(Q) is set to TS(Ti).

اسلاید 22: Example Use of the ProtocolA partial schedule for several data items for transactions withtimestamps 1, 2, 3, 4, 5T1T2T3T4T5read(Y)read(X) read(Y)write(Y) write(Z) read(Z) read(X) abort read(X) write(Z) abort write(Y) write(Z)

اسلاید 23: Correctness of Timestamp-Ordering ProtocolThe timestamp-ordering protocol guarantees serializability since all the arcs in the precedence graph are of the form: Thus, there will be no cycles in the precedence graphTimestamp protocol ensures freedom from deadlock as no transaction ever waits. But the schedule may not be cascade-free, and may not even be recoverable.transactionwith smallertimestamptransactionwith largertimestamp

اسلاید 24: Recoverability and Cascade FreedomProblem with timestamp-ordering protocol:Suppose Ti aborts, but Tj has read a data item written by TiThen Tj must abort; if Tj had been allowed to commit earlier, the schedule is not recoverable.Further, any transaction that has read a data item written by Tj must abortThis can lead to cascading rollback --- that is, a chain of rollbacks Solution 1:A transaction is structured such that its writes are all performed at the end of its processingAll writes of a transaction form an atomic action; no transaction may execute while a transaction is being writtenA transaction that aborts is restarted with a new timestampSolution 2: Limited form of locking: wait for data to be committed before reading itSolution 3: Use commit dependencies to ensure recoverability

اسلاید 25: Thomas’ Write RuleModified version of the timestamp-ordering protocol in which obsolete write operations may be ignored under certain circumstances.When Ti attempts to write data item Q, if TS(Ti) < W-timestamp(Q), then Ti is attempting to write an obsolete value of {Q}. Rather than rolling back Ti as the timestamp ordering protocol would have done, this {write} operation can be ignored.Otherwise this protocol is the same as the timestamp ordering protocol.Thomas Write Rule allows greater potential concurrency. Allows some view-serializable schedules that are not conflict-serializable.

اسلاید 26: Validation-Based ProtocolExecution of transaction Ti is done in three phases. 1. Read and execution phase: Transaction Ti writes only to temporary local variables 2. Validation phase: Transaction Ti performs a ``validation test to determine if local variables can be written without violating serializability. 3. Write phase: If Ti is validated, the updates are applied to the database; otherwise, Ti is rolled back.The three phases of concurrently executing transactions can be interleaved, but each transaction must go through the three phases in that order.Assume for simplicity that the validation and write phase occur together, atomically and seriallyI.e., only one transaction executes validation/write at a time. Also called as optimistic concurrency control since transaction executes fully in the hope that all will go well during validation

اسلاید 27: Validation-Based Protocol (Cont.)Each transaction Ti has 3 timestampsStart(Ti) : the time when Ti started its executionValidation(Ti): the time when Ti entered its validation phaseFinish(Ti) : the time when Ti finished its write phaseSerializability order is determined by timestamp given at validation time, to increase concurrency. Thus TS(Ti) is given the value of Validation(Ti).This protocol is useful and gives greater degree of concurrency if probability of conflicts is low. because the serializability order is not pre-decided, andrelatively few transactions will have to be rolled back.

اسلاید 28: Validation Test for Transaction TjIf for all Ti with TS (Ti) < TS (Tj) either one of the following condition holds:finish(Ti) < start(Tj) start(Tj) < finish(Ti) < validation(Tj) and the set of data items written by Ti does not intersect with the set of data items read by Tj. then validation succeeds and Tj can be committed. Otherwise, validation fails and Tj is aborted.Justification: Either the first condition is satisfied, and there is no overlapped execution, or the second condition is satisfied andthe writes of Tj do not affect reads of Ti since they occur after Ti has finished its reads.the writes of Ti do not affect reads of Tj since Tj does not read any item written by Ti.

اسلاید 29: Schedule Produced by ValidationExample of schedule produced using validationT14T15read(B)read(B)B:= B-50read(A)A:= A+50read(A)(validate)display (A+B)(validate)write (B)write (A)

اسلاید 30: Multiple GranularityAllow data items to be of various sizes and define a hierarchy of data granularities, where the small granularities are nested within larger onesCan be represented graphically as a tree (but dont confuse with tree-locking protocol)When a transaction locks a node in the tree explicitly, it implicitly locks all the nodes descendents in the same mode.Granularity of locking (level in tree where locking is done):fine granularity (lower in tree): high concurrency, high locking overheadcoarse granularity (higher in tree): low locking overhead, low concurrency

اسلاید 31: Example of Granularity Hierarchy The levels, starting from the coarsest (top) level aredatabaseareafilerecord

اسلاید 32: Intention Lock ModesIn addition to S and X lock modes, there are three additional lock modes with multiple granularity:intention-shared (IS): indicates explicit locking at a lower level of the tree but only with shared locks.intention-exclusive (IX): indicates explicit locking at a lower level with exclusive or shared locksshared and intention-exclusive (SIX): the subtree rooted by that node is locked explicitly in shared mode and explicit locking is being done at a lower level with exclusive-mode locks.intention locks allow a higher level node to be locked in S or X mode without having to check all descendent nodes.

اسلاید 33: Compatibility Matrix with Intention Lock ModesThe compatibility matrix for all lock modes is: ISIXSS IXX ISIXSS IXX 

اسلاید 34: Multiple Granularity Locking SchemeTransaction Ti can lock a node Q, using the following rules:The lock compatibility matrix must be observed.The root of the tree must be locked first, and may be locked in any mode.A node Q can be locked by Ti in S or IS mode only if the parent of Q is currently locked by Ti in either IX or IS mode.A node Q can be locked by Ti in X, SIX, or IX mode only if the parent of Q is currently locked by Ti in either IX or SIX mode.Ti can lock a node only if it has not previously unlocked any node (that is, Ti is two-phase).Ti can unlock a node Q only if none of the children of Q are currently locked by Ti.Observe that locks are acquired in root-to-leaf order, whereas they are released in leaf-to-root order.

اسلاید 35: Multiversion SchemesMultiversion schemes keep old versions of data item to increase concurrency.Multiversion Timestamp OrderingMultiversion Two-Phase LockingEach successful write results in the creation of a new version of the data item written.Use timestamps to label versions.When a read(Q) operation is issued, select an appropriate version of Q based on the timestamp of the transaction, and return the value of the selected version. reads never have to wait as an appropriate version is returned immediately.

اسلاید 36: Multiversion Timestamp OrderingEach data item Q has a sequence of versions <Q1, Q2,...., Qm>. Each version Qk contains three data fields:Content -- the value of version Qk.W-timestamp(Qk) -- timestamp of the transaction that created (wrote) version QkR-timestamp(Qk) -- largest timestamp of a transaction that successfully read version Qkwhen a transaction Ti creates a new version Qk of Q, Qks W-timestamp and R-timestamp are initialized to TS(Ti). R-timestamp of Qk is updated whenever a transaction Tj reads Qk, and TS(Tj) > R-timestamp(Qk).

اسلاید 37: Multiversion Timestamp Ordering (Cont)Suppose that transaction Ti issues a read(Q) or write(Q) operation. Let Qk denote the version of Q whose write timestamp is the largest write timestamp less than or equal to TS(Ti).If transaction Ti issues a read(Q), then the value returned is the content of version Qk.If transaction Ti issues a write(Q)if TS(Ti) < R-timestamp(Qk), then transaction Ti is rolled back. if TS(Ti) = W-timestamp(Qk), the contents of Qk are overwrittenelse a new version of Q is created.Observe thatReads always succeedA write by Ti is rejected if some other transaction Tj that (in the serialization order defined by the timestamp values) should read Tis write, has already read a version created by a transaction older than Ti.Protocol guarantees serializability

اسلاید 38: Multiversion Two-Phase LockingDifferentiates between read-only transactions and update transactionsUpdate transactions acquire read and write locks, and hold all locks up to the end of the transaction. That is, update transactions follow rigorous two-phase locking.Each successful write results in the creation of a new version of the data item written.each version of a data item has a single timestamp whose value is obtained from a counter ts-counter that is incremented during commit processing.Read-only transactions are assigned a timestamp by reading the current value of ts-counter before they start execution; they follow the multiversion timestamp-ordering protocol for performing reads.

اسلاید 39: Multiversion Two-Phase Locking (Cont.)When an update transaction wants to read a data item:it obtains a shared lock on it, and reads the latest version. When it wants to write an itemit obtains X lock on; it then creates a new version of the item and sets this versions timestamp to .When update transaction Ti completes, commit processing occurs:Ti sets timestamp on the versions it has created to ts-counter + 1Ti increments ts-counter by 1Read-only transactions that start after Ti increments ts-counter will see the values updated by Ti. Read-only transactions that start before Ti increments the ts-counter will see the value before the updates by Ti. Only serializable schedules are produced.

اسلاید 40: Deadlock HandlingConsider the following two transactions: T1: write (X) T2: write(Y) write(Y) write(X)Schedule with deadlockT1T2lock-X on Xwrite (X) lock-X on Ywrite (X) wait for lock-X on Xwait for lock-X on Y

اسلاید 41: Deadlock HandlingSystem is deadlocked if there is a set of transactions such that every transaction in the set is waiting for another transaction in the set.Deadlock prevention protocols ensure that the system will never enter into a deadlock state. Some prevention strategies :Require that each transaction locks all its data items before it begins execution (predeclaration).Impose partial ordering of all data items and require that a transaction can lock data items only in the order specified by the partial order (graph-based protocol).

اسلاید 42: More Deadlock Prevention StrategiesFollowing schemes use transaction timestamps for the sake of deadlock prevention alone.wait-die scheme — non-preemptiveolder transaction may wait for younger one to release data item. Younger transactions never wait for older ones; they are rolled back instead.a transaction may die several times before acquiring needed data itemwound-wait scheme — preemptiveolder transaction wounds (forces rollback) of younger transaction instead of waiting for it. Younger transactions may wait for older ones.may be fewer rollbacks than wait-die scheme.

اسلاید 43: Deadlock prevention (Cont.)Both in wait-die and in wound-wait schemes, a rolled back transactions is restarted with its original timestamp. Older transactions thus have precedence over newer ones, and starvation is hence avoided.Timeout-Based Schemes :a transaction waits for a lock only for a specified amount of time. After that, the wait times out and the transaction is rolled back.thus deadlocks are not possiblesimple to implement; but starvation is possible. Also difficult to determine good value of the timeout interval.

اسلاید 44: Deadlock DetectionDeadlocks can be described as a wait-for graph, which consists of a pair G = (V,E), V is a set of vertices (all the transactions in the system)E is a set of edges; each element is an ordered pair Ti Tj. If Ti  Tj is in E, then there is a directed edge from Ti to Tj, implying that Ti is waiting for Tj to release a data item.When Ti requests a data item currently being held by Tj, then the edge Ti Tj is inserted in the wait-for graph. This edge is removed only when Tj is no longer holding a data item needed by Ti.The system is in a deadlock state if and only if the wait-for graph has a cycle. Must invoke a deadlock-detection algorithm periodically to look for cycles.

اسلاید 45: Deadlock Detection (Cont.)Wait-for graph without a cycleWait-for graph with a cycle

اسلاید 46: Deadlock RecoveryWhen deadlock is detected :Some transaction will have to rolled back (made a victim) to break deadlock. Select that transaction as victim that will incur minimum cost.Rollback -- determine how far to roll back transactionTotal rollback: Abort the transaction and then restart it.More effective to roll back transaction only as far as necessary to break deadlock.Starvation happens if same transaction is always chosen as victim. Include the number of rollbacks in the cost factor to avoid starvation

اسلاید 47: Insert and Delete OperationsIf two-phase locking is used :A delete operation may be performed only if the transaction deleting the tuple has an exclusive lock on the tuple to be deleted.A transaction that inserts a new tuple into the database is given an X-mode lock on the tupleInsertions and deletions can lead to the phantom phenomenon.A transaction that scans a relation (e.g., find all accounts in Perryridge) and a transaction that inserts a tuple in the relation (e.g., insert a new account at Perryridge) may conflict in spite of not accessing any tuple in common. If only tuple locks are used, non-serializable schedules can result: the scan transaction may not see the new account, yet may be serialized before the insert transaction.

اسلاید 48: Insert and Delete Operations (Cont.)The transaction scanning the relation is reading information that indicates what tuples the relation contains, while a transaction inserting a tuple updates the same information. The information should be locked.One solution: Associate a data item with the relation, to represent the information about what tuples the relation contains.Transactions scanning the relation acquire a shared lock in the data item, Transactions inserting or deleting a tuple acquire an exclusive lock on the data item. (Note: locks on the data item do not conflict with locks on individual tuples.)Above protocol provides very low concurrency for insertions/deletions.Index locking protocols provide higher concurrency while preventing the phantom phenomenon, by requiring locks on certain index buckets.

اسلاید 49: Index Locking ProtocolEvery relation must have at least one index. Access to a relation must be made only through one of the indices on the relation.A transaction Ti that performs a lookup must lock all the index buckets that it accesses, in S-mode.A transaction Ti may not insert a tuple ti into a relation r without updating all indices to r.Ti must perform a lookup on every index to find all index buckets that could have possibly contained a pointer to tuple ti, had it existed already, and obtain locks in X-mode on all these index buckets. Ti must also obtain locks in X-mode on all index buckets that it modifies.The rules of the two-phase locking protocol must be observedGuarantees that phantom phenomenon won’t occur

اسلاید 50: Weak Levels of ConsistencyDegree-two consistency: differs from two-phase locking in that S-locks may be released at any time, and locks may be acquired at any timeX-locks must be held till end of transactionSerializability is not guaranteed, programmer must ensure that no erroneous database state will occur]Cursor stability: For reads, each tuple is locked, read, and lock is immediately releasedX-locks are held till end of transactionSpecial case of degree-two consistency

اسلاید 51: Weak Levels of Consistency in SQLSQL allows non-serializable executionsSerializable: is the defaultRepeatable read: allows only committed records to be read, and repeating a read should return the same value (so read locks should be retained)However, the phantom phenomenon need not be preventedT1 may see some records inserted by T2, but may not see others inserted by T2Read committed: same as degree two consistency, but most systems implement it as cursor-stabilityRead uncommitted: allows even uncommitted data to be read

اسلاید 52: Concurrency in Index StructuresIndices are unlike other database items in that their only job is to help in accessing data.Index-structures are typically accessed very often, much more than other database items. Treating index-structures like other database items leads to low concurrency. Two-phase locking on an index may result in transactions executing practically one-at-a-time.It is acceptable to have nonserializable concurrent access to an index as long as the accuracy of the index is maintained.In particular, the exact values read in an internal node of a B+-tree are irrelevant so long as we land up in the correct leaf node.There are index concurrency protocols where locks on internal nodes are released early, and not in a two-phase fashion.

اسلاید 53: Concurrency in Index Structures (Cont.)Example of index concurrency protocol:Use crabbing instead of two-phase locking on the nodes of the B+-tree, as follows. During search/insertion/deletion:First lock the root node in shared mode.After locking all required children of a node in shared mode, release the lock on the node.During insertion/deletion, upgrade leaf node locks to exclusive mode.When splitting or coalescing requires changes to a parent, lock the parent in exclusive mode.Above protocol can cause excessive deadlocks. Better protocols are available; see Section 16.9 for one such protocol, the B-link tree protocol

اسلاید 54: End of Chapter

اسلاید 55: Partial Schedule Under Two-Phase Locking

اسلاید 56: Incomplete Schedule With a Lock Conversion

اسلاید 57: Lock Table

اسلاید 58: Tree-Structured Database Graph

اسلاید 59: Serializable Schedule Under the Tree Protocol

اسلاید 60: Schedule 3

اسلاید 61: Schedule 4

اسلاید 62: Schedule 5, A Schedule Produced by Using Validation

اسلاید 63: Granularity Hierarchy

اسلاید 64: Compatibility Matrix

اسلاید 65: Wait-for Graph With No Cycle

اسلاید 66: Wait-for-graph With A Cycle

اسلاید 67: Nonserializable Schedule with Degree-Two Consistency

اسلاید 68: B+-Tree For account File with n = 3.

اسلاید 69: Insertion of “Clearview” Into the B+-Tree of Figure 16.21

اسلاید 70: Lock-Compatibility Matrix