ICS (072)Concurrency Control Techniques1 Concurrency Control Techniques Chapter 18 Dr. Muhammad Shafique

ICS (072)Concurrency Control Techniques2 Outline Granularity of data items Concurrency control techniques 1.Locking techniques 2.Timestamp ordering techniques Using locks for concurrency control in indexes Other concurrency control issues Summary

ICS (072)Concurrency Control Techniques3 Categories of concurrency techniques 1.Locking techniques 2.Timestamp ordering techniques 3.Multi-version concurrency control techniques 4.Optimistic concurrency control techniques 5.Advanced concurrency control techniques

ICS (072)Concurrency Control Techniques4 Granularity of Data Items Data item granularity Size or granule of data for concurrency control. It could be An attribute value A database record A disk block A whole file The whole database Data item granularity and degree of concurrency Fine granularity --- refers to small item sizes Coarse granularity --- refers to large item sizes Larger the data item size, lower the degree of concurrency Data item size depends on the types of transactions

ICS (072)Concurrency Control Techniques5 Concurrency Control Using Locks Lock A variable associated with a data item Describes status of the data item with respect to operations that can be performed on it Types of locks Binary locks Locked/unlocked Enforces mutual exclusion Multiple-mode locks: Each data item can be in one of three lock states 1.Read lock or shared lock 2.Write lock or exclusive lock 3.Unlock Lock table managed by lock manager subsystem of DBMS

ICS (072)Concurrency Control Techniques6 Lock and Unlock Operations for Binary Locks

ICS (072)Concurrency Control Techniques7 Locking Rules 1.T must issue read_lock(X) or write_lock(X) before any read_item(X) operation is performed in T 2.T must issue write_lock(X) before any write_item(X) operation is performed in T 3.T must issue unlock(X) after all read_item(X) and write_item(X) operations are completed in T 4.T will not issue a read_lock(X) if it already holds a read lock or write lock on X (may be relaxed) 5.T will not issue a write_lock(X) if it already holds a read lock or write lock on X (may be relaxed) 6.T will not issue unlock (X) request unless it holds a read lock write lock on X

ICS (072)Concurrency Control Techniques8 Conversion of Locks A transaction T that holds a lock on item X, under certain conditions, is allowed to convert the lock from one state to another Upgrade Convert read_lock to write_lock Downgrade Convert write_lock to read_lock Using locks does not guarantee serializability of schedules on its own

ICS (072)Concurrency Control Techniques9 Two Transactions T1 read_lock(Y); read_item(Y); unlock(Y); write_lock(X); read_item(X); X:=X+Y; write_item(X); unlock(X); T2 read_lock(X); read_item(X); unlock(X); write_lock(Y); read_item(Y); Y:=X+Y; write_item(Y); unlock(Y); Let’s assume serial schedule S1: T1,T2 Initial values: X=20, Y=30  Result: X=50, Y=80

ICS (072)Concurrency Control Techniques10 Locks Alone Don’t Insure Serializability T1 read_lock(Y); read_item(Y); unlock(Y); write_lock(X); read_item(X); X:=X+Y; write_item(X); unlock(X); T2 read_lock(X); read_item(X); unlock(X); write_lock(Y); read_item(Y); Y:=X+Y; write_item(Y); unlock(Y); Non-serializable! Result: X=50, Y=50 unlocked too early! Let’s run T1 and T2 in interleaved fashion Schedule S

ICS (072)Concurrency Control Techniques11 Two-Phase Locking (2PL) Protocol Transaction is said to follow the two-phase-locking protocol if all locking operations precede the first unlock operation Expanding (growing) phase Shrinking phase During the shrinking phase no new locks can be acquired Downgrading ok Upgrading is not Example

ICS (072)Concurrency Control Techniques12 2PL Example T1’ read_lock(Y); read_item(Y); write_lock(X); unlock(Y); read_item(X); X:=X+Y; write_item(X); unlock(X); T2’ read_lock(X); read_item(X); write_lock(Y); unlock(X); read_item(Y); Y:=X+Y; write_item(Y); unlock(Y); Both T1’ and T2’ follow the 2PL protocol Any schedule including T1’ and T2’ is guaranteed to be serializable Limits the amount of concurrency

ICS (072)Concurrency Control Techniques13 Locking Techniques for Concurrency Control Quick review of ideas discussed so far Locks and system lock tables Lock table and lock manager Binary locks --- lock & unlock operations (critical sections) Shared/exclusive locks --- read-lock, write-lock, and unlock Conversion of locks Upgrade the lock Downgrade the lock Concurrency control subsystem generates read-lock and write-lock requests on behalf of the transactions Using locks in transactions does not guarantee serializability of schedules on its own

ICS (072)Concurrency Control Techniques14 Locking Techniques for Concurrency Control (Cont.) Two-phase locking protocol (2PL) All lock operations precede the first unlock operation Expanding phase and shrinking phase Upgrading of locks must be done in expanding phase and downgrading of locks must be done in shrinking phase If every transaction in a schedule follows 2PL protocol then the schedules is guaranteed to be serializable. Variants of 2PL Basic, conservative, strict, and rigorous

ICS (072)Concurrency Control Techniques15 Locking Techniques for Concurrency Control (Cont.) 1.Basic 2PL All lock operations before the first unlock operation 2.Conservative 2PL or static 2PL Lock all the items it accesses before the transaction begins execution. 1.Deadlock free protocol 2.Read-set and write-set of the transaction should be known 3.Strict 2PL No exclusive lock will be unlocked until the transaction commits or aborts Strict 2PL guarantees strict schedules 4.Rigorous 2PL No lock will be unlocked until the transaction commits or aborts

ICS (072)Concurrency Control Techniques16 Dealing with Deadlocks and Starvation in 2PL 2PL can produce deadlocks Deadlock and starvation in 2PL Deadlock occurs when each transaction T in a set of two or more transactions is waiting for some item that is locked by some other transaction T’ in the set. Example (next slide)

ICS (072)Concurrency Control Techniques17 Dealing with Deadlocks and Starvation in 2PL

ICS (072)Concurrency Control Techniques18 Dealing with Deadlocks and Starvation in 2PL Deadlock prevention protocols Conservative 2PL, lock all needed items in advance Ordering all items in the database Possible actions if a transaction is involved in a possible deadlock situation Block and wait Abort and restart Preempt and abort another transaction

ICS (072)Concurrency Control Techniques19 Dealing with Deadlocks and Starvation in 2PL Two schemes that prevent deadlock (Timestamp based) Wait-die An older transaction is allowed to wait on a younger transaction whereas a younger transaction requesting an item from held by an older transaction is aborted and restarted with the same timestamp Wound-wait A younger transaction is allowed to wait on an older transaction whereas an older transaction requesting an item from held by a younger transaction preempts the younger transaction by aborting it.

ICS (072)Concurrency Control Techniques20 Dealing with Deadlocks and Starvation in 2PL Two schemes that prevent deadlock and do not require timestamps No waiting Abort immediately and restart after a certain time delay Cautious waiting If a transaction holding the lock is not waiting for another item to be locked then allow T to wait otherwise abort and restart T

ICS (072)Concurrency Control Techniques21 Dealing with Deadlocks and Starvation in 2PL Deadlock detection and timeouts Construct and maintain a wait-for graph Victim selection Timeouts Starvation A transaction cannot proceed for an infinite period of time while other transactions in the system continue normally Unfair waiting scheme Victim selection

ICS (072)Concurrency Control Techniques22 Concurrency Control Based on Timestamp Ordering Timestamp --- a unique identifier created by the DBMS to identify a transaction read-TS(X) The largest timestamp among all the timestamps of transactions that have successfully read item X write-TS(X) The largest timestamp among all the timestamps of transactions that have successfully written item X Timestamp Ordering (TO) algorithms Basic timestamp ordering Strict timestamp ordering Thomas’s write rule

ICS (072)Concurrency Control Techniques23 Concurrency Control Based on Timestamp Ordering (Cont.) Basic timestamp ordering algorithm Order transactions based on their timestamps 1.T issues a write(X) 1.If read-TS(X) > TS(T) or if write-TS(X) > TS(T) the abort and rollback T and reject the operation. 2.If condition above does not occur then execute the operation and set write-TS(X) = TS(T) 2.T issues a read(X) 1.If write-TS(X) > TS(T) then abort and rollback T and reject the operation. 2.If write-TS(X)  TS(T) then execute the operation and set read-TS(X) = max ( read-TS(X), TS(T) ) The schedules produced by basic TO are guaranteed to be conflict serializable No deadlocks but cyclic restart are possible (hence starvation)

ICS (072)Concurrency Control Techniques24 Concurrency Control Based on Timestamp Ordering (Cont.) Strict Timestamp Ordering (strict TO) A transaction T that issues a read-item(X) or write-item(X) such that TS(T) > write-TS(X) has its read or write operation delayed until the transaction T’ that wrote the value of X (hence TS(T’ ) = write-TS(X)) has committed or aborted. No deadlocks, since T waits for T’ only if TS(T) > TS(T’) Strict TO ensures that the schedules are both strict (for easy recoverability) and (conflict) serializable

ICS (072)Concurrency Control Techniques25 Concurrency Control Based on Timestamp Ordering (Cont.) Thomas’s write rule It rejects fewer write operations, by modifying the basic TO checks for the write-item(X) operation as follows: 1.If read-TS(X) > TS(T), then abort and roll back T and reject the operation. 2.If write-TS(X) > TS(T), then do not execute the write operation but continue processing. [ This is because some transaction with timestamp greater than TS(T)—and hence after T in the timestamp ordering—has already written the value of X. Hence, we must ignore the write_item(X) operation of T because it is already outdated and obsolete. Notice that any conflict arising from this situation would be detected by case (1).] 3.If neither the condition in part (1) nor the condition in part (2) occurs, then execute the write-item(X) operation of T and set write- TS(X) to TS(T). Thomas’ write rule does not enforce conflict serializability

ICS (072)Concurrency Control Techniques26 Using Locks for Concurrency Control in Indexes Two phase locking can be applied to indexes. Here the nodes of an index correspond to disk pages. Tree structure of the index can be taken advantage of A conservative approach for insertions can be to lock the root node in exclusive mode. Index locking using B-link tree rather than B+ tree In B-link tree, sibling nodes at the same level are linked together at every level

ICS (072)Concurrency Control Techniques27 Other Concurrency Control Issues Insertion, deletion, and phantom records Insertion operation New data item inserted in the database Deletion operation Phantom record Use index locking Interactive transactions No rollback Delay output of transactions until they have committed Latches Locks held for a short duration are called latches Latches are used to, for example, write a buffer to a page

ICS (072)Concurrency Control Techniques28 Summary Granularity of data items Concurrency control techniques 1.Locking techniques 2.Timestamp ordering techniques Using locks for concurrency control in indexes Other concurrency control issues Thank you