Concurrency Control: 18.4 Locking Systems with Several Lock Modes CS257 Spring/2009 Professor: Tsau Lin Student: Suntorn Sae-Eung ID: 212 1

18.4 Locking Systems with Several Lock Modes In 18.3, if a transaction must lock a database element (X) either reads or writes, – No reason why several transactions could not read X at the same time, as long as none write X Introduce locking schemes – Shared/Read Lock ( For Reading) – Exclusive/Write Lock( For Writing) 2

Shared & Exclusive Locks Transaction Consistency – Cannot write without Exclusive Lock – Cannot read without holding some lock – Consider lock for writing is “stronger” than for reading This basically works on 2 principles 1. A read action can only proceed a shared or an exclusive lock 2. A write lock can only proceed a exclusive lock All locks need to be unlocked before commit 3

Shared & Exclusive Locks (cont.) Two-phase locking (2PL) of transactions T i Notation: sl i (X)– T i requests shared lock on DB element X xl i (X)– T i requests exclusive lock on DB element X u i (X)– T i relinquishes whatever lock on X 4 Lock  R/W  Unlock

Shared & Exclusive Locks (cont.) Legality of Schedules – An element may be locked by: one write transaction or by several read transactions shared mode, but not both

Compatibility Matrices A convenient way to describe lock- management policies – Rows correspond to a lock held on an element by another transaction – Columns correspond to mode of lock requested. – Example : 6 Lock requested SX Lock in hold SYESNO X

Upgrading Locks A transaction (T) taking a shared lock is friendly toward other transaction. When T wants to read and write a new value X, 1. T takes a shared lock on X. 2. performs operations on X (may spend long time) 3. When T is ready to write a new value, “Upgrade” shared lock to exclusive lock on X. 7

Upgrading Locks (cont.) Observe the example T 1 cannot take an exclusive lock on B until all locks on B are released. ‘B’ is released T1 retry and succeed

Upgrading Locks (cont.) Upgrading can simply cause a “Deadlock”. – Both the transactions want to upgrade on the same element 9 Both transactions will wait forever !!

Update locks The third lock mode resolving the deadlock problem, which rules are – Only “Update lock” can be upgraded to a write (exclusive) lock later. – An “Update lock” is allowed to grant on X when there are already shared locks on X. – Once there is an “Update lock,” it prevents additional any kinds of lock, and later changes to a write (exclusive) lock. Notation: ul i (X) 10

Update locks (cont.) Example

Update locks (cont.) Compatibility matrix (asymmetric) 12 Lock requested SXU Lock in hold SYESNOYES XNO U

Increment Locks A useful lock for transactions which increase/decrease value. e.g. money transfer between two bank accounts. If 2 transactions (T 1, T 2 ) add constants to the same database element (X), – It doesn’t matter which goes first, but no reads are allowed in between transaction processing Let see on following exhibits 13

Increment Locks (cont.) A=5 A=15 A=17 A=7 T 1 : INC (A,2) T 2 : INC (A,10) CASE 1 CASE 2

Increment Locks (cont.) What if A=5 A=15 A=7 T 1 : INC (A,2) T 2 : INC (A,10) A=5A=7 A != 17 A=5

Increment Locks (cont.) INC (A, c) – – Increment action of writing on database element A, which is an atomic execution consisting of 1. READ(A,t); 2. t = t+c; 3. WRITE(A,t); Notation: – il i (X)– action of T i requesting an increment lock on X – inc i (X)– action of T i increments X by some constant; don’t care about the value of the constant.

Increment Locks (cont.) Example

Increment Locks (cont.) Compatibility matrix 18 Lock requested SXI Lock in hold SYESNO X I YES