Notes

Slide Show

Outline

1
2	Chapter 18 Concurrency Control Techniques
3	Chapter 18 Outline Databases Concurrency Control 1 Purpose of Concurrency Control 2 Two-Phase locking 5 Limitations of CCMs 6 Index Locking 7 Lock Compatibility Matrix 8 Lock Granularity
4	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.
5	Database Concurrency Control Two-Phase Locking Techniques Locking is an operation which secures (a) permission to Read or (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.
6	Database Concurrency Control Two-Phase Locking Techniques: Essential components Two locks modes (a) shared (read) and (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
7	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.
8	Database Concurrency Control Two-Phase Locking Techniques: Essential components Database requires that all transactions should be well-formed. A transaction is well-formed if:
9	Database Concurrency Control
10	Database Concurrency Control
11	Database Concurrency Control
12	Database Concurrency Control
13	Database Concurrency Control
14	Database Concurrency Control
15	Database Concurrency Control
16	Database Concurrency Control
17	Database Concurrency Control
18	Database Concurrency Control
19	Database Concurrency Control
20	Database Concurrency Control
21	Database Concurrency Control
22	Database Concurrency Control
23	Database Concurrency Control
24	Database Concurrency Control
25	Database Concurrency Control
26	Database Concurrency Control
27	Database Concurrency Control
28	Database Concurrency Control
29	Database Concurrency Control
30	Database Concurrency Control
31	Database Concurrency Control
32	Database Concurrency Control
33	Database Concurrency Control
34	Database Concurrency Control
35	Database Concurrency Control
36	Database Concurrency Control
37	Database Concurrency Control
38	Database Concurrency Control
39	Database Concurrency Control
40	Database Concurrency Control
41	Database Concurrency Control
42	Database Concurrency Control
43	Database Concurrency Control
44	Database Concurrency Control
45	Database Concurrency Control
46	Database Concurrency Control