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7.24- Lock Based Protocols- Shared And Exclusive Locks | Concurrency Control | DBMS Free Course

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2 Concurrency Control Manager (CCM)
The concurrency control manager synchronizes the concurrent access of database transactions
to shored objects in the database. It is responsible for maintaining the guarantees regarding
the effects of concurrent access to the shored database. It will make sure that the result of
transactions running in parallel is identical to the result of some serial execution of the same
transactions.
The Concurrency Control Manager grant locks to different transactions. Concurrency
Control manager is basically a programme.
8.5.1.3 Types of Lock
There are two types of locks that can be implemated on a transaction.
(i) Shared lock : Shared lock is Read-Only lock. If a transaction Ti
has obtained shared
lock on data item A then Ti can read A but cannot modify A. It is denoted by S
and Ti is said to be in Shared lock mode.
(ii) Exclusive lock : Exclusive lock is Read-Write lock. If a transaction Ti
has obtained
exclusive lock on data item A then Ti can both read and modify (write) A. It is
denoted by X and Ti is said to be in Exclusive lock mode.
8.5.1.4 Compatible Lock Modes
Suppose transaction Ti has obtained a lock on data item V in mode A. If transaction Tj
can also obtain lock on V in mode B then, A is compatible with B or these two lock modes
are compatible. Matrix of compatibility of shared and Exclusive lock modes are shown in
Figure 8.6.
S X
S Possible Not possible
X Not possible Not possible
FIGURE 8.6. Compatibility of shared and exclusive lock mode.
The table shows that if a transaction Ti
has shared lock on data item V then any
other transaction Tj
can obtain only shared lock on V at the same time. All other possible
combinations are incompatible.
S - lock(A) — Shared lock on data item A
X - lock(A) — Exclusive lock on data item A
read(A) — Read operation on A
354 Introduction to Database Management System
write(A) — Write operation on A
unlock(A) — A is free now.
A transaction Ti
can hold only those items which are used in it. If any data item V is
not free then Ti
is made to wait until V is released. It is not desirable to unlock data item
immediately after its final access.