Concurrency Control in Distributed Database.

Presented By:
MAYANK SETH
ARUN BRIJWASI

CONTENTS
Meaning of distributed database
Introduction to concurrency control
Locking based protocols
Timestamping

Distributed databases
 A distributed database is a single logical database that is
spread physically across the computer in multiple location
that are connected by data communication links.
 Distributed database is a kind of virtual database whose
components parts are physically stored in a number of
distinct real database at a number of distinct location.

CONCURRENCY
A situation in which two or more persons access the same
records simultaneously is called concurrency
Concurrency control involve the synchronization of access to
the distributed database, such that the integrity of the
database is maintained.

Concurrency control
Concurrency control can be achieved
through:
Locking protocols
Timestamping

Lock-Based Protocols
 A lock is a mechanism to control concurrent access to
a data item.
 Data items can be locked in two modes :
 Exclusive (X) mode. Data item can be both read as
well as written. X-lock is requested using lock-X
instruction.
 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.

Lock-Based Protocols (Cont.)
A transaction may be granted a lock on an item if the
requested lock is compatible with locks already held on the
item by other transactions
Any 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.

Lock based protocols
Lock based concurrency control
can either be :
One phase locking protocol
Two phase locking protocol

One phase locking protocol
In this method each transaction
locks an item before use and
releases the lock as soon as it has
finished using it.

Two phase locking protocol
This protocol requires that each transaction
issue lock and unlock request in 2 phases:
 First phase: a transaction only acquires all
the locks it needs and do not release any
lock. This phase is called growing phase.
Second phase: a transaction releases the
locks and cannot request any new locks. This
phase is called shrinking phase.

Various locking protocols
The various locking protocols that can be used in
distributed databases in following manner:
Single lock manager approach
Distributed lock manager approach
Primary copy
Majority protocol

Single lock manager protocol
The system maintains a single lock manager
that resides in a single chosen site—say Si .
All lock and unlock requests are made at site
Si .
When a transaction needs to lock a data
item, it sends a lock request to Si .

The lock manager determines whether
the lock can be granted immediately.
 If the lock can be granted, the lock
manager sends a message to that effect
to the site at which the lock request was
initiated.
Otherwise, the request is delayed until it
can be granted, at which time a message
is sent to the site at which the lock
request was initiated.

Distributed lock manager
The lock-manager function is distributed over
several sites.
Each site maintains a local lock manager whose
function is to administer the lock and unlock
requests for those data items that are stored in
that site.
When a transaction wishes to lock a data item Q
that is not replicated and resides at site Si , a
message is sent to the lock manager at site Si
requesting a lock (in a particular lock mode).

 If data item Q is locked in an incompatible
mode, then the request is delayed until it can
be granted.
 Once it has determined that the lock request
can be granted, the lock manager sends a
message back to the initiator indicating thatit
has granted the lock request.

Primary copy
When a system uses data replication, we can choose one
of the replicas as the primary copy.
For each data item Q, the primary copy of Q must reside in
precisely one site, which we call the primary site of Q.
When a transaction needs to lock a data item Q, it requests
a lock at the primary site of Q.
The response to the request is delayed until it can be
granted.
The primary copy enables concurrency control for
replicated data to be handled like that for unreplicated data.

Majority protocol
If data item Q is replicated in n different sites, then a
lock-request message must be sent to more than one-
half of the n sites in which Q is stored.
 Each lock manager determines whether the lock can
be granted immediately.
The response is delayed until the request can be
granted.
The transaction does not operate on Q until it has
successfully obtained a lock on a majority of the
replicas of Q.

Timestamp-Based Protocols
Each transaction is given a unique timestamp that the system
uses in deciding the serialization order.
The protocol manages concurrent execution such that the
timestamps 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.

In generalizing the centralized scheme to a distributed
scheme is to develop a scheme for generating unique
timestamps.
There are two primary methods for generating unique
timestamps, one centralized and one distributed.
 In the centralized scheme, a single site distributes the
timestamps.
In the distributed scheme, each site generates a unique
local timestamp by using either a logical counter or the
local clock.
 We obtain the unique global timestamp by
concatenating the unique local timestamp with the site
identifier, which also must be unique.

Concurrency Control in Distributed Database.

Concurrency Control in Distributed Database.

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