Download Chapter 10 - Emunix Emich

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Chapter 10
Protecting Data
Integrity in a
Multiuser
Environment
10
Chapter Objectives
• Develop code that implements database
transactions
• Develop code that commits and rolls back
transactions
• Identify the types of problems that occur
when multiple users access a database
concurrently
10
Chapter Objectives
• Implement database, recordset, page-level,
and record-level locking strategies
• Develop error-handling procedures that
support concurrent use of a database
• Replicate and synchronize replicas and
partial replicas of a database
10
Using Transactions to Protect
Against System Failure
• Transaction
 Logical unit of work
 Each unit of work comprises one or more
physical database operations
• When a transaction completes successfully,
changes that the transaction made to the
database are committed to the database
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Using Transactions to Protect
Against System Failure
• Whenever a transaction aborts, changes that
it made to the database before aborting are
not committed to the database
 Changes are rolled back
 Database fields that contain changes return to their
original state
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Transaction Processing in
VBA
• Three methods of the Connection object to
define the beginning or end of a transaction
in VBA:
 BeginTrans
 Denotes the beginning of the transaction
 CommitTrans
 Denotes the end of a successful transaction
 Rollback Trans
 Denotes the end of an unsuccessful transaction
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Additional Transaction
Features and Caveats
• UseTransaction property
 When set to Yes, a transaction is automatically
defined for the query
 Therefore, BeginTrans and CommitTrans methods
are unnecessary
• FailOnError property
 When set to Yes, the query results are
automatically rolled back when the first error
occurs
10
Additional Transaction
Features and Caveats
Figure 10-1 Setting the Use Transaction and Fail On Error properties
in query Design view
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Additional Transaction
Features and Caveats
• Access supports a maximum of five levels
of nested transactions
• When one transaction is nested inside
another, commitments of transactions are
temporary until the final CommitTrans
method executes
 If that method does not execute or the
corresponding RollbackTrans method executes,
all nested transactions are rolled back
10
Additional Transaction
Features and Caveats
• Connection object
 Controls a transaction
• Recordsets that use a particular connection are
rolled back when the RollbackTrans method is
applied to the Connection object
• When data are updated through bound forms,
Access developers have little control over the
definition of a transaction
 Access controls the updates
10
Solving Concurrent
Processing Problems
• Concurrent transactions
 Transactions that overlap
• Inconsistent analysis
 Type of concurrency issue
• Locking mechanisms
 Prevent more than one transaction at a time
from modifying a data object
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Solving Concurrent
Processing Problems
• Granularity
 Level of locking
• Write lock
 Allows other users to read but not update the
locked records
 Prevents the lost update problem, but not
prevent the inconsistent analysis problem
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Solving Concurrent
Processing Problems
• Read lock
 Prevents other users from reading or writing
locked records
 Prevents both the inconsistent analysis problem
and the lost update problem
• Pessimistic locking
 Locks a page or record as soon as the first field
in the record is changed and releases a lock
after the record is written
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Solving Concurrent
Processing Problems
• Optimistic locking
 Locks a page or record when the Update method is
encountered in VBA and releases the lock after record
is written
• Isolation level
 Indicates the degree to which the transaction should be
independent of other transactions
• Dirty read
 Prevented by specifying that transaction should not read
changes made by another transaction until the other
transaction is complete
10
Locking Records at the
Database Level
• You can lock the entire database through the Open
window
Figure 10-2
Opening a
database in
Exclusive made
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Locking Records at the
Database Level
• You can
lock
databases
through
startup
command
lines and
VBA code
Figure 10-3 Record locking defaults within Advanced tab of Options dialog box
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Locking Records at the
Database Level
Figure 10-4
Opening a
database in
Exclusive mode
through a
shortcut
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Locking Records at the
Database Level
• Mode property of the Connection object
 Used to specify whether other users can access
the database
 When set to the intrinsic constant
adModeShareExclusive, other users cannot
open a connection to the database
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Locking Records at the
Database Level
Table 10-1 Legal values for the Mode property
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Setting the Locking
Characteristics of Bound Forms
• RecordsLocks property of a form and Advanced
tab of the Options dialog box are used to set
locking characteristics of bound forms
Table 10-2 Setting the locking characteristics of bound forms
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Setting the Locking
Characteristics of Bound Forms
• Recordset-level locks
 Specified by setting the RecordLocks property to All
Records in the form property sheet or to 1 in VBA
• Edited Record option
 Locks a record (or page) as soon as a user begins to
change the value of a field
• No Locks option
 Does not lock the record until all of the changes have
been made
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Record-Level Locking
Techniques in VBA
• Locktype property
 Used to adjust locking strategy of recordsets
other than those associated with bound forms
 adLockPessimistic
 Employs pessimistic locking
 adLockOptimistic
 Employs optimistic locking
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The CursorType Property
• CursorType property of a recordset object
 Indicates how to respond when others make changes
• Values of CursorType relevant for Jet Engine
 adOpenStatic indicates that once recordset is
created, it should not reflect changes, new records,
or deletions made by other users
 adOpenKeyset indicates that the transaction cannot
view the results of another transaction until the other
transaction is committed
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The IsolationLevel Property
• IsolationLevel property
 Used to control how Access behaves in a
multiple-user environment
 Indicates how you should respond when others
make changes
 Property of a Connection object
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The IsolationLevel Property
• Specifies when recordsets can view changes
by users
 If it is set to adXactReadUncommitted:
 Recordset can view changes by others before they
are committed in a transaction
 A dirty read is possible
 adXactReadCommitted
 Indicates that the transaction cannot view the results
of another transaction until the other transaction is
committed
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The IsolationLevel Property
Table 10-3
IsolationLevel
values and
descriptions
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Problems with Locking
• Deadlock
 Situation where each transaction is waiting for the other
to release a locked resource and neither transaction can
finish
• Update Retry Interval
 The time, in milliseconds, that Access waits before
trying to obtain the record again
• Number of Update Retries
 Number of times Access will attempt to obtain a record
10
Handling Concurrency Errors
• If an error handler detects a locked database,
recordset, or record, the error-handling routine
typically waits and then repeats the statement
through the use of the Resume statement
• SQLState property of an Error object
 Returns a value that identifies the error according
to the ANSI SQL standard
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Handling Concurrency Errors
Table 10-4
Commonly
used values
of SQL State
10
Handling Concurrency Errors
• Number property of an Error object
 Returns a long integer that is determined by the
database provider and the connection
• Status property of a recordset object
 May be consulted to gather information about
the success or failure of an operation
10
Handling Concurrency Errors
Table 10-5 Values of the recordset Status property
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Handling Concurrency Errors
Table 10-5 Values of the recordset Status property (continued)
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Database Replication
• Database replication
 Creates copies of all or part of a database
 Each copy cab reside on a different computer
• Synchronization
 A process used to periodically distribute
updates from one copy to other copies of the
database
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An Overview of Replication
and Synchronization Steps
• Design Master
 Copy of the database that can propagate structural
changes
• Two issues must be resolved during the process of
creating the Design Master:
 The design of the database should be analyzed to
determine whether it adequately supports replication
 You must determine which objects will be replicated
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An Overview of Replication
and Synchronization Steps
• Design Master replica set
 Consists of databases that share the same
Design Master database
• Types of replicas:
 Complete replica
 Contains all of the data from replicable tables
 Partial replica
 Contains only the rows that meet a particular
criterion
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An Overview of Replication
and Synchronization Steps
• Types of views replicas can have:
 Global replicas
 Can be used to create other replicas and can be
synchronized with any replica
 Can create and then serve as a synchronization hub
for local replicas
 Local replicas
 Can create only other local replicas and must be
synchronized with the global replica that was used
to create it
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An Overview of Replication
and Synchronization Steps
• Replicas created from a particular global replica,
directly or transitively through a local replica,
define a replica set
Figure 10-6
Replica sets
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An Overview of Replication
and Synchronization Steps
• During synchronization, a conflict occurs
when:
 The same record and column in two different
replicas have been updated
 Two records share the same primary key and when
one replica deletes a record that has a referential
integrity relationship with a record in another
replica
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Replication Within VBA
• Microsoft Jet and Replication Object
Library (JRO)
 Contains object that support methods that
perform activities similar to the activities
carried out when you replicate and synchronize
through the menus
 Must be checked in the References window
before you can use the replication objects
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Replication Within VBA
Table 10-6
Replication
objects and
methods
10
Replication Within VBA
• You can create partial replicas by using the
CreateReplica, Append, and
PopulatePartial methods
 Partial replicas are not populated through the
CreateReplica method
 PopulatePartial method adds data to the replica
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Chapter Summary
• Features such as transactions, locking, and
replication help Access developers prepare an
application to function in complex environments
• Notion of a transaction
 Controls how concurrent access occurs
• Transaction
 Logical grouping of database operations that define a
unit of work
10
Chapter Summary
• One way that transactions control database
processing is by defining when changes
affected by the transaction may actually be
applied to the database
• CommitTrans and RollbackTrans
connection methods
 Handle problems with committing a
transaction’s effects to a database
10
Chapter Summary
• In Access, to control problems caused by
the actions of multiple users, transactions
use a locking scheme
 Pessimistic locking
 Locks a record as soon as it is edited
 Optimistic locking
 Locks a record when it is about to be updated
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Chapter Summary
• Replication
 Used when more than one site needs access to
an application, but network access is not
available or desired
 Replica
 Copy of an application
 Partial replica
 Filters a table, so that the records in the replica are a
subset of the original table’s records
10
Chapter Summary
• Synchronization of replicas
 Process where updates made in one replica are
propagated to other replicas
• Replication difficulties occur when two
replicas modify the same column and record
• A conflict resolution function is used to
determine whether the correct value was
placed in the database