Microsoft Access as an ODBC Client Application

A Presentation for the

Microsoft Access SIG

of the North Texas PC User Group

and the Metroplex Access Developers

by Larry Linson.

© Copyright 2000 by L.M. Linson, all rights reserved

What is Access?

“A nice little desktop database” First popular Windows database For the novice For the power user For the developer A File Server database with Jet

Access can build

A standalone database A multi-user database A client linked to ODBC databases A direct client to SQL Server A web interface to database

Specifically

Microsoft Access 2, 97, 2000 .MDB or .MDE file format Jet database engine and Data Access

Objects (DAO) Tables LINKed via ODBC Driver to Server database (e.g. SQL Server)

Specifically NOT

Access Data Project (ADP) Data Access Pages (DAP) ActiveX Data Objects (ADO) Other Web Interface (e.g., Front

Page) ODBCDirect

How Many Users?

Multi-User– With everything wrong – 4, 5, or fewer

– With everything right – 90 to 150 ODBC Client

– Easily hundreds, limited only by

– Concurrent connections to server

– Essentially unlimited

Multi-User Layout

Tables on Server– Available To All Users

On Users’ Workstations– Access (or Runtime)

– Application Part

– Fetch .DLLs, Objects Locally

Client-Server Layout

Server Database Engine on Server– Extraction, manipulation done here

On Users’ Workstations– Access (or Runtime) and Jet database engine– Application

» Queries, Forms, Reports, Macros, Modules» Linked TableDefs and Local Tables

– ODBC Drivers, ODBC Data Source Information

Jet is a File-Server

Retrieve, Extract, and Manipulate on User’s Workstation

Every I/O Done Across Network– That would normally be to local hard drive– Not whole database, nor necessarily whole table

– Just what Access finds necessary Index may be enough to find exact records

Server Databases

Retrieval, extraction, and manipulation on the Server

Much less Network I/O– (Just) Requests from user to server

– (Just) Results from server to user

Multi-user Layout . . .

DATAON

SERVER

PROCESSON USER

WORKSTATION

PROCESSON USER

WORKSTATION

. . . PROCESSON USER

WORKSTATION

Client-Server Layout . . .

DATASTORED

INSERVER DB

CREATE REQUESTSEND / RECEIVE

DATA

PROCESSREQEST

IN SERVER DB

Request &Data

U I

Comparison . . .

Server(Not Necessarily Windows)Windows

U I

U IU I

Data Only

Server Database with DataAccess, Jet, ODBC

Access, Jet

What’s Different? . . .

Data in Server DB– Access User Interface to Server Databases– Performance – potentially better; possibly worse– Corruption – not really a factor

Multiple users of same data– Collisions (Add / Update) – Locking handled on server– Seeing Other Users Update – Refreshing– Updating Related Tables – Transactions handled jointly– Who Am I? – Identify Users to Access and to Server

What’s Different? . . .

Retrieval, extraction, and manipulation on the Server

Fewer Problems – Network failures

– Power failures

– Individual User and Machine failures

Updating

What’s Different? . . .

Access security

Server security – Tables on Server

» Accessing, manipulating, updating data

Security

Queries MacrosForms ModulesReports Local Tables

What’s Different? . . .

Rules and triggers at the Server Don’t generate Access error

messages Your application has to have

– pre-validation and

– user notification

Data Validation

What’s Different? . . .

As defined by Server database Server types may

– match directly to Access types

– not have Access counterpart » Text date fields

– not have all Access types» Counter, OLE, Memo

Data Types

What’s Different? . . .

Defined by Server database,– Implemented on Server

Often enforced only by – Triggers

» Written manually

» Generated by Data Modeler

– Some (e.g., Microsoft SQL Server)» Automated, specified similar to Access

Referential Integrity

What’s Different? . . .

Most servers do not permit– updating tables joined in SQL

ODBC workaround (sometimes) – Generate / send multiple updates

Pass-through Queries– Must adhere to server specifications

Updateable Queries/Views

Performance Factors

Hardware Environment» More Memory» Faster Processors» Faster Hard Drives

Software Environment» Not Too Much Else Running

Network Environment» Faster is Better

Performance

Server is moreimportant thanuser’s machine!

Not as important

as in Multi-User

}

More Factors . . .

Reduce Network Traffic – Application Design

Local tables (refresh from server)

Use Queries not Tables– Limit both Tables and Columns

– Database Implementation Queries, not DAO Code (When Feasible) Queries, not RecordsetClone.FindFirst

Performance

More Factors . . .

Built-in performance factors – Re-planning query execution

Also called “compiling” “preparing” State of data is important

Stored procedures Like saved Queries in Access But with logic like code, too

Performance

More Factors . . .

Tuning at the Server – Spread data over different disks– Monitor Performance

» Analyze» Add, remove, modify indexes

– Database Administration» More TLC» That’s why there are DBA’s

Performance

But . . .

Jet can be smart – Return only part of large recordset– Create separate query for FindFirst

It may “save us from ourselves”

Performance

Views Restrict user access

– Only the specified columns– Only the specified rows

Like Access’ “Saved Queries” Appear as Tables to Jet / Access Also assist performance

– Force joins and selection at server

Data Integrity

Record Locking . . . Done “co-operatively” in C/S

– Jet passes the request– Locking actually done by the server– Considerations same as Multi-User

» Except: you may lock out more users» May be a smaller time window for some

So, let’s just “fly through” – the repeated slides

Data Integrity

Record Locking . . . In Bound Forms

– No Locks (aka Optimistic Locking)» Only in the instant the record is saved

– Edited Record (aka Pessimistic Locking) » As soon as user begins to edit

– All Records» All records in the entire recordset» Batch Updates» Administrative Maintenance

Data Integrity

Record Locking . . . Possibilities by Object

– All Three Options» Table datasheets; Select, Crosstab, Union Query, Forms, OpenRecordset

– Lock Edited Record or All Records » Update, Delete, Make-Table, Append

– Lock All Records » Data Definition Queries

– No Locks or Lock All Records» Reports

Data Integrity

Record Locking . . .

Default Record Locking Option– Established by menu Tools | Options

Applies to All, Except– Data Definition Queries – All Records– Open Recordset – Edited Record

Data Integrity

Record Locking . . . OpenRecordset

– dbDenyRead or dbDenyWrite– Overrides .LockEdits property– Native Access Tables Only

.LockEdits Property– True = Edited Record (the default)– False = No Locks

Data Integrity

Record Locking . . . Advantages of Pessimistic Locking

– Simple to Develop, Prevents Overwriting

Disadvantages of Pessimistic– Lock Multiple Records, Less Concurrency

Advantages of Optimistic Locking– Simple to Use, Better Concurrency, Less Lockout

Disadvantages of Optimistic– Can be Confusing to User, Allows Overwriting

Data Integrity

Record Locking

Generally, use Optimistic– Minimize User Lockout

Or, Mixed, by Specific Object– Most Optimistic, but– Critical Information, Pessimistic

» Example: Quantity on Hand in Inventory

Data Integrity

Transactions . . .

Changes in a batch All Succeed or All Fail Same or Related Tables

– Allocate Stock to Order, Deduct Stock on Hand – Credit New Account and Debit Old Account

Data Integrity

Transactions . . .

Server can resolve problems– From failure occurring any time– Full audit trail– Hot backup – second copy of data

No– Orphan locks in .LDB– Data loss, lockout, corruption due to

» Errors that occur while committing

Data Integrity

Transactions

Consider client-server if– Processing funds transactions– Performing critical inventory updates– Etc.– Regardless of other considerations

» Performance

» Number of users

Data Integrity

Identifying Users Typically secure the database

– Everyone Logs In

Viewing and Reporting– MSLDBUSR.DLL by Microsoft– Unsupported

Server Security– Log in to connect and link

Miscellaneous

Some Cautions . . . Jet may lose track of record

– When using server equivalent of AutoNumber– Data entry works fine– When record saved, all show “#Deleted”

Countermeasure– Table of tables and next id number– Stored procedure to return next id– Never use “data entry”, always editing

Miscellaneous

Cautions . . . “Complex” queries

– More than two joins – Jet sometimes decides– Brings back a flood of data

Countermeasure– More than two joins, performance slow– Consider creating a view– View appears as Table to Jet– Forces joins and extraction on server

Miscellaneous

Cautions . . . “Complex” queries (alternative)

– More than two joins – Jet sometimes decides– Brings back a flood of data

Countermeasure– Create a passthrough Query– Tells Jet, “Hands off!”– Forces joins and extraction on server

Miscellaneous

Cautions . . . “Order by” not a key, not indexed

– “Order By field must be in Select Clause”– Message is erroneous

Countermeasure– May have to make it a key or index– May be able to use a passthrough Query

Miscellaneous

Cautions . . . Some joins won’t work in Access

Query

Miscellaneous

tabid

tabname

colid

tabid

colname

TableName

ColumnName

.

.

.

•continuedServer Tables

Access Table

Cautions . . . Countermeasure – “intermediary”

Miscellaneous

tabid

tabname

colid

tabid

colname

tabname

colname

Server Tables Access Tables

tabname

colname

TableName

ColumnName

Query

Resources . . .

Access 97 Developer’s Handbook by Getz, Litwin, Gilbert published by SYBEX ISBN: 0 - 7821 - 1941 - 7 Chap 15 – Developing Client-Server

Applications

Resources . . .

Access 2000 Developer’s Handbook, Volume 2: Enterprise Edition

by Getz, Litwin, Gilbert published by SYBEX ISBN: 0 - 7821 - 2372 - 4 Chap 3 – Developing Client-Server

Applications (also other chapters this volume)

Resources . . .

Building Applications with Microsoft Access 97

Manual, comes with Office Developer or the MSDN Library

publisher: Microsoft Chapter 10 – Creating Multi-User

Databases

Resources

Microsoft Office Visual Basic Programmer’s Guide

Manual, comes with Office 2000 Developer or the MSDN Library

publisher: Microsoft Chapter 16 –Multi-User Database

Solutions