The LA Fox Developer

A Newsletter for FoxPro Application Developers in Southern California

Visit our WebSite at: http://www.mesllc.com/lafox.html

November 1997

An Introduction to Microsoft

Transaction Server

by Gaiy DeWitt

At DevCon ‘97 Microsoft made quite a big deal about Microsoft

Transaction Server (MTS) and the support for it in Tahoe, the

next version of Visual FoxPro. Lots of VFP developers were

nonpiussed, wondering what the big deal was. To see why it can

be a big deal, consider an example using bank accounts.

Assume a three-tier, object-oriented approach.

One object is the Account. In the real world you can deposit

money into an account at any time. You may withdraw money

at any time as long as you are not attempting to withdraw more

than the balance. So I create an object to handle an account. It

has two public methods: Deposit() and Withdraw(). Those

methods are wrapped in a transaction because we need 100%

reliability in case of disconnects, crashes, etc. In addition, the

Withdraw() method can fail if the amount being withdrawn is

greater than the baance. This is an easy thing to wrap in

transaction as the database can have a rule that the balance

cannot be less than 0. Note that this is a database constraint,

not a business rule. So we start a transaction and commit the

record. The transaction fails because of the rule violation so we

rollback the transaction and report to the front end. Not too hard

so far.

Now let’s complicate the scenario. What about transferring

money from one account to another? This requires depositing in

one account and withdrawing from another. Both transactions

must succeed or both must fail. The money must go directly

from one account to another with no in-between state. There

cannot be a state where money has been withdrawn from one

account but not deposited in the other. Can I use my existing

Account objects, one depositing and one withdrawing? Maybe.

But chances are I have to rewrite them to handle multiple

transactions. Because each individual account’s transaction

must be wrapped in the transfer transaction. So if A.Withdraw()

succeeds, but B.Deposit() fails I have to roll back A. To do this

in VFP requires quite a bit of logic to determine the nesting level

of transactions.

(Con't, page 7)

In this issue

SQL Views in VFP

FoxPro Q&A

VFP Conversion Tips

At LA Fox

November 17, 1997, 7:30 PM - Rick Strahl. Rick is President

of West Wind Technologies, a company providing Internet-based

application development using Visual FoxPro. Rick has been

extensively involved in Web-based application development, and

is author of Web Connection for Visual FoxPro, a tool that

allows Visual FoxPro to be used as the back-end to build Web

applications. Rick is frequently published in FoxPro-related

magazines and bociks.

December 15, 1997, 7:30 PM - Annual LA Fox Christmas

Party. It’s coming up on that time of the year again. We’ll be

having a White Elephant Gift Exchange again this year. As

always, there’ll be plenty of tasty treats along with good cheer

and fellowship. And a special guest. For complete details,

except for the special guest, see the article on page 9.

January 19, 1998, 7:30 PM - Les Pinter. While working on a

Ph.D. in economics at Rice University in Houston, Texas, Les

joined with two high-school friends to develop the Magic Wand,

one of the first word processing programs for microcomputers.

In 1990 he sold the source code to Microsoft, which used it in

the development of Microsoft Word. During the development of a

dBase Ill application for LucasFilm, Les discovered that

FoxBase ran his applications 7 times faster, and was

immedately hooked on Fox. He began The Pinter FoxPro Letter,

(now in its eighth year) a newsletter published both in the US

and in Moscow (a Russian language edition).Les has written 6

books on programming in FoxPro for McGraw-Hill. He is a

Contributing Editor for Point DBF, the French data base maga-

zine, a technical editor for IDG books, and writes “The Cutting

(Con’t, page 2)

Page 3

Page 4

Page 8

...and much more

Out and About

by Barry R. Lee

The November meeting of LA Fox will be

held on Nov.17, 1997, at 7:30 PM at our

regular meeting place (the Torrance Airport,

3301 Airport Drive, in Torrance). For details

on how to get there, see the map on the back

page.