The LA Fox Developer

A Newsletter for FoxPro Application Developers in Southern California

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 In addition, the Withdraw() method can fail if the amount being withdrawn is greater than the baance.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 There cannot be a state where money has been withdrawn from one account but not deposited in the other.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)

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

(Con’t, page 2)

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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.