The LA Fox Developer

A Newsletter for FoxPro Application Developers in Southern California

Visit our WebSite at: http://www.lafox.org

May 2000

Multi-User Processing

By:

Charlie Parker,

Rocky Mountain Fox User Group

If you have an application where many different users may be

making changes at the same time, then you need to be able to

provide the ability to merge changes from these users. Here is a

routine that does just that. The routine below can be put into a

VFP method or used as a procedure in a program library.

An ideal application gives the user the ability to make changes,

then cancel or save the changes. The easiest way to do this is

to use VFP’s table buffering. A table buffer is actually a copy of

the records that a user has changed, added or deleted. At some

point (e.g. when the user clicks a “Save” button or exits the

screen), you need to tell VFP to take what is in the buffer and

write it out to the actual table on disk. However, what if a

different user has also made a change to the same record since

we buffered it? Then when we write out our buffer, we may undo

or overwrite changes made by that other user.

Here is a simple example. We have a table with one record that

has 2 fields - a name and a phone number. The name is “John

Smith” and the phone number is “555-1212”. User A buffers the

record and changes the name (in the buffered record) to “Mary

Smith”. User B buffers the record (which still says “John Smith”)

and changes the phone number to “555-111 1”. User A saves his

change. Then user B saves his change. What is in the record?

Name = “John Smith” and phone number = “555-1111”. We just

undid the change that user A made.

The solution? When a user saves the changes he has made,

then we need to compare his buffered records on a field-by-field

basis with the data in the table on disk. If someone else has

made a change that is not reflected in our buffered data, then

we update our buffered data to reflect these changes BEFORE

we write our buffered records to the table.

Using our same example above, here is how it would work. User

A buffers the record and changes the name (in the buffered

record) to “Mary Smith”. User B buffers the record (which still

says “John Smith”) and changes the phone number to “555-

1111”. User A saves his change. Then user B saves his change.

But this time, before we write the buffer out to disk, we check to

(Con’t, page 8)

At LA Fox

May 15, 2000, 7:30 PM. Michael Cummings. Mike will be

speaking about Visual FoxPro’s advanced database features.

Building Smart Databases:

We will look at Rules, Triggers, Stored Procedures, and Default

Values. We can use these features to build some smarts into

VFP databases and make it easier to build applications. This is

a practical “Show Me the Code” session.

June 19, 2000, 7:30 PM. FoxFire Unleashed! Bill Wood,

Program Manager and primary developer of the latest, just-

released version of FoxFire! will be here to dazzle and delight.

July 17, 2000, 7:30 PM. Kristyne McDaniel. Kris will be here

talking about configurable toolbars.

Elsewhere

May 14-18, 2000, VFP DevCon 2000, New Orleans

Sheraton, New Orleans, LA. This new event is going to have

strong support from Microsoft and will be on the same scale!

level as Advisor’s DevCon in Miami. But it is offering several

innovative concepts. Content and direction has been deter-

mined by an advisory board made up primarily of speakers.

People attending this conference will be allowed to “cross-

pollinate” with the SQL conference, running concurrently at the

same hotel, free of charge. This means that while an attendee

(Con’t, page 2)

In this issue

ADO Jumpstart

MSDN Presents

Hot Tips

Page 3

Page 7

Page 7

Out and About

by Barty R. Lee

The May meeting of LA Fox will be held on

May 15, 2000, at 7:30 PM at our new meeting

place (NCR, 100 N. Sepulveda Blvd., El

Segundo, CA). For details on how to get

there, see the map on the back page.

and much more