[AccessD] Using Global Connections

Mark,

I also use globals for the db connection.  This is a case where the
connection is specific, a penalty is paid for creating / deleting it, and it
is needed all over the app.  The connection is not something that is going
to be updated (written to) by pieces of the application, it is going to be
created once and then used all over.  That is a good candidate for a global.


Globals tend to cause problems when they are *updated* from all over the
place, and the reason is that is quite difficult to track down who is doing
the updating when it is incorrectly updated.  If for example a serial port
is opened and closed by 10 different functions, one function tries to use
the serial port and it is closed.  Who closed it?  It's supposed to be
opened, but some process closed it unexpectedly.  These kinds of problems
can be a real bear to track down and fix.  

It becomes a bigger problem if the system is written and maintained by a
team of programmers where nobody knows the whole story.  Globals by
definition are available to anyone at any time.  

Scope exists for a reason.  If a variable is only used by a specific
function, then it's variables have no business being global to the program
(or even the module).  If a variable is only used by code inside an instance
of a class but needs to be accessed by several functions inside the class,
then it should be defined inside the class (in the header) as a PRIVATE
variable.  Thus only code inside the class can modify the variable.  If a
variable needs to be shared by many functions inside of a module, but only
these functions modify the variable, then the variable should be PRIVATE to
that module.  By making any of these variables truly Global to the project,
you end up with things being (able to be) set by code that has no business
setting the variable.

On the other hand, if as in your example, a database connection is needed,
and a function calls a function which calls a function, which needs that
connection, why pass the connection through 3 functions that don't even need
the connection simply so that the fourth function that does need it can have
access to the connection?

In a case like this, I tend to make the connection a private variable inside
a module, with a function that initializes the variable when the db opens,
and then define a function that READS the connection variable and returns
the value.  This in effect makes it read only.  Anyone can call the function
that gets the value, but since the variable is PRIVATE to the module, no one
outside the module can write to it.

John W. Colby
Colby Consulting
www.ColbyConsulting.com

-----Original Message-----
From: accessd-admin at databaseadvisors.com
[mailto:accessd-admin at databaseadvisors.com]On Behalf Of Mark L. Breen
Sent: Friday, March 14, 2003 12:37 PM
To: accessd at databaseadvisors.com
Subject: Re: [AccessD] Using Global Connctions


Hello All,

I was not aware that a previous debate (sounds a little heated) had been
already carried out.  Sorry for duplicating the discussion.

In the last few emails, Marcus was the only one that explicitly stated that
he likes the idea of using Globals for a db connection.

So leaving aside passing around an order nunber or a customer id as a global
(which I would never even consider), what do you guys do to hand around a
database connection?  Do you use Globals or do you make the connction each
time or do you use some kind of class.  Any sample code would be interesting
to read.

What I do is have the following code in a module

' This will be the global connection that we will share, we open it once
only and share it throughout the applications life
Global gccnn As New ADODB.Connection
Global Const gcstrConStr As String = "Provider=SQLOLEDB.1;Integrated
Security=SSPI;Persist Security Info=False;Initial Catalog=spps40;Data
Source=PC1"


and then in the first form_load I have

    ' Give it a connect string
    gccnn.ConnectionString = gcstrConStr

    ' And make the connection
    gccnn.Open



What do you guys think of this strategy, should I use a class to pass it
around.  And if I am not using a class, do you think that it is a little bit
untidy or is downright bad practice.

Again, I am just curious what the rest of the world is doing in their apps.

Thanks

Mark


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