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I want to easily piece complex applications
together from pre-existing parts. |
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I want the write the parts of the program
assigned to me in a language I’m comfortable with and still have my code be
compatible with the rest of the application. |
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I want to provide my pieces to the program in
packaged parts which hide the implementation details from the rest of the
developers. |
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I want the system to handle all the storing and
loading of my program pieces and I want to be able to use them anytime and
anywhere on the network. |
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I want to be able to purchase pre-built pieces
for my application if I find suitable alternatives to writing them
myself. If I go this route, I want
to easily be able to integrate some strangers code with mine no matter what
language he wrote his piece in. |
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If I come up with an ingenious piece of code, I
want to make it available to use in future projects no matter the
development environment. |
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Scratch that last line, I want to be able to use
it in other projects NOW, but I only want it stored once on my system so I
don’t have to update it twice whenever I change it. |
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Each client application bound directly to the
database |
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When the database structure changes (field
names, relationships, etc…) EVERY client application needs to be
re-installed to update changes |
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Every client machine maintains it’s own
connections to the database. This
requires a large amount of resources on the client machines themselves |
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Drawbacks: Not very scalable, not very
maintainable |
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Much less maintenance on the client
computers. All that is required is
a browser |
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Easier to maintain the entire application from a
network perspective. The entire
application runs on a small number of servers. |
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Changes made to these servers affect EVERY user. |
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Drawbacks:
These few servers must support a large number of users. If the web server is down, the entire
application is down. |
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Many types of clients can easily be supported by
the same infrastructure |
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All clients share the business logic stored in
objects on the object server and data stored in the database. |
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No client makes requests of the database
directly. All database interaction
takes place through the object server. |
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The object server (usually Microsoft Transaction
Server or COM+ services) provides a number of performance benefits in this
situation: |
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Database Connection Pooling |
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Multi-Threaded (Apartment) execution |
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Object Pooling |
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Distributed Transaction Support |
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COM (Component Object Model) |
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‘Binary’ object standard making it language
independent |
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Interface based |
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All objects are registered in the Windows
registry |
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Easy to develop in a large number of languages |
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Visual C++ |
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Visual Basic |
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Borland Delphi |
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Visual J++ |
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VBScript/Jscript functions can be exposed and used as COM Objects (although they
are not technically binary COM objects) |
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Location independent |
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Objects can execute on any machine on the
network transparently to the calling application |
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All objects stored as DLL’s or EXE’s in the
Windows file system. Loaded and
executed on demand based on information based in the registry. All this is handled transparently by the
operating system making it extremely easy to use any COM objects registered
on your system. |
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COM+ |
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Microsoft’s Marketing guru’s added a ‘+’ |
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Tightly integrated with the Windows 2000
operating system providing for increased performance |
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COM+ is identical to COM at a binary object
level. The same COM principles
still apply. |
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The ‘+’ denotes a number of additional features
and services added by the Windows 2000 operating system. |
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Queued Components (transparent integration with
MSMQ) |
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Component Load Balancing (AppCenter Server) |
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Robust Eventing System |
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Component Services (Next version of MTS built
into operating system) |
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Created as ActiveX EXE’s or ActiveX DLL’s |
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Each ‘Class’ within the VB project will be
exposed as a COM object. |
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Each ‘Class’ has an ‘Instancing’ property which
determines how it can be created by external programs. |
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‘Private’ – This class does not appear to other
programs and objects of this type cannot be created outside of the host VB
Class itself |
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‘PublicNotCreatable’ – This class can be viewed
and manipulated by an external program but the external program cannot
create a new instance of it. |
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‘Multiuse/GlobalMultiuse’ – This class can be
created and used by external programs |
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Heavy use of Public/Private/Friend keywords. |
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‘Public’ properties and methods are exposed to
all programs using this object |
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‘Friend’ properties and methods are exposed only
to other classes or modules within the same VB project |
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‘Private’ properties and methods can only be
used from within the same VB class as the property or method |
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Make heavy use of Encapsulation |
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It is possible to to simple, ‘Interface’ based
inheritance using the ‘Implements’ keyword |
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Can use ‘Event’s to inform client application of
changes in state or other events. |
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COM objects can be ‘wrappers’ around collections
of other objects |
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An individual property can return another object |
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Examples: |
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mobjPerson.Locations(2).City = “Ballwin” |
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mobjPerson.Relatives.Item(1).Locations.Item(3).State=“Nebraska” |
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For each mobjLocation in mobjPerson.Locations |
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Debug.Print mobjLocation.State |
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Next |
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What is Persistence? |
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The ability for an object to save, or ‘Persist’,
it’s state to some form of storage medium (i.e., File, Database, etc) |
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The ability for an object to restore a
previously saved state from the same medium. |
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Persisting objects to a database |
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Database tables with columns that map to object
properties |
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Proper conversion of VB property types to
database column data types |
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VB String Properties convert to SQL Server
nvarchar() types, etc… |
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Unique identifier for each object |
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Object Hierarchies can be easily representing in
a Relational Database |
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Since all data access is hidden from the user of
a COM object, it is possible to load an object and all it’s child objects
in one step. |
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For example: |
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mobjPerson.Load 23 ‘This command would load the data into the mobjPerson object
as well as all it’s child collections without having the COM client worry
about loading the children separately |
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Connect to the database directly from the client
side application |
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Public Sub Set_Name(ByVal ID as Long, ByVal Name
as String) |
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Dim mobjRS as ADODB.Recordset |
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Set mobjRS = New ADODB.Recordset ‘Create a new
ADO recordset |
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mobjRS.Open “Select * From PersonTable Where
ID=” & ID, cnConnection, _ |
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adOpenKeyset, adLockOptimistic |
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mobjRS.Fields(“Name”) = Name ‘Set our property |
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mobjRS.Update |
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mobjRS.Close |
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Set mobjRS = Nothing |
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End Sub |
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Use a simple COM object which wraps up our data: |
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Public Sub Set_Name(ByVal ID as Long, ByVal Name
as String) |
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Dim mobjPerson as Person |
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Set mobjPerson = New Person ‘Create a new
instance of Person class |
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With mobjPerson |
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.Load ID |
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.Name = Name |
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.Save |
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End With |
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Set mobjPerson = Nothing |
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End Sub |
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When you request a COM object in VB the system
looks through the current VB project references to find which COM libraries
are referenced and then looks through the registry to find the location of
the file containing that COM library.
Once the file is loaded the COM system dynamically allocates memory
and loads the correct object from the library into the memory of the
computer which is being used to run the object. It then returns to VB a pointer to that instance of that
object in memory. |
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All the above happens transparently to the user
EVERY time a COM object is requested by a client application. |
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All the programmer has to do is request the
object through COM. |
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VB exposes three ways to create a COM object: |
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New – The standard ‘New’ keyword (I.e. Set
mobjObject = New Person) |
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CreateObject(“Library.Object”,”Computer”) – This
method requires you to specify both the entire name of the object requested
as well as the computer on which it should run. Note that the object must already be installed on the
computer it is to run on. |
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CreateInstance – This is used to maintain
transactional integrity in MTS and I won’t go into it much here. But it is
a way to create COM objects in VB nonetheless. |
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Almost everything in VB 5 and 6 is based on COM. |
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VB is also a very simple tool to rapidly create
your own COM objects without much knowledge of the underlying COM
infrastructure. |
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When you compile an ActiveX Dll project in VB
containing any number of objects, VB handles all the registering of the
object and it’s interfaces behind the scenes. All you have to do is click on ‘File->Make MyDLL.dll…’ and
specify a location to save your newly created DLL. VB automatically creates a language
neutral COM object and handles all the registration process behind the
scenes. |
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As far as the COM system is concerned “ocx”
files and COM “dll” files are all binary COM objects. The “ocx” extension indicates that this
COM object exposes a graphical interface and exposes a certain set of
properties required by the ActiveX specification (I.e. Top, Left,
Forecolor, Backcolor, etc, etc…) |
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Since ActiveX components make up most of the
user interface of a large number of programs (I.e. Buttons, Toolbars, etc),
you can see how heavily the operating system depends on the underlying COM
infrastructure. |
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In the previous code examples, where did the
‘Person’ object run? |
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How could we have ran it on another computer? |
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Public Sub Set_Name(ByVal ID as Long, ByVal Name
as String) |
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Dim mobjPerson as Person |
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Set mobjPerson = CreateObject(“PersonDLL.Person”,”KevinsComputer”)
‘Create a new instance of Person class |
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With mobjPerson |
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.Load ID |
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.Name = Name |
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.Save |
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End With |
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Set mobjPerson = Nothing |
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End Sub |
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Notice the rest of the code functions no
differently no matter where the object runs. |
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Msdn.Microsoft.com – The complete source of
everything Microsoft and the complete online MSDN library |
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www.devx.com – The people who write VBPJ – an
excellent source for the latest news and information on the programming
world |
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VBPJ – Visual Basic Programmers Journal – Get it
if you can |
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www.vbchat.com - Great discussion boards and an
IRQ chat for VB developers |
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ANY WROX books which interests you |
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Learn to Program Objects With Visual Basic 6 by John Smiley (WROX Press) |
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Visual Basic 6.0 Business Objects by Rockford
Lhotka (WROX Press) |
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Enterprise Application Architecture with VB, ASP
and MTS by Joseph Moniz (WROX
Press) |
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Understanding and Programming COM+: A Practical
Guide to Windows 2000 DNA by Robert
Oberg (Prentice Hall) |
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Me |
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kvgros@sseinc.com (Work) |
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kvgross@swbell.net (Home) |
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Feel free to ask any questions you might have |
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Notes