ComponentObjectModel, DCOM and Software Components

1
ComponentObjectModel, DCOMandSoftware Components

• Nat Brown
• COM Program Management
• Microsoft Corporation
• With additions by R. Cook

2
How to Compose Applications?

• From components written in any language
• From components executing on multiple machines
• Allow an application to delegate UI actions and
  screen space to components (e.g. embed
  spreadsheet in a document)
• Allow applications to be used as components to
  build other applications (automation)

3
What is Automation?

• Automation is the ability to control an
  application from another.
• Sometimes referred to as scripting.
• For example, script can start Excel, load a
  spreadsheet, add data, save, quit Excel.
• Visual Basic for Applications (VBA) is a
  scripting language which makes use of automation.
• Perl (and PerlScript) makes use of automation.

4
What is a COM Component?

• Supports the IUnknown abstract interface
• May support other interfaces
• No inheritance or other language dependencies
• Every interface has a UNIQUE 128-bit (uuidgen
  in Windows and OS/X)
• Every copy of an interface is reference-counted,
  although this does not preclude a high-level
  implementation of garbage collection

5
What is a COM Component?

• Every class has a UNIQUE 128-bit CLSID
• A class (implementation) can support multiple
  interfaces
• A class can have a text name, referred to as a
  PROGID

6
What is a COM Component?
C namespace EsriSamples Guid("97FDB5D7-41D
8-4260-BF72-3EB2CDD36747") ProgID("ZoomInCmd.Z
oomIn") public class ZoomIn ICommand ...
7

• class CMyClass  public Iunknown //C
• private    ULONG m_cRef  // COM reference count
• public
• STDMETHODIMP QueryInterface(REFIID riid, void pp
  vObject) 
• IUnknown punk  nullptr
•         if (riid  IID_IUnknown)
•             punk  static_castltIunknown gt(this)
• // TODO List other implemented interfaces here.
•         ppvObject  punk
•         if (punk nullptr) return E_NOINTERFACE
  
•         punk-gtAddRef()
•         return S_OK    
• STDMETHODIMP_(ULONG) AddRef()
  return m_cRef 
• STDMETHODIMP_(ULONG) Release()
•         ULONG cRef  --m_cRef
•         if (cRef  0) delete this
•         return cRef

8
Interface Definition Language - IDL

• object,
• uuid(75D873CD-7B63-11D3-9D43-00C0F031CDDE),
• helpstring("IServer Interface"),
• pointer_default(unique)
• interface IServer IUnknown
• HRESULT HelloWorld(in, string char
  pszMessage)

9
The COM Programming ModelA scalable programming
model
10
DCOM ArchitectureFlexible and extensible

• Pluggable Transports

Client Machine
Server Machine
D C O M
D C O M
COM Object
Clients
11
DCOM ArchitectureFlexible and extensible

• Pluggable Security

Server Machine
Client Machine
D C O M
D C O M
COM Object
Clients
12
DCOM ArchitectureEfficient and scalable

• Multiplexing - Single Port per-protocol, per
  server process, regardless of of objects
• Scalable - Connection-Less Protocols like UDP
  Preferred
• Established Connection-Oriented (TCP) Sessions
  Reused by same client

Server
Client
Client
13
DCOM ArchitectureEfficient and scalable

• Low Bandwidth
• Keep-Alive Messages bundled for all connections
  between Machines

Server Machine
Client Machine
Keep-Alive Traffic for all connections
Client 1
Server
Logical Connections or Sessions
Client 2
14
Ease-of-Use First Steps
Clients
Network
Receiver
Queue
Connections
Context
Security
Management
Configuration
Thread Pool
Service Logic
Synchronization
Shared Data
Server
MTS easier servers
Easier components?
15
Additional Technologies

• COM
• MTS - Microsoft Transaction Server
• MSMQ - Microsoft Message Queue
• ADS - Active Directory Service
• As distributed registry
• As namespace abstraction
• All Microsoft products are COM based
• IIS - Internet Information Server
• Exchange
• Internet Explorer
• Word, Excel, etc.

16
First Steps

• Basically there is a general model of use
• 1) A typical controller process will request that
  a COM server generate a COM object.
• 2) The server is loaded or located, the request
  is submitted, a response is returned.
• 3) If request results in a valid COM object then
  controller process interacts with the object.
• 4) Destroy COM object.

17
First Steps

• Lets say we need to change the title and subject
  of a Microsoft Word document

1) Need to somehow run Word 2) Need to load up
the document 3) Need to change the title and
subject 4) Need to save the document 5) Need to
quit Word
18
First Steps

• How would we implement such a system?
• Request a Word application COM object
• Query the appropriate interface
• Call a function in the Word application COM
  object which loads a document. It returns a Word
  document COM object
• Modify the Title and Subject properties from the
  Word document COM object
• Call into the Word document COM object to save to
  disk
• Destroy both the document and application COM
  objects