MAPI, SAPI, and TAPI Developer's Guide

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What Is WOSA?
The Windows Open Services Architecture was developed by Microsoft to "...provide a single, open-ended interface to
enterprise computing environments." The concept of WOSA is to design a way to access extended services from the
Windows operating system that require having only a minimum amount of information about the services. For example,
the MAPI (Message API) model is designed to allow programmers to develop applications that use the message services
without having to understand the complexities of the hardware and software routines that implement messaging on
various Windows platforms. The same is true for the Speech (SAPI) and Telephony (TAPI) services we will discuss in
this book.
The WOSA model goes beyond the idea of exposing services in a uniform way across Windows operating systems.
WOSA is also designed to work in a mixed operating system environment. For example, the Microsoft Rpc (Remote
Procedure Call) interface is a WOSA service that is designed to work with the Open Software Foundation's DCE
(Distributed Computing Environment) Rpc model. The design of Microsoft Rpc allows programmers to design software
that will safely interface with any product that uses the DCE model, regardless of the operating system with which the
software must interact.
In order to attain this flexibility, the WOSA model defines two distinct interfaces-the Client API and the Server SPI.
These interfaces are linked by a single interface module that can talk to both API and SPI applications. As a result, all
client applications need to do is conform to the API rules and then to the universal interface. All server applications need
to do is conform to the SPI rules and then to the universal interface. No matter what changes are made to the client or
server applications, both software modules (client and server) will be compatible as long as they both continue to
conform to the API/SPI model and use the universal interface.
The next two sections of this chapter outline the WOSA model in detail and give several examples of WOSA services
currently available.

The WOSA Model
The WOSA model consists of three distinct pieces. Each of these pieces plays an important, and independent, role in
providing programming services to your applications. The three WOSA components are
l The Client API-the application programming interface used by the program requesting the service.
l The Server SPI-the service provider interface used by the program that provides the extended service (for
example, e-mail, telephony, speech services, and so on).
l The API/SPI Interface-the single module that links the API and SPI calls. This is usually implemented as a
separate DLL in the Windows environment.

Each of the components has an important job to do. Even though they perform their tasks independently, the components
work together to complete the service interface. This is the key to the success of the WOSA model-distinct, independent
roles that together provide the whole interface.

The Client API Makes Requests
The Client API is the interface for the application requesting the service. API sets are usually implemented at the
Windows desktop. The Message API (MAPI) is a good example of a WOSA client API. Each client API defines a stable
set of routines for accessing services from the back-end service provider. For example, the operations of logging into an
e-mail server, creating an e-mail message, addressing it, and sending it to another e-mail client are all defined in the
MAPI set. These services are requested by the client. The actual services are provided by the server-side application.