C# 4 and .NET 4

Windows services architecture ❘ 669
The service program implements the functionality of
the service. It needs three parts:
SCM
➤
➤
➤
A main function
A service-main function
A handler
start service process
Service
Before discussing these parts, we need to quickly
introduce you to the Service Control Manager (SCM).
The SCM plays an important role for services —
sending requests to your service to start and to stop it.
register service-mains
service-main
service Control Manager
The SCM is the part of the operating system that
communicates with the service. Figure 25-2 illustrates
how this communication works with a sequence diagram.
At boot time, each process for which a service is set to
figure 25-2
start automatically is started, and so the main function of
this process is called. The service has the responsibility
of registering the service-main function for each of its services. The main function is the entry point of the service
program, and in this function the entry points for the service-main functions must be registered with the SCM.
Main function, service-Main, and Handlers
register handler
The main function of the service is the normal entry point of a program, the Main() method. The main
function of the service might register more than one service-main function. The service-main function
contains the actual functionality of the service. The service must register a service-main function
for each service it provides. A service program can provide a lot of services in a single program; for
example, \system32\services.exe is the service program that includes Alerter, Application
Management, Computer Browser, and DHCP Client, among other items.
The SCM now calls the service-main function for each service that should be started. One important task of
the service-main function is to register a handler with the SCM.
The handler function is the third part of a service program. The handler must respond to events from the
SCM. Services can be stopped, suspended, and resumed, and the handler must react to these events.
After a handler has been registered with the SCM, the service control program can post requests to the
SCM to stop, suspend, and resume the service. The service control program is independent of the SCM and
the service itself. The operating system contains many service control programs, for example, the MMC
Services snap-in that you saw earlier. You can also write your own service control program; a good example
of this is the SQL Server Configuration Manager shown in Figure 25-3.
figure 25-3
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