What is a 'multiple instance'
One of the interesting points that was raised by one correspondent, Daniel
Lohmann is that the term "multiple instance" was not well-defined in
an earlier version of this essay. He points out that "multiple instance"
can mean different things in the multiuser environment of NT. He points
out that it is possible to use API calls such as LogonUser, CreateProcessAsUser,
CreateDesktop and SwitchDesktop, and therefore it is possible to
have multiple users and multiple desktop sessions running under different accounts.
Therefore, there are several issues about why one would not want multiple instances
of an application. He categorized them as
- Avoiding multiple instances started in the same user session, no matter how
many desktops that user session has, but allowing multiple instances to run
concurrently for different user sessions.
- Avoiding multiple instances started in the same desktop, but allowing multiple
instances to run as long as each one is in a separate desktop.
- Avoiding multiple instances started for the same user account, no matter
how many desktops or sessions running under this account exist, but allowing
multiple instances to run concurrently for sessions running under a different
user account.
- Avoiding multiple instances started on the same machine. This means that
no matter how many desktops are used by an arbitrary number of users, there
can be at most one instance of the program running.
He observes that many programmers actually mean (a), and sometimes mean (b)
or (c). The technique outlined in the earlier version of this essay always interpreted
the question as (d). Note that in the case of Win9x/Millenium, only (d)
exists, since there is only one user session and one desktop per machine.
Thus, as you read the essay below, be aware that we are answering only (d)
using this technique. Afterwards, I will present
his proposed solutions for being able to extend this technique to include (a)-(c).
He observes that certain "service-oriented" applications that fit into
the class of (d) are often better done as System Services. The service may be
started on-demand or at system startup, but all users and all desktops share
the same common service.