I have a massive DCOM service written in Visual C++ 6.0 + MFC that manages user logins & their sessions. When the service is running live, how does one upgrade it, without kicking the currently logged in users? Is it possible to replicate the service to implement resiliency & failsafe/faileover mechanisms?

Would it have been better to implement a REAL client/server approach for multiple servers for resiliency & failsafe/failover mechanisms?

This was what application center 2000 was written to do (cluster COM/DCOM applications, especially COM web apps), but it's no longer a mainstream product and was made no longer available for procurement as of October 2006. However, with System Center 2007 and Windows Server 2008, AppCenter's functionality will be replicated (and hopefully improved upon). You can't cluster COM applications, so there's not a really good way to do this without App Center 2000 SP2 (or wait for Server 2008 and System Center 2007). Note that you'll likely have to make sure you consider migrating to a .NET version of your app for it to work properly with System Center 2007 and Server 2008).

This was what application center 2000 was written to do (cluster COM/DCOM applications, especially COM web apps), but it's no longer a mainstream product and was made no longer available for procurement as of October 2006. However, with System Center 2007 and Windows Server 2008, AppCenter's functionality will be replicated (and hopefully improved upon). You can't cluster COM applications, so there's not a really good way to do this without App Center 2000 SP2 (or wait for Server 2008 and System Center 2007). Note that you'll likely have to make sure you consider migrating to a .NET version of your app for it to work properly with System Center 2007 and Server 2008).

So in other words, DCOM is dead.

What if one doesn't want to migrate to .NET? Is the alternative normal straight-forward client/server architecture?