InformationWeek

When objects are called with COM+, security is automatically implemented if security roles have been assigned. No additional code is required for a developer to implement security within an application. Developers can define roles within their applications using definitions created with declarative statements in their application code, or leave them to be defined at installation.

Previous versions of Windows NT used a simple challenge-and-response system based on Microsoft LAN Manager. Windows 2000 and COM+ add support for Kerberos and Secure Sockets Layer security, as well as a negotiated setting where the best security that's supported by both client and server is chosen. This is primarily for support of "legacy" NT 4 clients and applications.

COM+ also allows for control over the level of access COM components have when interacting with the rest of the operating system. A COM+ application can be configured to take on the security attributes of the user who launched it or that of the person logged on to the console of the computer it's running on: the "interactive user." This security model is called "impersonation"-the application retains the credentials of the user it's impersonating until the program ends or those credentials expire.

Alternatively, a COM+ component can be configured to negotiate security connections with a specific user account and password, as many Windows NT applications that run as operating-system services currently do. This approach is preferable for server applications and other long-running processes, because the credentials of an "impersonator" application will expire within eight hours with the default configuration of Windows 2000's implementation of Kerberos, causing the application to crash.

Balance The Load
While Windows NT 4.0 Service Pack 4 and MTS provided basic load-balancing capabilities for applications running within the MTS environment, COM+ provides built-in dynamic application load balancing across a cluster of up to eight servers. Incoming requests for server application objects are redirected to a Component Load Balancing Server, which determines which system in the cluster is under the least load. The call is routed to that server, which in theory can be anywhere on the network.

In this respect, COM+ operates as an object request broker, providing scalability similar to Corba components. COM+ makes it possible to use clusters of low-cost hardware and existing software to build scalable Internet and multitier client-server applications.

The Queued Component service of COM+ is an expansion of the capabilities of MSMQ. It provides for guaranteed delivery of information from one application component to another, even when there's no readily available network connection between the two. Queued components let applications such as sales-force automation programs, database clients, and messaging software operate in a disconnected or infrequently connected state and send data to remote components when a connection is available, asynchronously.

When a client calls a queued component, an object called the Recorder packages the requested data and delivers it as a message to MSMQ to be sent to the application server. There, the message is retrieved and passed to the Queued Component player on the server, which plays back the client application's data. In a transaction situation, MSMQ guarantees that a transaction can be completed before it passes data to the Queued Component player.

COM+ also provides Windows 2000 with native publish-and-subscribe middleware. An application can generate events without needing to know the applications that use them and without having to direct event messages specifically to those applications. These messages can also be queued to guarantee delivery of messages to remote applications. The receiving applications operate as listeners for subscribed events; COM+ can use filters to limit the traffic to specific events, so that a stock ticker, for example, could alert subscribers to only the advances and declines they're looking for. Similarly, publish-and-subscribe events can be used to transmit information from system- management applications over low-bandwidth connections.