Microsoft Transaction Server (MTS) wassoftware that provided services toComponent Object Model (COM)software components, to make it easier to create large distributed applications. The major services provided by MTS were automatedtransaction management, instance management (orjust-in-time activation) and role-based security. MTS is considered to be the first major software to implementaspect-oriented programming.[1]
MTS was first offered in theWindows NT 4.0 Option Pack. InWindows 2000, MTS was enhanced and better integrated with the operating system andCOM, and was renamedCOM+. COM+ addedobject pooling,loosely-coupled events and user-defined simple transactions (compensating resource managers) to the features of MTS.
COM+ is still provided withWindows Server 2003 andWindows Server 2008, and the Microsoft.NET Framework provides a wrapper for COM+ in the EnterpriseServices namespace. TheWindows Communication Foundation (WCF) provides a way of calling COM+ applications withweb services. However, COM+ is based on COM, and Microsoft's strategic software architecture is now web services and .NET, not COM. There are pure .NET-based alternatives for many of the features provided by COM+, and in the long term it is likely COM+ will be phased out.
A basic MTS architecture comprises:
COM components that run under the control of the MTS Executive are called MTS components. In COM+, they are referred to as COM+ Applications. MTS components are in-processDLLs. MTS components are deployed and run in the MTS Executive which manages them. As with other COM components, anobject implementing the IClassFactoryinterface serves as a Factory Object to create new instances of these components.
MTS inserts a Factory Wrapper Object and an Object Wrapper between the actual MTS object and its client. This interposing of wrappers is calledinterception. Whenever the client makes a call to the MTS component, the wrappers (Factory and Object) intercept the call and inject their own instance-management algorithm called the Just-In-Time Activation (JITA) into the call. The wrapper then makes this call on the actual MTS component. Interception was considered difficult at the time due to a lack of extensible metadata.[1]
In addition, based on the information from the component's deployment properties, transaction logic and security checks also take place in these wrapper objects.
For every MTS-hosted object, there also exists a Context Object, which implements the IObjectContext interface. The Context Object maintains specific information about that object, such as its transactional information, security information and deployment information.Methods in the MTS component call into the Context Object through its IObjectContext interface.
MTS does not create the actual middle-tier MTS object until the call from a client reaches the container. Since the object is not always running, it does not use up many system resources (even though an object wrapper and skeleton for the object do persist).
As soon as the call comes in from the client, the MTS wrapper process activates its Instance Management algorithm called JITA. The actual MTS object is created "just in time" to service the request from the wrapper. And when the request is serviced and the reply is sent back to the client, the component either calls SetComplete()/SetAbort(), or its transaction ends, or the client calls Release() on the reference to the object, and the actual MTS object is destroyed. In short, MTS uses a stateless component model.
Generally, when a client requests services from a typical MTS component, the following sequence occurs on the server :
It is thus possible to implement high-latency resources as asynchronous resource pools, which should take advantage of the statelessJIT activation afforded by themiddleware server.
