Application Server consists of a single Windows service—Application Host (OCSAppServerMaster.exe)—and one or more instances of another Windows service—OCSAppServerHost.exe. Each instance of OCSAppServerHost.exe on a server hosts an Application Server application unique to that server. There will never be multiple instances of any particular Application Server application on a server.

In the Enterprise Edition consolidated pool topology, all Front End Servers run identical instances of each Application Server application selected at the time of Office Communications Server installation.

The Application Server applications themselves are written as .NET Framework 3.5 managed assemblies. All OCSAppServerHost.exe process instances and the OCSAppServerMaster.exe process run under the RTCComponentService account security context.

Each Application Server application leverages the following Office Communications Server 2007 R2 components:

  • WMI – Each application uses the lcwmi WMI provider to store/retrieve global and pool settings. For each Application Server application, all instances in a pool use a common set of settings stored in the backend RTCConfig database. Global settings are stored in Active Directory Domain Services.

  • Unified Communications Managed APIs (UCMA) 2.0 –Application Server applications interface these APIs to access the SIP and media stack on each front end server. Communications between the Application Server applications and other SIP endpoints all route through a SIP Proxy in the same pool as the application (but not necessarily on the same physical server).

  • Contact Objects/Trusted Services – Application Server applications are SIP Application Endpoints and as such require a SIP Address of Record (AOR) in the form of a SIP URI or Tel URI. This is accomplished by assigning an Active Directory Domain Services Contact object to each application and setting the application as a Trusted Service. These assignments enable the application to act as a SIP User Agent without having to authenticate or register, and enables them to subscribe to the presence of registered users and publish presence on behalf of registered users.

  • Management – The Office Communications Server 2007 R2 Management Console exposes service start-up and shut-down control of the Application Server services on a per-server basis in the same way it provides control of the other Office Communications Server 2007 R2 services, such as the Front End services or Conferencing Servers. If an Application Server application requires pool-level configuration, the Management Console can list the application under the new Applicationsoption under the pool’s Propertiesmenu in Office Communications Server 2007 R2. This will launch a new snap-in for managing that application’s pool level settings. (In R2, only the Response Group feature requires pool-level configuration.) The Management Console also can expose property sheets on the Forest container as needed to configure global properties associated with the Application Server application. In Office Communications Server 2007 R2, Communicator 2007 R2 Attendant is the only Application Server application that requires configuration at this level. Because all Application Server application instances in a pool are identical, configuration of Application Server applications on a per-server basis is unnecessary.

  • Installation/Activation – the Application Server environment and each Application Server application are packaged as Windows installer (.MSI) files called from the main Office Communications Server installation program (if one or more Application Server applications were selected for installation). After you install it, they are activated or deactivated in AD DS using lcscmd.exe /server:<appserver> /action:Activate|Deactivate /Role:AppServer.

  • Logging –Application Server applications can expose new components to the Office Communications Server 2007 R2 Logging Tool (OCSLogger.exe) for tracing communications with the application.

Other Key Application Server Characteristics

Distributed Workload

The Office Communications Server Application Host process does not listen on the network at all, but like the Office Communications Server Conferencing Servers, each Application Server application has a TCP port assigned to it. All servers in the pool listen on those assigned ports for each the specific Application Server application they host, and the hardware load balancer also listens on those ports and distributes inbound traffic to Application Server application instances on a server in the pool. All SIP communications with an Application Server application occurs over TLS between a SIP Proxy in its pool using the pool’s SSL certificate, even if the Application Server application instance happens to be on the same physical server as the SIP Proxy. As with other UC endpoints, media communications are direct between the client and the Application Server application instance that took the call. For example, once a call has been routed to a Conferencing Attendant instance, the audio media flows directly between it and the Mediation Server that handled the call.


Application Server applications are stateless in that they do not persist data between sessions, and each instance operates autonomously of the other instances of that application in the pool or across pools.

All Application Server instances of a particular application are equivalent across servers in pool and do not communicate with each other or another server process (unlike the Conferencing Servers, which communicate state information to the Conferencing Server Factory). Furthermore, Application Server applications do not register with the SIP Registrar and thereby do not support multiple points-of-presence. A call to an Application Server application is routed to only one Application Server instance and the other instance remain unaware of that call.

The Office Communications Server Application Host service (OCSAppServerMaster.exe) also does not monitor the state of the Application Server applications on its server, other than monitoring whether the instances are running and if it detects an unhandled exception in the application, it will restart it automatically.