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INFO: COM Servers Activation and NT Windows Stations

This article was previously published under Q169321
SUMMARY
When a client requests a class object for a registered class, COM eitherreturns an existing class object or launches a process that is registeredas containing the requested class object. The process of obtaining a classobject reference for a requesting client (whether or not that results inprocess creation or "launching") is called "activation."

Under certain conditions, COM may launch a new server process even when anexisting class object is running and has been registered as multiple use.Moreover, when COM creates a new process that process may be launched in anew security environment known as a "window station" rather than sharing anexisting window station such as the interactive window station. (For moreinformation on window stations, search the Win32 SDK documentation for thatphrase.)

Understanding COM's algorithms for creating new processes and windowstations during an activation request is important for several reasons.First, COM may create more than one process instance of a multiple-useclass object because of security issues. Second, "single-use" servers willalways be launched in separate processes, but they may or may not belaunched in separate window stations. This difference might manifest itselfto application code in certain unusual cases, such as when two COM serverstry to communicate via window messages or secure communication facilitiessuch as COM or RPC. Third, since the number of window stations that cansimultaneously be created in Windows NT is limited, it is important to knowwhen your COM server gets a new window station.

This article examines different activation scenarios and explains when newprocesses and window stations are created.
MORE INFORMATION
When COM creates a new server process or assigns a new window station to anew server process depends on several factors:
  1. The security identity under which the COM class is set to start. The identity of a class can be set via the DCOMCNFG tool and is specified by the RunAs named value under the AppID key for the class.
  2. Single-use vs multiple-use registration by class objects.
  3. The Security Identifier(SID) of the client process (this is the numeric value that represents a particular user account/security identity/principal in a Windows NT environment).
  4. The Logon Identifier (LUID) of the client process.(A new logon identifier is generated for each unique logon to a computer running Windows NT. This logon could be either through a user typing in a user name and password at the NT logon prompt, or through a call to the win32 API LogonUser.)
  5. Local vs remote activation.
  6. The window station of the client.

Multiple-use Classes

A multiple-use class is a class that is registered with COM (viaCoRegisterClassObject() API) specifying the REGCLS_MULTIPLEUSE flag. Forsuch a class, COM normally uses the same instance of the server process forall client activation requests. However, for classes configured to rununder the security of the launching users and in a few other cases, COMlaunches a new instance of the server process to service activationrequests. When a new instance of the server process is thus launched, it ispossible that the server process gets a new window station as well. We willexamine the various scenarios below, but first we will discuss briefly whyCOM launches new processes containing requested class objects when aninstance of the class object is already registered as a "multiple-use"class.

First, when a COM class (more accurately, when an AppID associated with aCOM class) is registered with the system to be run as "the launching user,"the system administrator has set a certain security policy with respect tothat class. The policy is that an activator should receive a class objectinside a process that is running in the same security context as theactivating code.

This security policy can come into conflict with the server-definedbehavior of having only one class factory object for all activationrequests (as indicated by REGCLS_MULTIPLEUSE). COM prioritizes the securitypolicy over application behavior. As a result, multiple-use serversregistered to run as "the launching user" will not behave according to thenormal rules of multiple-use. A new process will be launched for eachactivating security principal.

Second, if a process not launched by COM running in a security contextdifferent from the one specified for the given CLSID registers that CLSID,that registration will fail(CoRegisterClassObject will return an error codeCO_E_WRONG_SERVER_IDENTITY in this case). And, if an activation requestlater arrives a new process will be launched by COM with the securitycontext specified for the CLSID/AppID. COM can't trust code callingCoRegisterClassObject() (an unsecured operation), it can only trust theregistry setting (the registry is a secure database) to determine whichclass object to use and how to run it. This behavior prevents illicitmachine-wide spoofing of class objects by unauthorized users.

With that in mind, we now return to the issue of when new processes andwindow stations are created when multiple-use servers are launched by COM.Notice that the LUID of the client does not matter in any way in themultiple-use class case.
  1. "Interactive user" security identity. In the case where a multiple-use COM class's AppID is configured to run as the "Interactive User" identity, the very first server process and its class object will be used by COM to service all subsequent activation requests. This server instance will have the interactive window station, if one exists (if no user is logged on locally then all activation requests will fail). As noted above, if a process not launched by COM and not running in the interactive window station registers the CLSID, that registration will fail. And, if an activation request later arrives, a new process will be launched with the security context of the Interactive User. This behavior prevents illicit spoofing of class objects. Since no additional server processes are ever started by COM, the question of new window stations is moot. The SID of the client, its LUID or whether it is local or remote does not matter in this case.
  2. "This user" security identity. Similarly, if a multiple-use COM class's AppID is configured to run as "this user" (a predefined security ID), the first server process and its class object will be used by COM to service all subsequent activation requests. This first server instance will have its own window station created as part of the first process creation. Since no additional server processes are ever started by COM, the question of additional window stations is moot. The SID of the client, its LUID or whether it is local or remote does not matter in this case. Note that a new window station will be created when you launch the first instance of a class/AppID configured to run as "This user," even if the same user is currently logged on in the interactive winstation. COM never uses the interactive winstation to launch a server configured to run as "This user" because that would result in different behavior for the class depending upon the unrelated issue of the identity of the currently logged on user. As noted above, if a process not launched by COM and not running in the account specified in "This user" attempts to register the CLSID, that registration will fail and if an activation request later arrives a new process will be launched in the "This user" account. This behavior prevents illicit spoofing of class objects. On the other hand, if the process registered for a given CLSID/APPID is configured to run as "This user," is created in the appropriate user account by some agent other than COM (for example, it is run by the interactive user when the interactive user is the same user as "This user" or it is started via CreateProcess() by a service running in the same security context as "this user"), and it then registers its REGCLS_MULTIPLEUSE class objects, COM will use the existing running class object to satisfy subsequent incoming activation requests from any client.
  3. "Launching user" security identity. In this case, the class's AppID is set to launch under the "Launching User" idenitity (this is also known as an "Activate as Activator" class). a. Local client. First consider the local machine case. There are two rules: 1. Each different activation client SID will cause COM to launch a new instance of the server process even in the same window station. 2. Even for matching SIDs (such as the case where the same user is logged on more than once to a single NT machine), each different local client window station will cause COM to launch a new instance of the server process. In other words, launching user identity multiple-use servers are never shared across window stations. All client processes with the same SID and same window stations will share a single server process in the same window station. Since the server shares the window station of the client no new window stations are created. For example, assume that you are interactively logged on as a_domain\a_user. You run multiple instances of the client, all of which will connect to the same instance of the server(which has the interactive window station). Now let's say you have a client, which is a service, that is set to launch start under a_domain\a_user. This service will launch a new instance of the COM server. This happens because COM will cause a new instance of the server to be launched since the service process has a window station other than the interactive window station--even though the identity of the service(client) process is the same as the identity of the running server process (a_domain\a_user). However, note that no new window station is created for the COM server process. The server will still inherit the window station of the service. If the service is set to start under LocalSystem and to interact with the desktop (refer to "Allow Service to Interact with Desktop" check box in the Service applet in control panel), then the service will run in the interactive window station or winsta0. In this case COM will still start a new instance of the server (client's SID which is LocalSystem in this case is different from that of the server's SID which is a_domain\a_user)in the interactive window station. b. Remote client. In the case of remote activation, COM ignores the window station of the client because the client is remote, unlike the local case above. The rule here is that each new client SID will cause a new instance of the server process to be launched and each new server process will get a new window station. Subsequent activation requests by remote clients with the same SID will reuse the existing registered class object as well as its process and window station. For example, assume that you are logged on as a_domain\a_user on 10 different machines. The clients from all these machines will connect to the same instance of the server on the server machine. A client from a a_domain\b_user machine will launch a new server instance and a new window station. Remote callers with the same SID as a local user that has launched the COM server registered for the requested CLSID will reuse the existing class object. However, the order in which the COM server is started is important in this case. If the server is launched by the local client first, then the remote client with the same SID will connect to this server. On the other hand, if the server is launched by the remote client first, then a local client with the same SID will start a new instance of the server. This goes back to the window station rules mentioned above. For local clients, the window station of the client and the server must match, for remote clients the window station of the client is ignored. For example, if local client a_domain\a_user launches the server first, then remote client a_domain\a_user will connect to the server. Conversely, if remote client a_domain\a_user launches the server first, then local client a_domain\a_user will launch a new server instance and a new window station. The LUID of the client does not matter in this case.
  4. Service-based COM servers. A COM class/AppID that is packaged in a Win32 service is by practical necessity a multiple-use server since services can be launched only once. In this case, at the first activation request a new process is launched in its own window station. There are two exceptions to this: a. If the service is set to launch under the LocalSystem account, it will inherit a system pre-defined window station. b. If the service is set to launch under the LocalSystem account and can interact with the desktop, it will inherit the interactive window station or winsta0. All subsequent activation requests, whether local or remote, are handled by the service's class object. As noted above, if a process not launched by COM and not running as the specified service registers the CLSID, that registration will fail and if an activation request later arrives the registered service will be launched. This behavior prevents illicit spoofing of class objects.

Single-Use Classes

NOTE: Single-Use classes should be avoided as much as possible. Single-useregistration is a legacy setting and is intended to support older COMapplications and to ease the porting of legacy non-COM applications to COM.It is strongly recommended that new classes be designed to support multiple-use class object registration. This is especially true in the case ofservers with "This User" identity, where single-use classes cause the exactopposite effect of multi-use classes. They create a new server process andnew window station per activation and, as we discuss below, this can causeresource problems under Windows NT.

A single-use class is one that is registered with COM (via theCoRegisterClassObject() API) specifying the REGCLS_SINGLEUSE flag. For sucha class, COM will always start a new instance of the class's server processfor each activation request from any client (local or remote). For purposesof this article, the remaining question is: when will the server get a newwindow station as well?
  1. "Interactive user" security identity. Take the case in which the single-use class is set via its AppID to start under the "Interactive User" identity. For this case, each new instance of the server process will always share the interactive window station, if one exists (if no user is logged on locally then all activation requests will fail). No new window station will be created by COM.
  2. "This user" security identity. Now consider the case where the single-use COM class's AppId is set to run under the "This User" identity. In this case, the rule is very simple. Every new client activation launches a new process in a new window station. This is true regardless of whether the user specified as "This User" is logged on to the interactive window station at the time of any of the activation requests.
  3. "Launching user" security identity. a. Local client. In the local machine activation scenario, the server process will always get the window station of the client. No new window stations are ever created. For example, assume that you are interactively logged on as a_domain\a_user and you run multiple instances of the client program. Each new resulting instance of the server will get the interactive window station. Now assume that the client is a service running under the local system account. The COM server in this case will share the window station of the service process. b. Remote client. In the remote activation case, COM ignores the window station of the client since the client is remote. As always, a new server instance process will be started for each remote activation. The rules are: 1. For each new remote client SID, a new window station will be created for the server process. 2. For each new remote client LUID, a new window station will be created for the server process. 3. All remote clients with the same SID/LUID pair will create servers that will share the same window station. For example, you are logged on the remote client machine as a_domain\a_user. This client launches the remote server that will get a new window station. Now, if a_domain\a_user starts a second instance of the client application from the same client machine which in turn launches a new copy of the server on the remote machine, this server will share the original server's window station. Now assume you log on to another machine again as a_domain\a_user, and run the client from there. The corresponding server instance will have a new window station. This demonstrates that even if the client SIDs are the same, since the second client has a different LUID, its server process will get a new window station.
  4. Service-based COM servers. Single-use classes should never be implemented as Windows NT services. It makes no sense, because it is not possible to run multiple instances of an Windows NT service process.

Table Summarizing Scenarios

---------------------------------------------------------------------------
       |                      Multiple-Use Server       |             (class factory has requested reuse)       |                       Activation Modes       |-------------------------------------------------------------------       |   Interactive     | As "This       |As "Launching    | Win32Client |      User         |   User"        |   user"         | service				
Local  | Process launched  | Process        | Process         | Service       | in the            | launched in a  | launched client | started on       | interactive window| new window     | window station  | first       | station on first  | station on     | on first        | activation       | activation        | first          | request,        | request       | request,          | activation     | subsequent      | (new winsta       | subsequent        | request,       | requests from   | or system/        | requests get      | subsequent     | the same SID/   | interactive       | existing class    | requests get   | window station  | winsta       | object,           | existing class | get existing    | depending       | activations fail  | object         | class object, no| on service       | if no user logged |                | sharing of class| config),       | on locally        |                | objects across  | subsequent       |                   |                | window stations | requests       |                   |                | even if same SID| get-------|                   |                |-----------------| existingRemote |                   |                | Process launched| class       |                   |                | in new winsta on| objects       |                   |                | first activation|       |                   |                | request by a    |       |                   |                | SID, subsequent |       |                   |                | remote requests |       |                   |                | by the same SID |       |                   |                | get existing    |       |                   |                | class object;   |       |                   |                | class launched  |       |                   |                | by local user   |       |                   |                | reused by remote|       |                   |                | callers with    |       |                   |                | same SID        |				
---------------------------------------------------------------------------

---------------------------------------------------------------------------
       |                      Single-Use Server       |          (new process created for each activation request)       |                       Activation Modes       |-------------------------------------------------------------------       |   Interactive     | As "This       |As "Launching    | Win32Client |      User         |   User"        |   user"         | service				
Local  | Process always    | Process always | Process always  | N/A(only       | launched in the   | launched in a  | launched in     | one       | interactive       | new window     | the window      | activation       | window station,   | station        | station of      | possible)       | if no interactive |                | client process  |       | window station    |                |                 |       | activation fails  |                |                 |-------|                   |                |-----------------|Remote |                   |                | if both SID and |       |                   |                | LUID of the     |       |                   |                | client match    |       |                   |                | previous client |       |                   |                | activation,     |       |                   |                | process launched|       |                   |                | in existing     |       |                   |                | window station, |       |                   |                | otherwise in new|       |                   |                | windowstation   |				
---------------------------------------------------------------------------

Window Stations and Windows NT Resources

In this section we will examine the implications of creating new windowstations under Windows NT and how that should affect the configuration ofyour COM server. As you will see, there is a limit to the number of windowstations that can be created on a Windows NT machine. When that limit isexceeded, Windows NT will fail an attempt by COM to launch a new instanceof the server process. Typically, an error message like the followingappears:
Initialization of the dynamic link library
d:\winnt\system32\kernel32.dll failed. The process is
terminating abnormally.
Under Windows NT, each window station has at least one desktop associatedwith it. Windows NT uses a special memory heap for all windows applicationsrunning on a desktop. By default, each desktop heap consumes 3MB of memory.Windows NT has a non-configurable limit of 48MB for creating desktop heaps.This means that the maximum number of window stations that can be createdon a Windows NT machine is 16 (probably less because a window station cancontain more than one desktop). To increase this number, you can reduce thedefault desktop heap size by editing the registry using the RegistryEditor.

WARNING: Using Registry Editor incorrectly can cause serious, system-wideproblems that may require you to reinstall Windows NT to correct them.Microsoft cannot guarantee that any problems resulting from the use ofRegistry Editor can be solved. Use this tool at your own risk.

The named value you need to edit appears under the following key:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session   Manager\SubSystems				
You need to edit the "Windows" named value. It is a REG_SZ string. Edit thestring and look for "Shared Section=1024,3072". Modify this to read "SharedSection=1024,3072,512". You need to restart the machine for this change totake effect. By making this change, you are specifying 3MB (default) heapsize for the interactive window station's desktop and 512 KB for all non-interactive desktops (the first parameter is obsolete but should not bechanged). This change will allow the creation of approximately less than48MB/512KB or 96 window stations.

NOTE: A window station can contain multiple desktops within it. In thediscussion of "Launching User" servers above, wherever the window stationof the local client process is mentioned, it should be considered as ashorter form for "window station and desktop". "Launching User" setting isreally meant for legacy non-DCOM aware servers and should be used rarely.Such legacy servers expect to run in their own desktops. Thus, forMULTIPLEUSE "Launching User" servers, each client process in a differentdesktop within the same window station causes a new server process to bestarted in that window station/desktop. For SINGLEUSE "Launching User"servers, again, the server inherits the windows station/desktop of theclient process.

In Windows NT 4.0 Service Pack 4.0, COM attempts to re-use window stations. Prior to this, if a server is set to RunAs a specific user and if multiple instances of the server process are launched, each process will get its own window station. This leads to the window station limitations describe above. In SP4, COM will attempt to create all RunAs (that is, not "Activate as Activator" or "Launching User") servers that are set to run under the same user identity (or token) in the same window station.

This eliminates the need for adjusting the desktop heap size in those cases where multiple server processes run with the same token. If, in your configuration, all servers are set to RunAs the same user, or multiple instances of the same RunAs server process run, then you should not reduce the desktop heapsize as suggested in the article. It is recommended that you leave it at the default value (3M) in this case. This is because, if you reduce the desktop heap, then the system can create more window stations/desktops; however, the number of processes that can run in a window station/desktop become progressively smaller.

On the other hand, in your configuration, if you have multiple servers running with different tokens, then you will face the window station limitations. In this case, you should follow the suggestions in the article for reducing the desktop heap size.
REFERENCES
For additional information about the desktop heap issue, please see thefollowing article in the Microsoft Knowledge Base:

142676 How to correct common User32.dll file errors
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Article ID: 169321 - Last Review: 07/11/2005 19:06:47 - Revision: 2.5

Microsoft OLE 4.0

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