TOPIC
    about_Transactions

SHORT DESCRIPTION
    Describes how to manage transacted operations in Windows PowerShell. 

LONG DESCRIPTION
    Transactions are supported in Windows PowerShell beginning
    in Windows PowerShell 2.0. This feature enables you to start
    a transaction, to indicate which commands are part of the
    transaction, and to commit or roll back a transaction.
    
    
  ABOUT TRANSACTIONS

      In Windows PowerShell, a transaction is a set of one or more
      commands that are managed as a logical unit. A transaction can
      be completed ("committed"), which changes data affected by the
      transaction. Or, a transaction can be completely undone ("rolled back")
      so that the affected data is not changed by the transaction. 

      Because the commands in a transaction are managed as a unit,
      either all commands are committed, or all commands are rolled
      back. 

      Transactions are widely used in data processing, most notably
      in database operations and for financial transactions. Transactions
      are most often used when the worst-case scenario for a set of
      commands is not that they all fail, but that some commands succeed
      while others fail, leaving the system in a damaged, false, or
      uninterpretable state that is difficult to repair.

            
  TRANSACTION CMDLETS

      Windows PowerShell includes several cmdlets designed for managing
      transactions.

      Cmdlet                 Description
      --------------         ---------------------------------    
      Start-Transaction      Starts a new transaction. 

      Use-Transaction        Adds a command or expression to the
                             transaction. The command must use
                             transaction-enabled objects.

      Undo-Transaction       Rolls back the transaction so that
                             no data is changed by the transaction.

      Complete-Transaction   Commits the transaction. The data
                             affected by the transaction is changed.

      Get-Transaction        Gets information about the active
                             transaction.


      For a list of transaction cmdlets, type:

          get-command *transaction

      For detailed information about the cmdlets, type:

	  get-help <cmdlet-name> -detailed

      For example:
	
	  get-help use-transaction -detailed


  TRANSACTION-ENABLED ELEMENTS

      To participate in a transaction, both the cmdlet and the provider
      must support transactions. This feature is built in to the objects
      that are affected by the transaction.
   
      The Windows PowerShell Registry provider supports transactions
      in Windows Vista. The TransactedString object 
      (Microsoft.PowerShell.Commands.Management.TransactedString) works
      with any operating system that runs Windows PowerShell. 

      Other Windows PowerShell providers can support transactions. To 
      find the Windows PowerShell providers in your session that support
      transactions, use the following command to find the "Transactions"
      value in the Capabilities property of providers:

	  get-psprovider | where {$_.Capabilities -like "*transactions*"}

      For more information about a provider, see the Help for the provider.
      To get provider Help, type:

	  get-help <provider-name>

      For example, to get Help for the Registry provider, type:

	  get-help registry
  


  THE USETRANSACTION PARAMETER

      Cmdlets that can support transactions have a UseTransaction
      parameter. This parameter includes the command in the active
      transaction. You can use the full parameter name or its alias,
      "usetx".

      The parameter can be used only when the session contains an
      active transaction. If you enter a command with the UseTransaction
      parameter when there is no active transaction, the command fails.

      To find cmdlets with the UseTransaction parameter, type:

	  get-help * -parameter UseTransaction    

      In Windows PowerShell core, all of the cmdlets designed to work
      with Windows PowerShell providers support transactions. As a
      result, you can use the provider cmdlets to manage transactions. 

      For more information about Windows PowerShell providers, 
      see about_Providers.    
 

  THE TRANSACTION OBJECT

      Transactions are represented in Windows PowerShell by a
      transaction object, System.Management.Automation.Transaction.

      The object has the following properties:

      RollbackPreference:  
          Contains the rollback preference set for the current
          transaction. You can set the rollback preference when you
          use Start-Transaction to start the transaction. 

          The rollback preference determines the conditions under
          which the transaction is rolled back automatically. Valid
          values are Error, TerminatingError, and Never. The default
          value is Error.

      Status:              
         Contains the current status of the transaction. Valid
         values are Active, Committed, and RolledBack.


      SubscriberCount:              
         Contains the number of subscribers to the transaction. A
         subscriber is added to a transaction when you start a 
         transaction while another transaction is in progress. The
         subscriber count is decremented when a subscriber commits
         the transaction.
    

  ACTIVE TRANSACTIONS

      In Windows PowerShell, only one transaction is active at a 
      time, and you can manage only the active transaction. Multiple
      transactions can be in progress in the same session at the same
      time, but only the most-recently started transaction is active. 

      As a result, you cannot specify a particular transaction when 
      using the transaction cmdlets. Commands always apply to the
      active transaction.

      This is most evident in the behavior of the Get-Transaction cmdlet. 
      When you enter a Get-Transaction command, Get-Transaction always
      gets only one transaction object. This object is the object that 
      represents the active transaction. 

      To manage a different transaction, you must first finish the active
      transaction, either by committing it or rolling it back. When you
      do this, the previous transaction becomes active automatically. 
      Transactions become active in the reverse of order of which they are
      started, so that the most recently started transaction is always 
      active.


  SUBSCRIBERS AND INDEPENDENT TRANSACTIONS

      If you start a transaction while another transaction is in progress,
      by default, Windows PowerShell does not start a new transaction. 
      Instead, it adds a "subscriber" to the current transaction.

      When a transaction has multiple subscribers, a single 
      Undo-Transaction command at any point rolls back the entire
      transaction for all subscribers. However, to commit the transaction,
      you must enter a Complete-Transaction command for every subscriber.

      To find the number of subscribers to a transaction, check the
      SubscriberCount property of the transaction object. For example,
      the following command uses the Get-Transaction cmdlet to get
      the value of the SubscriberCount property of the active transaction:

          (Get-Transaction).SubscriberCount
        
      Adding a subscriber is the default behavior because most transactions
      that are started while another transaction is in progress are related
      to the original transaction. In the typical model, a script that
      contains a transaction calls a helper script that contains its own
      transaction. Because the transactions are related, they should be 
      rolled back or committed as a unit.

      However, you can start a transaction that is independent of the 
      current transaction by using the Independent parameter of the 
      Start-Transaction cmdlet.

      When you start an independent transaction, Start-Transaction
      creates a new transaction object, and the new transaction becomes
      the active transaction. The independent transaction can be 
      committed or rolled back without affecting the original transaction.

      When the independent transaction is finished (committed or rolled 
      back), the original transaction becomes the active transaction 
      again.


  CHANGING DATA

      When you use transactions to change data, the data that is affected
      by the transaction is not changed until you commit the transaction.
      However, the same data can be changed by commands that are not 
      part of the transaction. 

      Keep this in mind when you are using transactions to manage shared
      data. Typically, databases have mechanisms that lock the data while
      you are working on it, preventing other users, and other commands,
      scripts, and functions, from changing it.

      However, the lock is a feature of the database. It is not related
      to transactions. If you are working in a transaction-enabled
      file system or other data store, the data can be changed while
      the transaction is in progress.


EXAMPLES
    The examples in this section use the Windows PowerShell Registry 
    provider and assume that you are familiar with it. For information
    about the Registry provider, type "get-help registry".

  EXAMPLE 1: COMMITTING A TRANSACTION

    To create a transaction, use the Start-Transaction cmdlet. The
    following command starts a transaction with the default settings.
 
 	start-transaction

    To include commands in the transaction, use the UseTransaction
    parameter of the cmdlet. By default, commands are not included
    in the transaction,      	 

    For example, the following command, which sets the current 
    location in the Software key of the HKCU: drive, is not included
    in the transaction.

        cd hkcu:\Software

    The following command, which creates the MyCompany key, uses the
    UseTransaction parameter of the New-Item cmdlet to include the
    command in the active transaction.

        new-item MyCompany -UseTransaction

    The command returns an object that represents the new key, but
    because the command is part of the transaction, the registry
    is not yet changed.

        Hive: HKEY_CURRENT_USER\Software

        SKC  VC Name                           Property
        ---  -- ----                           --------
          0   0 MyCompany                      {}


    To commit the transaction, use the Complete-Transaction cmdlet.
    Because it always affects the active transaction, you cannot specify
    the transaction.

	complete-transaction    


    As a result, the MyCompany key is added to the registry.

	dir m*
       
        Hive: HKEY_CURRENT_USER\software

        SKC  VC Name                           Property
        ---  -- ----                           --------
         83   1 Microsoft                      {(default)}
          0   0 MyCompany                      {}
	 


  EXAMPLE 2: ROLLING BACK A TRANSACTION

    To create a transaction, use the Start-Transaction cmdlet. The
    following command starts a transaction with the default settings.
 
 	start-transaction

    The following command, which creates the MyOtherCompany key, uses the
    UseTransaction parameter of the New-Item cmdlet to include the
    command in the active transaction.

        new-item MyOtherCompany -UseTransaction

    The command returns an object that represents the new key, but
    because the command is part of the transaction, the registry
    is not yet changed.

        Hive: HKEY_CURRENT_USER\Software

        SKC  VC Name                           Property
        ---  -- ----                           --------
          0   0 MyOtherCompany                 {}


    To roll back the transaction, use the Undo-Transaction cmdlet.
    Because it always affects the active transaction, you do not specify
    the transaction.

	Undo-transaction    

    The result is that the MyOtherCompany key is not added to the registry.

	dir m*
       
        Hive: HKEY_CURRENT_USER\software

        SKC  VC Name                           Property
        ---  -- ----                           --------
         83   1 Microsoft                      {(default)}
          0   0 MyCompany                      {}


 
  EXAMPLE 3: PREVIEWING A TRANSACTION

    Typically, the commands used in a transaction change data. However,
    the commands that get data are useful in a transaction, too, because
    they get data inside of the transaction. This provides a preview of the 
    changes that committing the transaction would cause.

    The following example shows how to use the Get-ChildItem command
    (the alias is "dir") to preview the changes in a transaction.


    The following command starts a transaction.

	start-transaction

    The following command uses the New-ItemProperty cmdlet to add the
    MyKey registry entry to the MyCompany key. The command uses the
    UseTransaction parameter to include the command in the transaction.


        new-itemproperty -path MyCompany -Name MyKey -value 123 -UseTransaction

    The command returns an object representing the new registry entry,
    but the registry entry is not changed.

        MyKey
        -----
        123


    To get the items that are currently in the registry, use a Get-ChildItem
    command ("dir") without the UseTransaction parameter. The following 
    command gets items that begin with "M."

	dir m*


    The result shows that no entries have yet been added to the MyCompany key.

        Hive: HKEY_CURRENT_USER\Software

        SKC  VC Name                           Property
        ---  -- ----                           --------
        83   1 Microsoft                      {(default)}
         0   0 MyCompany                      {}


    To preview the effect of committing the transaction, enter a Get-ChildItem
    ("dir") command with the UseTransaction parameter. This command has a view
    of the data from within the transaction.


	dir m* -useTransaction


    The result shows that, if the transaction is committed, the MyKey entry
    will be added to the MyCompany key.


        Hive: HKEY_CURRENT_USER\Software

        SKC  VC Name                           Property
        ---  -- ----                           --------
        83   1 Microsoft                      {(default)}
         0   1 MyCompany                      {MyKey}


     
  EXAMPLE 4: COMBINING TRANSACTED AND NON-TRANSACTED COMMANDS

    You can enter non-transacted commands during a transaction. The
    non-transacted commands affect the data immediately, but they do
    not affect the transaction.

    The following command starts a transaction in the HKCU:\Software
    registry key.

	start-transaction


    The next three commands use the New-Item cmdlet to add keys to
    the registry. The first and third commands use the UseTransaction
    parameter to include the commands in the transaction. The second
    command omits the parameter. Because the second command is not
    included in the transaction, it is effective immediately.

        new-item MyCompany1 -UseTransaction

        new-item MyCompany2

        new-item MyCompany3 -UseTransaction


    To view the current state of the registry, use a Get-ChildItem ("dir")
    command without the UseTransaction parameter. This command gets items
    that begin with "M."

	dir m*

    The result shows that the MyCompany2 key is added to the registry, but
    the MyCompany1 and MyCompany3 keys, which are part of the transaction,
    are not added.
     
        Hive: HKEY_CURRENT_USER\Software

        SKC  VC Name                           Property
        ---  -- ----                           --------
        83   1 Microsoft                      {(default)}
        0    0 MyCompany2                     {}


    The following command commits the transaction. 

        complete-transaction

    Now, the keys that were added as part of the transaction appear in the
    registry.

	dir m*

     
        Hive: HKEY_CURRENT_USER\Software

        SKC  VC Name                           Property
        ---  -- ----                           --------
        83   1 Microsoft                      {(default)}
        0    0 MyCompany1                     {}
        0    0 MyCompany2                     {}
        0    0 MyCompany3                     {}



  EXAMPLE 5: USING AUTOMATIC ROLLBACK

    When a command in a transaction generates an error
    of any kind, the transaction is automatically rolled
    back. 

    This default behavior is designed for scripts that
    run transactions. Scripts are typically well tested
    and include error-handling logic, so errors are not
    expected and should terminate the transaction. 

    The first command starts a transaction in the HKCU:\Software
    registry key.

	start-transaction

    The following command uses the New-Item cmdlet to
    add the MyCompany key to the registry. The command uses
    the UseTransaction parameter (the alias is "usetx") to include
    the command in the transaction.

	New-Item MyCompany -UseTX

    Because the MyCompany key already exists in the registry,
    the command fails, and the transaction is rolled back.        

        New-Item : A key at this path already exists
        At line:1 char:9
        + new-item <<<<  MyCompany -usetx

    A Get-Transaction command confirms that the transaction
    has been rolled back and that the SubscriberCount is 0.

        RollbackPreference   SubscriberCount   Status
        ------------------   ---------------   ------
        Error                0                 RolledBack 

        
  EXAMPLE 6: CHANGING THE ROLLBACK PREFERENCE

    If you want the transaction to be more error tolerant,
    you can use the RollbackPreference parameter of 
    Start-Transaction to change the preference.

    The following command starts a transaction with a
    rollback preference of "Never".

         start-transaction -rollbackpreference Never

    In this case, when the command fails, the transaction
    is not automatically rolled back.

	New-Item MyCompany -UseTX

        New-Item : A key at this path already exists
        At line:1 char:9
        + new-item <<<<  MyCompany -usetx
   
     
    Because the transaction is still active, you can
    resubmit the command as part of the transaction. 

	New-Item MyOtherCompany -UseTX



  EXAMPLE 7: USING THE USE-TRANSACTION CMDLET

    The Use-Transaction cmdlet enables you to do direct scripting
    against transaction-enabled Microsoft .NET Framework  objects.
    Use-Transaction takes a script block that can only contain commands
    and expressions that use transaction-enabled .NET Framework objects,
    such as instances of the 
    Microsoft.PowerShell.Commands.Management.TransactedString class.

    The following command starts a transaction.

         start-transaction

    The following New-Object command creates an instance of the
    TransactedString class and saves it in the $t variable.

         $t = New-Object Microsoft.PowerShell.Commands.Management.TransactedString

    The following command uses the Append method of the TransactedString
    object to add text to the string. Because the command is not part
    of the transaction, the change is effective immediately.

	 $t.append("Windows") 

    The following command uses the same Append method to add text, but
    it adds the text as part of the transaction. The command is enclosed
    in braces, and it is set as the value of the ScriptBlock parameter of
    Use-Transaction. The UseTransaction parameter (UseTx) is required.

	 use-transaction {$t.append(" PowerShell")} -usetx

    To see the current content of the transacted string in $t, use the
    ToString method of the TransactedString object.
                 
	 $t.tostring()      

    The output shows that only the non-transacted changes are effective.

	 Windows	

    To see the current content of the transacted string in $t from within
    the transaction, embed the expression in a Use-Transaction command.  

	 use-transaction {$s.tostring()} -usetx      

    The output shows the transaction view.

	 Windows PowerShell

    The following command commits the transaction.

	 complete-transaction

    To see the final string:

	 $t.tostring()
	
	 Windows PowerShell


  EXAMPLE 7: MANAGING MULTI-SUBSCRIBER TRANSACTIONS

    When you start a transaction while another transaction is in
    progress, Windows PowerShell does not create a second transaction
    by default. Instead, it adds a subscriber to the current 
    transaction. 

    This example shows how to view and manage a multi-subscriber 
    transaction.

    Begin by starting a transaction in the HKCU:\Software key.
	
        start-transaction
    
    The following command uses the Get-Transaction command to 
    get the active transaction.

	get-transaction


    The result shows the object that represents the active transaction.

        RollbackPreference   SubscriberCount   Status
        ------------------   ---------------   ------
        Error                1                 Active 
     
     
    The following command adds the MyCompany key to the registry.
    The command  uses the UseTransaction parameter to include the
    command in the transaction.
                 
        new-item MyCompany -UseTransaction


    The following command uses the Start-Transaction command to
    start a transaction. Although this command is typed at the command
    prompt, this scenario is more likely to happen when you run a
    script that contains a transaction.

        start-transaction


    A Get-Transaction command shows that the subscriber count on the
    transaction object is incremented. The value is now 2.     

        RollbackPreference   SubscriberCount   Status
        ------------------   ---------------   ------
        Error                2                 Active 
     

    The next command uses the New-ItemProperty cmdlet to add the 
    MyKey registry entry to the MyCompany key. It uses the UseTransaction
    parameter to include the command in the transaction.

        new-itemproperty -path MyCompany -name MyKey -UseTransaction


    The MyCompany key does not exist in the registry, but this 
    command succeeds because the two commands are part of the 
    same transaction.

    The following command commits the transaction. If it rolled back
    the transaction, the transaction would be rolled back for all the
    subscribers.

	complete-transaction


    A Get-Transaction command shows that the subscriber count on the
    transaction object is 1, but the value of Status is still Active
    (not Committed).     


        RollbackPreference   SubscriberCount   Status
        ------------------   ---------------   ------
        Error                1                 Active 


    To finish committing the transaction, enter a second Complete-
    Transaction command. To commit a multi-subscriber transaction,
    you must enter one Complete-Transaction command for each
    Start-Transaction command.

     	complete-transaction

      
    Another Get-Transaction command shows that the transaction
    has been committed.


        RollbackPreference   SubscriberCount   Status
        ------------------   ---------------   ------
        Error                0                 Committed 
 

  EXAMPLE 8: MANAGING INDEPENDENT TRANSACTIONS

    When you start a transaction while another transaction is in
    progress, you can use the Independent parameter of Start-Transaction
    to make the new transaction independent of the original transaction.

    When you do, Start-Transaction creates a new transaction object
    and makes the new transaction the active transaction.

    Begin by starting a transaction in the HKCU:\Software key.
	
        start-transaction
     
    The following command uses the Get-Transaction command to 
    get the active transaction.

	get-transaction


    The result shows the object that represents the active transaction.

        RollbackPreference   SubscriberCount   Status
        ------------------   ---------------   ------
        Error                1                 Active 
     
     
    The following command adds the MyCompany registry key as part of
    the transaction. It uses the UseTransaction parameter (UseTx) 
    to include the command in the active transaction.

	new-item MyCompany -use


    The following command starts a new transaction. The command uses
    the Independent parameter to indicate that this transaction
    is not a subscriber to the active transaction.

         start-transaction -independent

    When you create an independent transaction, the new (most-recently
    created) transaction becomes the active transaction. You can use
    a Get-Transaction command to get the active transaction. 

	get-transaction

    Note that the SubscriberCount of the transaction is 1, indicating
    that there are no other subscribers and that the transaction is 
    new.

        RollbackPreference   SubscriberCount   Status
        ------------------   ---------------   ------
        Error                1                 Active 
	
    The new transaction must be finished (either committed or rolled
    back) before you can manage the original transaction.

    The following command adds the MyOtherCompany key to the registry.
    It uses the UseTransaction parameter (UseTx) to include the command
    in the active transaction.

        new-item MyOtherCompany -usetx

    Now, roll back the transaction. If there were a single
    transaction with two subscribers, rolling back the transaction 
    would roll back the entire transaction for all the subscribers.

    However, because these transactions are independent, rolling
    back the newest transaction cancels the registry changes and 
    makes the original transaction the active transaction.

        undo-transaction

    A Get-Transaction command confirms that the original
    transaction is still active in the session.


	get-transaction

        RollbackPreference   SubscriberCount   Status
        ------------------   ---------------   ------
        Error                1                 Active 
     
    The following command commits the active transaction.

        complete-transaction


    A Get-ChildItem command shows that the registry has been
    changed.

	dir m*


        Hive: HKEY_CURRENT_USER\Software

        SKC  VC Name                           Property
        ---  -- ----                           --------
        83   1 Microsoft                      {(default)}
         0   0 MyCompany                      {}


SEE ALSO
    Start-Transaction
    Get-Transaction
    Complete-Transaction
    Undo-Transaction
    Use-Transaction
    Registry (provider)
    about_Providers
    Get-PSProvider
    Get-ChildItem

	