Describes how to manage transacted operations in Windows PowerShell. (about_transactions)

   
# 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

C:\Windows>powershell get-help about_trap -full

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