About Remote Jobs

Short Description

Describes how to run background jobs on remote computers.

Detailed Description

PowerShell concurrently runs commands and scripts through jobs. There are three jobs types provided by PowerShell to support concurrency.

  • RemoteJob - Commands and scripts run in a remote session.
  • BackgroundJob - Commands and scripts run in a separate process on the local machine. For more information, see about_Jobs.
  • PSTaskJob or ThreadJob - Commands and scripts run in a separate thread within the same process on the local machine. For more information, see about_Thread_Jobs.

Running scripts remotely, on a separate machine or in a separate process, provide great isolation. Any errors that occur in the remote job do not affect other running jobs or the parent session that started the job. However, the remoting layer adds overhead, including object serialization. All objects are serialized and deserialized as they are passed between the parent session and the remote (job) session. Serialization of large complex data objects can consume large amounts of compute and memory resources and transfer large amounts of data across the network.

Important

The parent session that created the job also monitors the job status and collects pipeline data. The job child process is terminated by the parent process once the job reaches a finished state. If the parent session is terminated, all running child jobs are terminated along with their child processes.

There are two ways work around this situation:

  1. Use Invoke-Command to create jobs that run in disconnected sessions. See the detached processes section of this article.
  2. Use Start-Process to create a new process rather than a job. For more information, see Start-Process.

Remote Jobs

You can run jobs on remote computers by using three different methods.

  • Start an interactive session on a remote computer. Then start a job in the interactive session. The procedures are the same as running a local job, although all actions are performed on the remote computer.

  • Run a job on a remote computer that returns its results to the local computer. Use this method when you want to collect the results of jobs and maintain them in a central location on the local computer.

  • Run a job on a remote computer that maintains its results on the remote computer. Use this method when the job data is more securely maintained on the originating computer.

Start a job in an interactive session

You can start an interactive session with a remote computer and then start a job during the interactive session. For more information about interactive sessions, see about_Remote, and see Enter-PSSession.

The procedure for starting a job in an interactive session is almost identical to the procedure for starting a background job on the local computer. However, all of the operations occur on the remote computer, not the local computer.

  1. Use the Enter-PSSession cmdlet to start an interactive session with a remote computer. You can use the ComputerName parameter of Enter-PSSession to establish a temporary connection for the interactive session. Or, you can use the Session parameter to run the interactive session in a PowerShell session (PSSession).

    The following command starts an interactive session on the Server01 computer.

    C:\PS> Enter-PSSession -computername Server01
    

    The command prompt changes to show that you are now connected to the Server01 computer.

    Server01\C:>
    
  2. To start a remote job in the session, use the Start-Job cmdlet. The following command runs a remote job that gets the events in the Windows PowerShell event log on the Server01 computer. The Start-Job cmdlet returns an object that represents the job.

    This command saves the job object in the $job variable.

    Server01\C:> $job = Start-Job -scriptblock {
      Get-Eventlog "Windows PowerShell"
    }
    

    While the job runs, you can use the interactive session to run other commands, including other jobs. However, you must keep the interactive session open until the job is completed. If you end the session, the job is interrupted, and the results are lost.

  3. To find out if the job is complete, display the value of the $job variable, or use the Get-Job cmdlet to get the job. The following command uses the Get-Job cmdlet to display the job.

    Server01\C:> Get-Job $job
    
    SessionId  Name  State      HasMoreData  Location   Command
    ---------  ----  -----      -----------  --------   -------
    1          Job1  Complete   True         localhost  Get-Eventlog "Windows...
    

    The Get-Job output shows that job is running on the "localhost" computer because the job was started on and is running on the same computer (in this case, Server01).

  4. To get the results of the job, use the Receive-Job cmdlet. You can display the results in the interactive session or save them to a file on the remote computer. The following command gets the results of the job in the $job variable. The command uses the redirection operator (>) to save the results of the job in the PsLog.txt file on the Server01 computer.

    Server01\C:> Receive-Job $job > c:\logs\PsLog.txt
    
  5. To end the interactive session, use the Exit-PSSession cmdlet. The command prompt changes to show that you are back in the original session on the local computer.

    Server01\C:> Exit-PSSession
    C:\PS>
    
  6. To view the contents of the PsLog.txt file on the Server01 computer at any time, start another interactive session, or run a remote command. This type of command is best run in a PSSession (a persistent connection) in case you want to use several commands to investigate and manage the data in the PsLog.txt file. For more information about PSSessions, see about_PSSessions.

    The following commands use the New-PSSession cmdlet to create a PSSession that is connected to the Server01 computer, and they use the Invoke-Command cmdlet to run a Get-Content command in the PSSession to view the contents of the file.

    $s = New-PSSession -computername Server01
    Invoke-Command -session $s -scriptblock {
      Get-Content c:\logs\pslog.txt}
    

Start a remote job that returns the results to the local computer (AsJob)

To start a job on a remote computer that returns the command results to the local computer, use the AsJob parameter of a cmdlet such as the Invoke-Command cmdlet.

When you use the AsJob parameter, the job object is actually created on the local computer even though the job runs on the remote computer. When the job is completed, the results are returned to the local computer.

You can use the cmdlets that contain the Job noun (the Job cmdlets) to manage any job created by any cmdlet. Many of the cmdlets that have AsJob parameters do not use PowerShell remoting, so you can use them even on computers that are not configured for remoting and that do not meet the requirements for remoting.

  1. The following command uses the AsJob parameter of Invoke-Command to start a job on the Server01 computer. The job runs a Get-Eventlog command that gets the events in the System log. You can use the JobName parameter to assign a display name to the job.

    Invoke-Command -computername Server01 -scriptblock {
      Get-Eventlog system} -AsJob
    

    The results of the command resemble the following sample output.

    SessionId   Name   State    HasMoreData   Location   Command
    ---------   ----   -----    -----------   --------   -------
    1           Job1   Running  True          Server01   Get-Eventlog system
    

    When the AsJob parameter is used, Invoke-Command returns the same type of job object that Start-Job returns. You can save the job object in a variable, or you can use a Get-Job command to get the job.

    Note that the value of the Location property shows that the job ran on the Server01 computer.

  2. To manage a job started by using the AsJob parameter of the Invoke-Command cmdlet, use the Job cmdlets. Because the job object that represents the remote job is on the local computer, you do not need to run remote commands to manage the job.

    To determine whether the job is complete, use a Get-Job command. The following command gets all of the jobs that were started in the current session.

    Get-Job
    

    Because the remote job was started in the current session, a local Get-Job command gets the job. The State property of the job object shows that the command was completed successfully.

    SessionId   Name   State      HasMoreData   Location   Command
    ---------   ----   -----      -----------   --------   -------
    1           Job1   Completed  True          Server01   Get-Eventlog system
    
  3. To get the results of the job, use the Receive-Job cmdlet. Because the job results are automatically returned to the computer where the job object resides, you can get the results with a local Receive-Job command.

    The following command uses the Receive-Job cmdlet to get the results of the job. It uses the session ID to identify the job. This command saves the job results in the $results variable. You can also redirect the results to a file.

    $results = Receive-Job -id 1
    

Start a remote job that keeps the results on the remote computer

To start a job on a remote computer that keeps the command results on the remote computer, use the Invoke-Command cmdlet to run a Start-Job command on a remote computer. You can use this method to run jobs on multiple computers.

When you run a Start-Job command remotely, the job object is created on the remote computer, and the job results are maintained on the remote computer. From the perspective of the job, all operations are local. You are just running commands remotely to manage a local job on the remote computer.

  1. Use the Invoke-Command cmdlet to run a Start-Job command on a remote computer.

    This command requires a PSSession (a persistent connection). If you use the ComputerName parameter of Invoke-Command to establish a temporary connection, the Invoke-Command command is considered to be complete when the job object is returned. As a result, the temporary connection is closed, and the job is canceled.

    The following command uses the New-PSSession cmdlet to create a PSSession that is connected to the Server01 computer. The command saves the PSSession in the $s variable.

    $s = New-PSSession -computername Server01
    

    The next command uses the Invoke-Command cmdlet to run a Start-Job command in the PSSession. The Start-Job command and the Get-Eventlog command are enclosed in braces.

    Invoke-Command -session $s -scriptblock {
      Start-Job -scriptblock {Get-Eventlog system}}
    

    The results resemble the following sample output.

    Id       Name    State      HasMoreData     Location   Command
    --       ----    -----      -----------     --------   -------
    2        Job2    Running    True            Localhost  Get-Eventlog system
    

    When you run a Start-Job command remotely, Invoke-Command returns the same type of job object that Start-Job returns. You can save the job object in a variable, or you can use a Get-Job command to get the job.

    Note that the value of the Location property shows that the job ran on the local computer, known as "LocalHost", even though the job ran on the Server01 computer. Because the job object is created on the Server01 computer and the job runs on the same computer, it is considered to be a local background job.

  2. To manage a remote job, use the Job cmdlets. Because the job object is on the remote computer, you need to run remote commands to get, stop, wait for, or retrieve the job results.

    To see if the job is complete, use an Invoke-Command command to run a Get-Job command in the PSSession that is connected to the Server01 computer.

    Invoke-Command -session $s -scriptblock {Get-Job}
    

    The command returns a job object. The State property of the job object shows that the command was completed successfully.

    SessionId   Name  State      HasMoreData   Location   Command
    ---------   ----  -----      -----------   --------   -------
    2           Job2  Completed  True          LocalHost   Get-Eventlog system
    
  3. To get the results of the job, use the Invoke-Command cmdlet to run a Receive-Job command in the PSSession that is connected to the Server01 computer.

    The following command uses the Receive-Job cmdlet to get the results of the job. It uses the session ID to identify the job. This command saves the job results in the $results variable. It uses the Keep parameter of Receive-Job to keep the result in the job cache on the remote computer.

    $results = Invoke-Command -session $s -scriptblock {
      Receive-Job -SessionId 2 -Keep
    }
    

    You can also redirect the results to a file on the local or remote computer. The following command uses a redirection operator to save the results in a file on the Server01 computer.

    Invoke-Command -session $s -command {
      Receive-Job -SessionId 2 > c:\logs\pslog.txt
    }
    

How to run as a detached process

As previously mentioned, when the parent session is terminated, all running child jobs are terminated along with their child processes. You can use remoting on the local machine to run jobs that are not attached to the current PowerShell session.

Create a new PowerShell session on the local machine. The use Invoke-Command to start a job in this session. Invoke-Command allows you to disconnect a remote session and terminate the parent session. Later, you can start a new PowerShell session and connect to the previously disconnected session to resume monitoring the job. However, any data that was returned to the original PowerShell session is lost when that session is terminated. Only new data objects generated after the disconnect are returned when re-connected.

# Create remote session on local machine
PS> $session = New-PSSession -cn localhost

# Start remote job
PS> $job = Invoke-Command -Session $session -ScriptBlock { 1..60 | % { sleep 1; "Output $_" } } -AsJob
PS> $job

Id     Name     PSJobTypeName   State         HasMoreData     Location      Command
--     ----     -------------   -----         -----------     --------      -------
1      Job1     RemoteJob       Running       True            localhost     1..60 | % { sleep 1; ...

# Disconnect the job session
PS> Disconnect-PSSession $session

Id Name         Transport ComputerName    ComputerType    State         ConfigurationName     Availability
-- ----         --------- ------------    ------------    -----         -----------------     ------------
1 Runspace1     WSMan     localhost       RemoteMachine   Disconnected  Microsoft.PowerShell          None

PS> $job

Id     Name     PSJobTypeName   State         HasMoreData     Location      Command
--     ----     -------------   -----         -----------     --------      -------
1      Job1     RemoteJob       Disconnected  True            localhost     1..60 | % { sleep 1;

# Reconnect the session to a new job object
PS> $jobNew = Receive-PSSession -Session $session -OutTarget Job
PS> $job | Wait-Job | Receive-Job
Output 9
Output 10
Output 11
...

For this example, the jobs are still attached to a parent PowerShell session. However, the parent session is not the original PowerShell session where Invoke-Command was run.

See also