Multitasking with background jobs in VB.NET

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Multitasking with background jobs
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And I can then get just that child job:
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PS C:\> get-job -name job12 WARNING: column "Command" does not fit into the display and was removed. Id -12 Name ---Job12 State ----Failed HasMoreData ----------False Location -------notonline
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As you can see, there are no results to retrieve because no output was ever created for this job. But the job s errors are stored in the results, and you can get them by using Receive-Job:
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PS C:\> receive-job -name job12 Receive-Job : [notonline] Connecting to remote server failed with the foll owing error message : WinRM cannot process the request. The following erro r occured while using Kerberos authentication: The network path was not fo und.
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The actual error is much longer; I ve truncated it here to save some space. You ll notice that the error includes the computer name that the error came from, [notonline]. What happens if only one of the computers can t be reached Let s try:
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PS C:\> invoke-command -command { nothing } -computer notonline,server-r2 -asjob -jobname ThisWillFail WARNING: column "Command" does not fit into the display and was removed. Id -13 Name ---ThisWillFail State ----Running HasMoreData ----------True Location -------notonline,se...
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After waiting for a bit, I ll run this:
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PS C:\> get-job WARNING: column "Command" does not fit into the display and was removed. Id -13 Name ---ThisWillFail State ----Failed HasMoreData ----------False Location -------notonline,se...
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The job still failed, but let s look at the individual child jobs:
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PS C:\> get-job -id 13 | select -expand childjobs WARNING: column "Command" does not fit into the display and was removed. Id -14 15 Name ---Job14 Job15 State ----Failed Failed HasMoreData ----------False False Location -------notonline server-r2
Common points of confusion
Okay, they both failed. I have a feeling I know why Job14 didn t work, but what s wrong with Job15
PS C:\> receive-job -name job15 Receive-Job : The term 'nothing' is not recognized as the name of a cmdlet , function, script file, or operable program. Check the spelling of the na me, or if a path was included, verify that the path is correct and try aga in.
Ah, that s right, I told it to run a bogus command. As you can see, each child job can fail for different reasons, and PowerShell will track each one individually.
12.9 Common points of confusion
Jobs are usually pretty straightforward, but there s one thing I ve seen folks do that does cause confusion. Don t do this:
PS C:\> invoke-command -command { Start-Job -scriptblock { dir } } -computername Server-R2
This is starting up a temporary connection to SERVER-R2 and starting a local job. Unfortunately, that connection immediately terminates, so there s no way to reconnect and retrieve that job. In general, then, don t mix and match the three ways of starting jobs. This one is also a bad idea:
PS C:\> start-job -scriptblock { invoke-command -command { dir } -computername SERVER-R2 }
That s completely redundant; just keep the Invoke-Command part and use the -AsJob parameter to have it run in the background. Less confusing, but equally interesting, are the questions my classroom students often ask about jobs. Probably the most important of these is, Can I see jobs started by someone else The answer is, no. Jobs are contained entirely within the PowerShell process, and although you could see that another user was running PowerShell, you wouldn t be able to see inside that process. It s just like any other application: you could see that another user was running Microsoft Office Word, for example, but you couldn t see what documents they were editing, because those documents exist entirely inside of Word s process. Jobs only last as long as your PowerShell session is open. After you close it, any jobs defined within it are gone. Jobs aren t defined anywhere outside of PowerShell, so they depend upon its process continuing to run in order to maintain themselves.
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