Block Retry using Powershell

Posted Feb 6, 2015 Updated Mar 28, 2021

By Alex Bevilacqua 2 min read

I’ve been doing a lot of Powershell scripting lately, and one of the features I’ve really been pining for is the ability to apply some form of retry logic to either a function or a block.

Take the following sample:

  
function RandomlyFail
{
    $rnd = Get-Random -minimum 1 -maximum 3
    if ($rnd -eq 2) {
        throw "OH NOES!!!"
    }
    $Host.UI.WriteLine("W00t!!!")
}

Depending on what the random number we get is, we’ll get one of two scenarios:

# Success
RandomlyFail

W00t!!!

# Failure
RandomlyFail

OH NOES!!!
At line:62 char:9
+         throw "OH NOES!!!"
+         ~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : OperationStopped: (OH NOES!!!:String) [], RuntimeException
    + FullyQualifiedErrorId : OH NOES!!!

Now, if this happened to be part of a larger script and we didn’t have everything wrapped in a try..catch block, execution could potentially stop there.

Since Powershell supports closures, we can write a function that accepts a script block as a parameter.

<# This function can be used to pass a ScriptBlock (closure) to be executed and returned. The operation retried a few times on failure, and if the maximum threshold is surpassed, the operation fails completely. Params: Command - The ScriptBlock to be executed RetryDelay - Number (in seconds) to wait between retries (default: 5) MaxRetries - Number of times to retry before accepting failure (default: 5) VerboseOutput - More info about internal processing (default: false) Examples: Execute-With-Retry { $connection.Open() } $result = Execute-With-Retry -RetryDelay 1 -MaxRetries 2 { $command.ExecuteReader() } #> function Execute-With-Retry { [CmdletBinding()] param( [Parameter(ValueFromPipeline,Mandatory)] $Command, $RetryDelay = 5, $MaxRetries = 5, $VerboseOutput = $false ) $currentRetry = 0 $success = $false $cmd = $Command.ToString() do { try { $result = & $Command $success = $true if ($VerboseOutput -eq $true) { $Host.UI.WriteDebugLine("Successfully executed [$cmd]") } return $result } catch [System.Exception] { $currentRetry = $currentRetry + 1 if ($VerboseOutput -eq $true) { $Host.UI.WriteErrorLine("Failed to execute [$cmd]: " + $_.Exception.Message) } if ($currentRetry -gt $MaxRetries) { throw "Could not execute [$cmd]. The error: " + $_.Exception.ToString() } else { if ($VerboseOutput -eq $true) { $Host.UI.WriteDebugLine("Waiting $RetryDelay second(s) before attempt #$currentRetry of [$cmd]") } Start-Sleep -s $RetryDelay } } } while (!$success); }

Now, if we retrofit our sample above:

  
Execute-With-Retry -RetryDelay 1 -VerboseOutput $true { RandomlyFail }

Failed to execute [ RandomlyFail ]: OH NOES!!!
DEBUG: Waiting 1 second(s) before attempt #1 of [ RandomlyFail ]
Failed to execute [ RandomlyFail ]: OH NOES!!!
DEBUG: Waiting 1 second(s) before attempt #2 of [ RandomlyFail ]
Failed to execute [ RandomlyFail ]: OH NOES!!!
DEBUG: Waiting 1 second(s) before attempt #3 of [ RandomlyFail ]
Failed to execute [ RandomlyFail ]: OH NOES!!!
DEBUG: Waiting 1 second(s) before attempt #4 of [ RandomlyFail ]
W00t!!!
DEBUG: Successfully executed [ RandomlyFail ]

The inspiration for this comes from an excellent article by Pawel Pabich.

Scripting

powershell

This post is licensed under CC BY 4.0 by the author.

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