Block Retry using Powershell

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);
	}

view raw Execute-With-Retry.ps1 hosted with ❤ by GitHub

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.