Synchronizing PowerShell script execution on multiple machines

I originally thought to call this “synchronizing watches” but since I am not changing system time I thought it might be misleading.  This post is all about synchronizing script execution on two different machines.

Some folks would use a central client and workflows (PowerShell v3) or PowerShell remoting to execute a script on remote machines at the same time.  Possibly even executing each one as a job so they happen at the same time. 

It also allows me to launch the script on server A and then move to the other side of the lab to launch its script and then begin the repro (or switch the monitor on the KVM).  I know that as long as they share a time service I can be confident in when the tracing began on the remote machine.

In my case I want to execute the script on each machine at the console.  My reasoning is that; the machines are not in the same domain making security messy, or I don’t want to modify firewall rules, or I am capturing network traffic and just don’t want the extra noise to filter through.

Here is my scenario: 

• I have multiple servers (a large, distributed application)
• Different things are happening on each server
• I want to enable a network trace at the same time (to have nicely correlated logs)
• The systems already have a common time source (the OS handles that after all)
• I need to manually trigger the reproduction of the bug after the tracing starts, or I need to trigger some event prior to the tracing beginning
• I only have two hands and two feet

Here is the synchronization snip at the beginning of my script:

Do {
    Start-Sleep ( 60 - (Get-Date -Format ss) )
    
    [string]$nowMinute = Get-Date -Format mm
} until ( $nowMinute[($nowMinute.Length - 1)] -eq "0" -or $nowMinute[($nowMinute.Length - 1)] -eq "5" )

The first thing that I do I describe as squaring up.  I invoke a Start-Sleep but I want to sleep until the end of the current minute, no longer.

Start-Sleep ( 60 - (Get-Date -Format ss) )

You could do this as two lines as well (or three if you like):

$secToZero = 60 - (Get-Date -Format ss)

Start-Sleep $secToZero

Then I get the current minute and evaluate it to the nearest 5 minute block.

I chose the 5 minute mark because it is easy to evaluate to (if the number ends in 0 or 5 it divides by 5).  I cast the returned integer to a string as well, so I can get that last digit.

[string]$nowMinute = Get-Date -Format mm

*Notice that if you use “MM” instead of “mm” you get the Month.

I do my math evaluation at the end of the Until.  Notice the –or between the 0 and 5 after the –eq.  That is important to force the evaluation against both numbers.

Here is where personal preference sets in.  I dreamed up three different ways to handle the evaluation in the Until block.

The first is a very literal evaluation of the second digit in the Seconds to see if it equals “0” or “5”.  the entire last line looks like this:

} until ( $nowMinute[($nowMinute.Length - 1)] -eq "0" -or $nowMinute[($nowMinute.Length - 1)] -eq "5" )

It is the ($nowMinute.Length –1) that puts us at position 1 ( position begins counting at 0) to perform the evaluation.  The position indicator for the array is the square brackets [].

Another way to handle this is through the PowerShell way to handle a Regular Expression (or a pattern match – a nice article on that is here.)

Based on that article I can keep my –or and simply indicate the second digit this way:

} until ( $nowMinute -match ".0" -or $nowMinute -match ".5" )

Or, I can do it more like a regular expression and state in a shorter line that the second digit can match either 0 or 5.  That looks like this:

} until ( $nowMinute -match ".[0,5]" )

The end result is all the same.  Much of the technique is all about personal preference.