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.
