All posts by Jim

Software engineer from Crete living in Switzerland; coffee addict; firearm lover; C# paladin; cryptography enthusiast; perpetually tormented by 3 beautiful women :-)

Powershell – file system operations within a transaction

Anyone who’s ever developed anything connected to a database knows about transactions. Using a transaction we can group data operations so that they happen “all or nothing”, i.e. either all of them succeed or no one does. One example is a transfer from one bank account to another: the complete transaction requires subtracting the amount to be transferred from one account and adding that same amount to the other. We wouldn’t want one to happen without the other, would we?

(yes, I know that’s not a complete description of what transactions are or do; that’s not my purpose here)

But what happens when we need the same from our filesystem?

Let’s take this scenario (which is a simplified version of a true story): we are receiving CSV files from solar panels (via SFTP) and we want to do some preprocessing and then store the data to a database. When processing them we have to generate a lot of intermediate files. After that, we want to move them to a different dir. But if something happens, say if the database is down, we want the whole thing to be cancelled so that, when we retry, we can start over.

Obviously a simple solution would be as follows:

try {
  # do a lot of hard work
  # store the data in the db
  # clean up intermediate files
  # move the CSV file to an "archive" dir
}
catch {
  # clean up intermediate files, potentially clean up any db records etc
}

That’s… well it can work but it’s not great. For example, the cleanup process itself -inside the catch block- might fail.

A much better process would be like that:

try {   
  # start a transaction
    # do a lot of hard work   
    # store the data in the db   
    # clean up intermediate files   
    # move the CSV file to an "archive" dir 
  # commit the transaction
} 
catch {   
  # rollback everything: files, db changes, the whole thing
}

That’s much cleaner! But is it possible to group db changes and filesystem changes (file create, move, delete & append, dir create & delete etc) all in one transaction?

Unix geeks are allowed to feel smug here: some flavors like HP-UX (though not Linux as far as I know) have this baked in from the get go. Your code doesn’t have to do anything special; the OS takes care of this on user request.

But as a matter of fact yes, it is also available on Windows, and it has been for some time now. The requirement is that you’re working on a file system that supports this, like NTFS.

But there’s a problem for the average .NET/Powershell coder: the standard methods, the ones inside System.IO, do not support any of this. So you have to go on a lower level, using the Windows API. Which for .NET coders, there’s no other way to put this, sucks. That’s also the reason why the Powershell implementation of file transactions (e.g. New-Item -ItemType File -UseTransaction) doesn’t work -it relies on System.IO.

I’m pretty sure that this is what crossed the minds of the developers that wrote AlphaFS which is just wonderful. It’s exactly what you’d expect but never got from Microsoft: a .NET implementation of most of System.IO classes that support NTFS’s advanced features that are not available in, say, ye olde FAT32. Chief among them is support for file system transactions.

So the example below shows how to do exactly that. I tried to keep it as simple as possible to highlight the interesting bits, but of course a real world example would be much more complicated, e.g. there would be a combined file system and database transaction, which would commit (or rollback) everything at the same time.

Note that there’s no need for an explicit rollback. As soon as the transaction scope object is disposed without calling Complete(), all changes are rolled back.

# To install:
# Prerequisite: nuget command line must be installed (see https://www.nuget.org/downloads).
#   If it's not in %path%, you need to change step 3 with its full path.
# 1. Create a directory to work into
# 2. Create a text file named "packages.config" with the following content (note the version which will change in the future):
#      <?xml version="1.0" encoding="utf-8"?>
#      <packages>
#        <package id="AlphaFS" version="2.2.6" targetFramework="net46" />
#      </packages>
# 3. Create a text file named "Install_AlphaFS.cmd" with the following content:
#      cd <your work dir>
#      nuget restore -PackagesDirectory .
# 4. Run Install_AlphaFS.cmd

$basePath = "$PSScriptRoot\..\alphatest"

# Stop on error
$ErrorActionPreference = "Stop"

# Make sure the path matches the version from step 2
Import-Module -Name "$PSScriptRoot\AlphaFS.2.2.6\lib\net40\AlphaFS.dll"

if (-not (Test-Path $basePath)) {
    New-Item -ItemType Directory -Path $basePath
}

# Check if the filesystem we're writing to supports transactions.
# On a FAT32 drive you're out of luck.
$driveRoot = [System.IO.Path]::GetPathRoot($basePath)
$driveInfo = [Alphaleonis.Win32.Filesystem.DriveInfo]($driveRoot)
if (-not $driveInfo.VolumeInfo.SupportsTransactions) {
    Write-Error "Your $driveRoot volume $($driveInfo.DosDeviceName) [$($driveInfo.VolumeLabel)] does not support transactions, exiting"
}

# That's some example data to play with.
# In reality you'll probably get data from a DB, a REST service etc.
$list = @{1="Jim"; 2="Stef"; 3="Elena"; 4="Eva"}

try {
    # Transaction starts here
    $transactionScope = [System.Transactions.TransactionScope]::new([System.Transactions.TransactionScopeOption]::RequiresNew)
    $fileTransaction = [Alphaleonis.Win32.Filesystem.KernelTransaction]::new([System.Transactions.Transaction]::Current)

    # Here we're doing random stuff with our files and dirs, just to show how this works.
    # The important thing to remember is that for the transaction to work correctly, ALL methods you use have to be -transacted.
    # I.e. you must not use AppendText() but AppendTextTransacted(), not CreateDirectory() but CreateDirectoryTransacted() etc etc.
    $logfileStream = [Alphaleonis.Win32.Filesystem.File]::AppendTextTransacted($fileTransaction, "$basePath\list.txt")
    foreach($key in $list.Keys) {
        $value = $list.$key
        $filename = "$([guid]::NewGuid()).txt"
        $dir = "$basePath\$key"

        Write-Host "Processing item $key $value"

        if (-not [Alphaleonis.Win32.Filesystem.Directory]::ExistsTransacted($fileTransaction, $dir)) {
            [Alphaleonis.Win32.Filesystem.Directory]::CreateDirectoryTransacted($fileTransaction, $dir)
        }
        [Alphaleonis.Win32.Filesystem.File]::WriteAllTextTransacted($fileTransaction, "$basePath\$key\$filename", $value)        
        $logfileStream.WriteLine("$filename;$key;$value")
    }
    $logfileStream.Close()
    
    # to simulate an error and subsequent rollback:
    Write-Error "Something not great, not terrible happened"
    
    # Commit transaction
    $transactionScope.Complete()
    Write-Host "Transaction committed, all modifications written to disk"
}
catch {
    Write-Host "An error occured and the transaction was rolled back: '$($_.Exception.Message)'"
    throw $_.Exception
}
finally {
    if ($null -ne $logfileStream -and $logfileStream -is [System.IDisposable]) {
        $logfileStream.Dispose()
        $logfileStream = $null
    }    
    if ($null -ne $transactionScope -and $transactionScope -is [System.IDisposable]) {
        $transactionScope.Dispose()
        $transactionScope = $null
    }    
}

Have fun coding!

Lefkogeia – a REST API test server

Lefkogeia is a server to test your REST APIs with. When it runs, it accepts every request made on the configured IP/port and returns an HTTP 200 “Thank you for your {method name, GET/POST etc}”. As you can imagine, I developed it for my own needs and then thought it would be handy for others, so I published it on Github.

You can download its first release here. The project’s intro page is here.

It logs all requests in a directory imaginatively called logs. It creates an access.txt file where all requests are written, and one file per request (000001.txt, 000002.txt etc) in which the request’s payload (e.g. an xml or a json) is written.

Usage

The primary use of Lefkogeia is to test/debug/troubleshoot REST API and web services clients. You run it (see release notes on that) and get your client to call it. It will log whatever was sent, allowing you to troubleshoot whatever problem you might have.

Configuration

To configure Lefkogeia, edit the appsettings.json file with a test editor.

An example appsettings.json file to serve multiple addresses & ports would be:

{
	"Logging": {
		"LogLevel": {
			"Default": "Debug",
			"System": "Information",
			"Microsoft": "Information"
		}
	},
	"Host": {
		"Url": [
			"http://localhost:6800",
			"http://server1:7777",
			"http://147.102.43.3:4545"
		]
	}
}

Paths are not yet supported in URLs, so if you change http://server1:7777 to http://server1:7777/testapi you will get an error. This is planned for the next release.

Also note that in order to use https:// you need to generate a certificate by running

dotnet dev-certs https --trust

For more info see https://go.microsoft.com/fwlink/?linkid=848054.

But why “Lefkogeia”, what does this even mean?

Because it’s such a beautiful place! Lefkogeia is a small village in southern Crete, Greece, with amazing beaches like Ammoudi, Shinaria, Klisidi and more. You can read more in Tripadvisor.

Bulk modify jobs in JAMS Scheduler

As I’ve mentioned before, at work we’re migrating all our scheduled tasks to JAMS. Now JAMS has a lot of flexibility to notify, sends emails etc but… you have to tell it to 🙂

And you can imagine that having to click-click-type-click in order to change, say, the email address in a few tens of jobs is not the creative work a developer craves for. Writing a powershell script to do that, though, is!

So here’s the script I wrote to change the email address for Warnings and Critical conditions, in bulk. Of course you can easily modify it to do whatever change you want (enable/disable a lot of jobs at once is a good example).

param(
    [string]$jamsServer = "myJamsServer", 
    [string]$jamsPath = "\somePath\someOtherPath"
)

# This script loops through all enabled JAMS jobs under a certain folder
# recursively, and changes the email address except for successes.

Import-Module Jams
$ErrorActionPreference = "Stop"
cls

try
{
    if ($null -eq (Get-PSDrive JD))
    {
        New-PSDrive JD JAMS $jamsServer -scope Local
    }
}
catch
{
    New-PSDrive JD JAMS $jamsServer -scope Local
}

$folders = New-Object System.Collections.ArrayList
$rootFolder = (Get-Item "JAMS::$($jamsServer)$($jamsPath)").Name
$folders.Add($rootFolder) | Out-Null
$childFolders = Get-ChildItem "JAMS::$($jamsServer)$($jamsPath)\*" -objecttype Folder -IgnorePredefined 
$childFolders | foreach { $folders.Add($_.Name) | Out-Null }

$rootJobs = New-Object System.Collections.ArrayList

foreach($f in $folders)
{
    Write-Host "Folder: $f"
    if ($f -eq $rootFolder)
    {
        $jobs = Get-ChildItem "JAMS::$($jamsServer)$($jamsPath)\*" -objecttype Job -IgnorePredefined -FullObject 
        $jobs | foreach { $rootJobs.Add($_.Name) | Out-Null }
    }
    else
    {
        $jobs = Get-ChildItem "JAMS::$($jamsServer)$($jamsPath)\$f\*" -objecttype Job -IgnorePredefined -FullObject 
    }

    # for test
    #$jobs | Format-Table -AutoSize

    foreach($job in $jobs)
    {
        #Write-Host "$($job.Name) : $($job.Properties["Enabled"])"
        #if you need a name filter as well, you can do:
        #if (($job.Name -notlike "*SomeString*") -or ($job.Properties["Enabled"].Value -eq $false))
        if ($job.Properties["Enabled"].Value -eq $false)
        {
            continue
        }

        $jobElements = $job.Elements
        $doUpdate = $false

        foreach($jobElement in $jobElements)
        {
            #Write-Host "$($job.Name) / $($jobElement.ElementTypeName) / $($jobElement.Description) / $($jobElement.ToString())"
            if (($jobElement.ElementTypeName -eq "SendEMail") -and ($jobElement.EntrySuccess -eq $false))
            {
                #Write-Host "$($job.Name) / $($jobElement.ElementTypeName) / $($jobElement.Description) / $($jobElement.FromAddress) / $($jobElement.ToAddress)"
                if ([string]::IsNullOrWhiteSpace($jobElement.ToAddress))
                {
                    $jobElement.FromAddress = "admin@superduperincrediblesoftware.com"
                    $jobElement.ToAddress = "someone@superduperincrediblesoftware.com;andhisdog@superduperincrediblesoftware.com"
                    $jobElement.MessageBody = "Uh, Houston, we've had a problem"      
                    $doUpdate = $true              
                }
            }
        }

        if ($doUpdate -eq $true)
        {
            $job.Update()
            Write-Host "Job $($job.Name) is updated"
        }
    }    
}

Have fun coding 🙂

Signs that you need coffee, #4

You go at the office machine and put an espresso cup in place, which can hold about 60 ml max.

You slide in an espresso capsule.

You press the “Large” button which, spoiler, produces around 110 ml coffee.

You stand in front of the machine in amazement while the coffee overflows the cup, wondering what went wrong.

You thank your good fortune that spared you the embarrassment as no colleague was in the company canteen at that time 🙂

Weird regional settings problems

If you’ve ever had to share files with data between different countries, you know that this can be problematic. For example, in Greece and the Netherlands the number “one thousand three hundred comma five” is written as “1 dot 300 comma 5”, in the UK it’s written as “1 comma 000 dot 5”, in Switzerland as “1 apostrophe 000 comma 5” etc etc. Same goes for dates.

So if you write software that is meant to be used in different countries, you have to be very careful and test thoroughly. And even then, you can run into problems. Just today I managed to solved a very weird one: Dutch-formatted numbers in an Excel file with Swiss settings caused an error message which, on the face of it, had nothing to do with formatting.

Y’know, 9/11 is the ninth of November in Greece

But the strangest, incomprehensible, 100% bang-your-head-on-the-wall problem I had was around 2005. My team wrote software that was meant to be multi-cultural and was used in Greece, Cyprus, Malta, Portugal, Turkey, Brasil and China (I may have missed a country or two after all these years).

So at some point me and my manager had to fly to Cyprus to test the software on-site; we went to a few of our customers and tried it out. And we were getting very, very, very strange error messages when doing simple, tried-and-true stuff. For a while we were flabbergasted.

After tearing my hair out and troubleshooting like crazy for hours on end, I noticed something which, while unusual, at first sight had nothing to do with our problems: our customers in Cyprus had set their Windows regional settings to use a dot as the thousand separator (according to the Greek settings) and… a dot (again) as the decimal separator (according to the UK settings).

Having tried virtually everything I changed it, just for the hell of it. I think I tried the normal Greek settings at first. And, like magic, everything was fixed! No errors whatsoever, everything ran smoothly!

You can imagine my astonishment.

I also tried a different setting (UK) and it was fine. I switched it back to the “special” Cyprus setting, and, sure enough, the problem started again. Now that I knew what to look for, I discovered that our software was “confused” (threw an error) when trying to understand just what kind of number 1 dot 234 dot 05 is.

I’m a translator for Skeptical Science

Short version: I’m honoured to be accepted as a volunteer translator for the Debunking Handbook and skepticalscience.com.

Hoaxes, myths, fake news. Unless you’ve been living under a rock (which is mildly unlikely given you’re reading a blog right now) you’ve encountered at least one, probably many. Is fluoridated water a plan to impose a communist government in the US? (no). Do airplanes spray us with chemicals to make us obedient? (no). Are the members of the UK Royal Family lizards? (no). Do vaccines cause autism? (hell no).

THEY are watching YOU (?)

Over the years, the issue has gotten me both fascinated and to the brink of despair. I’m genuinely fascinated in the way people think –or, as is unfortunately often the case, don’t. And I frequently despair when witnessing how easy it is for people, even ones that I think very highly of, to fall victims to the stupidest of conspiracy theories.

Hard as I try, I’m not immune to this myself –why would I? As a recent example, when reading that “people who curse are smarter” (yes I’m painfully aware of the irony) I immediately fell for it. It was only later that I found out that this is an existing but brutally misrepresented piece of research [link, in Greek].

Many times, when discussing with friends or family, I’ve heard yet another hoax, myth or conspiracy theory. I have then tried, and completely failed, to make my friend or relative aware of the misinformation or fallacy; and not for a lack of well-founded arguments. So I started looking for a way to effectively communicate science and, ultimately, truth.

That’s how I found the Debunking Handbook and skepticalscience.com. Upon reading it, in English, I immediately knew that that’s what I was looking for. The decision to help this effort by translating the handbook in Greek was almost a no-brainer.