.NET (PowerShell) Concurrent file usage (locking, while still allowing read access)

Question

I have an XML file that I need to have concurrent access to. I do not need concurrent write, but I do need concurrent read. The perscribed method for reading/modifying this XML file is by using a PowerShell script, which uses Linq to XML (XElement) to work with the XML file.

The ideal scenario is:

1. All users on the team will run this PowerShell script at the beginning of their workday, and it will remain running until the end of the workday.
2. Throughout the workday, any user on the team will enter pre-defined commands that will query the XML file to collect data. Each time the user runs one of these queries, it will open the XML file, perform the query, close the XML file, then display the results.
3. Occasionally, throughout the day, any of those users will need to modify the XML file. The user will enter a pre-defined command, specifying which node they want to modify, then they will enter all of the data they would like to add/modify. The script will then open the file (locking it, so no other user can open it read/write), read the XML file into an XElement object, perform the desired operations, then save the XElement back to the file, and then release the lock.
4. At any point, while a user is modifying a file (some operations may be lengthy), any other user who tries to make a modification must not be able to open the file (I will catch the exception, and present a "Please wait a few moments" type message). But, they must be able to open the file read-only to execute queries.

I have seen this related post, 'Concurrent file usage in C#', and attempted to implement this, using the below code. In one instance of PowerShell ISE, I execute the Read-Write function, and then while in the "Enter an item name" loop, I execute the Read-Only function in another instance of PowerShell ISE. When attempting to open the document read-only, I receive an exception stating The process cannot access the file 'C:\Path\To\file.xml' because it is being used by another process.

using namespace System.Linq.Xml
using namespace System.IO
Add-Type -AssemblyName 'System.Xml.Linq (rest of Assembly's Full Name)'

$filename = "C:\Path\To\file.xml"

function Read-Only()
{
    $filestream = [FileStream]::new($filename, [FileMode]::Open, [FileAccess]::Read)
    $database = [XElement]::Load($filestream)
    foreach($item in $database.Element("Items").Elements("Item"))
    {
        Write-Host "Item name $($item.Attribute("Name").Value)"
    }
    $filestream.Close()
    $filestream.Dispose()
}
function Read-Write()
{
    $filestream = [FileStream]::new($filename, [FileMode]::Open, [FileAccess]::ReadWrite, [FileShare]::Read)
    $database = [XElement]::Load($filestream)

    $itemname = Read-Host "Enter an item name ('quit' to quit)"
    while($itemname -ne 'quit')
    {
        $database.Element("Items").Add([XElement]::new([XName]"Item", [XAttribute]::new([XName]"Name", $itemname)))
        $itemname = Read-Host "Enter an item name ('quit' to quit)"
    }

    $filestream.Seek(0, [SeekOrigin]::Begin)
    $database.Save($filestream)

    $filestream.Close()
    $filestream.Dispose()
}

How can I lock a file for exclusive editing, while still allowing read-only access to any other client?

Edit: I have solved this problem by using the solution suggested by mklement0 - using a seperate file as a lock file. Code is posted here: https://pastebin.com/ytzGE7se

Answer 1

If you use [FileAccess]::Read without specifying a file-share mode explicitly, then if the file is already open, opening it again will only succeed if it was originally opened with file-share mode [FileShare]::ReadWrite (even though you're only asking for read access, the method defaults to requesting write access too) - whereas your Read-Write function (sensibly) uses just [FileShare]::Read.

Your immediate problem goes away if you explicitly open your read-only filestream with file-share mode [FileShare]::ReadWrite:

[System.IO.FileStream]::new(
  $path, 
  [System.IO.FileMode]::Open, 
  [System.IO.FileAccess]::Read, 
  [System.IO.FileShare]::ReadWrite  # !! required
)

This allows both other readers concurrent access as well as the one and only (read+)writer (which may have opened the file first).

However, a file getting rewritten while being read by others can be problematic, so for robust and predictable operation I suggest a different approach:

Note: This answer to a related question shows a simpler alternative to the solution below, which, however, takes longer to update the file.

---

Make modifications in a temporary copy of your file, then replace the original.

This requires explicit synchronization to coordinate between would-be updaters so as to serialize updates in order to prevent updates from overwriting each other.
You could achieve this with a separate lock file (sentinel file) named, say, updating, which acts as an indicator to other would-be writers that an update is in progress.

^{Mike Christiansen (the OP himself) ended up also storing the username of the locking user in that file, to provide feedback to other would-be lockers.}

When modification is requested:

• Keep trying in a loop until creating lock file updating succeeds, failing if the file already exists.

  • (Re)-creating the updating file signals to other would-be writers that a modification has started. Readers, by contrast, can continue to read at this point.

• Create a (temporary) copy of the current XML file and perform the modifications there.

  • Mike himself ended up simply modifying an in-memory copy of the file, which simplifies matters (if you have enough memory to read the file as a whole).

• Replace the original file with the modified copy - use a retry loop until copying over / rewriting the original succeeds, given that other readers may temporarily prevent deleting (re-creating) / rewriting the original file.

• Delete file updating, signaling to other would-be modifiers that the update has completed.

  • Be sure that the file is always cleaned up (try .. finally), because letting it linger would block future updates; you may also need a timeout-based mechanism that on waiting for a preexisting file to be deleted eventually forces deletion, if it can be assumed that the previous updater has crashed.

As for read access:

• While no modifications are taking place, concurrent read access should work fine.

• While the XML file is being replaced with the temporary copy / rewritten, opening the file for reading will fail for the duration of the file-copy operation / write operation, so you'll need a retry loop there too.

---

The code Mike ultimately used can be found here.

Answer 2

Maybe this can help. When you create a filestream there is an FileShare option. If you set it to ReadWrite, multiple processes can open that file

$fsMain = [System.IO.File]::Open("C:\stack\out.txt", "Open", "ReadWrite", "ReadWrite")
$fsReadOnly = [System.IO.File]::Open("C:\stack\out.txt", "Open", "Read", "ReadWrite")

Write-Host ("fsMain:  CanRead=" + $fsMain.CanRead + ", CanWrite=" + $fsMain.CanWrite)
Write-Host ("fsReadOnly:  CanRead= " + $fsReadOnly.CanRead + ", CanWrite=" + $fsReadOnly.CanWrite)

$fsMain.Close()
$fsReadOnly.Close()

Answer 3

May I suggest a slightly different approach to it all. You can do the following with PowerShell which allows you to avoid the issue on the reads. My XML navigation may be a little off, but it will work as described here.

function Read-Only()
{
    $database = [xml](Get-Content $filename)
    foreach($item in $database.Items)
    {
        Write-Host "Item name $item.Name"
    }
 }

Now you can keep your same write function or you can do it all with PowerShell. If you wanted to do it with PowerShell you can do something like this.

function Read-Write()
{
    $database = [xml](Get-Content $filename)

    $itemname = Read-Host "Enter an item name ('quit' to quit)"
    while($itemname -ne 'quit')
    {
        $newItem = $database.CreateElement("item")
        $newItem.SetAttribute("Name", $itemname)
        $database.Items.AppendChild($newItem)
        # You can also save all the changes up and write once.
        $database.Save($filename)
        $itemname = Read-Host "Enter an item name ('quit' to quit)"
    }
}