Question

I am writing a PowerShell provider in C#. The provider exposes an applications domain objects through a drive-like interface. For example:

my:\Users\[e-mail]
my:\Customers\Marty

This data ultimately comes from a database.

I have been unable to find any great guidance for when you should go to the database for data, and when you should cache it. I find that PowerShell calls methods like ItemExists and GetChildNames many times; often repeatedly for the same command. It is impractical to go to the database 5 or 6 times just because they hit Tab for auto-complete, for example.

But at the same time, as a user at the command prompt, if I type Get-ChildItem (dir) and see the list, then do something outside PowerShell so that I know the data is refreshed, taking another directory listing should expect to see any changes to the database.

I feel that if I knew the right term to describe my problem (in PowerShell parlance) I would be able to Google the answer or find an existing duplicate question, but I’m stuck.

Answer 1

This has very little to do with powershell and everything to do with your data, and how important it is to refresh it. A simple caching scheme would be to use a time based system whereby after N minutes, a request to your back end’s data layer would pull a fresh copy and reset the timer. It seems you already have an idea what your particular rules should be. I don’t think two successive “dir” commands should always result in two pulls from the backing store, but you do think so for your system. So make it so.

UPDATE

Perhaps a simple guiding principle might be that you should only refresh your data once per provider command issued. The list of built-in commands that operate on provider items consists of:

• Clear-Item
• Copy-Item
• Get-Item
• Invoke-Item
• Move-Item
• New-Item
• Remove-Item
• Rename-Item
• Set-Item

Additionally, the list of built-in commands that operate on provider item properties consists of:

• Clear-ItemProperty
• Copy-ItemProperty
• Get-ItemProperty
• Move-ItemProperty
• New-ItemProperty
• Remove-ItemProperty
• Rename-ItemProperty
• Set-ItemProperty

And finally, for reading/writing content, we use:

• Add-Content
• Clear-Content
• Get-Content
• Set-Content

Each of these commands has a corresponding method in NavigationCmdletProvider (for hierarchical datastores) and this is where you might want to refresh your data. When implementing the New/Move/Rename/Remove/Set/Clear and other data changing methods, you should use some kind of optimistic concurrency methodology as provider instances in PowerShell are not singletons; there may be one or more instances in play at any time.

I wrote a provider that takes its implementation from script that you may find easier to prototype things in. See http://psprovider.codeplex.com/

Hope this helps.