In Internet Information Services (IIS), application pools have two distinct but related startup controls:

• "Start application pool immediately": A basic setting checkbox that forces immediate initialization
• Start Mode (Advanced Setting): Offers "OnDemand" or "AlwaysRunning" configuration options

The "Start application pool immediately" setting performs a one-time action during IIS configuration changes. When checked, it:

• Triggers immediate startup of the worker process
• Doesn't persist after server reboots
• Primarily used for testing configurations

Contrast this with the Start Mode setting "AlwaysRunning", which:

• Persists across server reboots
• Guarantees the application pool starts automatically with IIS
• Works in conjunction with Application Initialization Module

Here's how to configure these settings programmatically using PowerShell:

# Set "Start immediately" behavior (one-time)
Import-Module WebAdministration
Start-WebAppPool -Name "MyAppPool"

# Configure persistent AlwaysRunning mode
Set-ItemProperty "IIS:\AppPools\MyAppPool" -Name startMode -Value "AlwaysRunning"

# Verify settings
Get-ItemProperty "IIS:\AppPools\MyAppPool" | Select-Object Name, State, StartMode

The "AlwaysRunning" mode significantly impacts:

• Cold start performance - eliminates initial load delay
• Memory usage - maintains worker processes continuously
• Application warmup - enables pre-loading of applications

For optimal performance in AlwaysRunning mode, combine with these applicationHost.config settings:

<applicationPools>
    <add name="MyAppPool" startMode="AlwaysRunning">
        <processModel idleTimeout="00:00:00" />
    </add>
</applicationPools>

<sites>
    <application path="/" applicationPool="MyAppPool">
        <virtualDirectory path="/" physicalPath="C:\sites\mysite" />
        <application preloadEnabled="true" />
    </application>
</sites>

In IIS (Internet Information Services) application pool configuration, two settings often cause confusion for developers and administrators:

• Start application pool immediately: A basic setting checkbox in the IIS Manager
• Start Mode: An advanced setting with "OnDemand" and "AlwaysRunning" options

"Start application pool immediately" affects the initial state when IIS starts:

// PowerShell equivalent for immediate start
Import-Module WebAdministration
Set-ItemProperty "IIS:\AppPools\MyAppPool" -Name autoStart -Value $true

Start Mode="AlwaysRunning" is part of the IIS Application Initialization module and works differently:

// Web.config implementation example
<applicationPools>
    <add name="MyAppPool" startMode="AlwaysRunning" />
</applicationPools>

For high-availability applications, combining both settings is often optimal:

// Recommended configuration for production
<applicationPools>
    <add name="CriticalAppPool" 
         autoStart="true"
         startMode="AlwaysRunning"
         startImmediately="true">
    </add>
</applicationPools>

• Start Immediately: Development environments where quick restarts are needed
• AlwaysRunning: Production environments requiring zero-downtime deployments
• Both: Enterprise applications with strict SLA requirements

If you're experiencing unexpected behavior:

# Check current settings in PowerShell
Get-ItemProperty "IIS:\AppPools\MyAppPool" | 
    Select-Object name, autoStart, startMode