What is the difference between host.json and application settings?

host.json controls the Function App runtime behavior — extension bundles, function timeout, concurrency limits, retry policies, and logging configuration. It is deployed with your code and affects all functions in the app. Application settings are environment variables configured in the Azure portal, CLI, or ARM/Bicep templates. They hold connection strings, secrets, and runtime configuration that varies between environments. Application settings marked as slot-sticky remain with the deployment slot during swaps.

How do I securely manage secrets in Function App configuration?

The recommended approach is to store secrets in Azure Key Vault and reference them in application settings using Key Vault references: @Microsoft.KeyVault(SecretUri=https://myvault.vault.azure.net/secrets/mysecret). The Function App accesses the secret through a managed identity, so no credentials are stored in configuration. This requires enabling system-assigned or user-assigned managed identity and granting it Key Vault Secret User (Get) permission. Key Vault references work with Consumption, Premium, and Dedicated plans.

What are extension bundles and when should I use them?

Extension bundles automatically include all standard trigger and binding extensions (Service Bus, Storage, Cosmos DB, Event Hubs, etc.) without requiring explicit NuGet package references. They are the default for non-.NET Function Apps and simplify dependency management. You configure the bundle version range in host.json. For advanced scenarios where you need a specific extension version or a custom extension, you can disable bundles and manage extensions manually.

Azure Function App Config Builder

ServerlessAzure

Build Azure Function App configuration with runtime, scaling, CORS, and app settings.

Azure Functions Configuration

Build Azure Function App configuration settings including runtime, scaling, CORS, and app settings.

Required Fields

nameresourceGroupruntimesiteConfigappSettings

Generated Output

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About This Tool

Azure Function Apps require careful configuration of hosting plan settings, runtime stack versions, application settings, connection strings, and deployment slots. The configuration varies significantly between Consumption, Premium, and Dedicated (App Service) plans, and each runtime (Node.js, Python, .NET, Java, PowerShell) has specific version and dependency requirements. The Function App Config Builder helps you assemble host.json settings, local.settings.json for development, application settings for production, and deployment configuration including managed identity, VNet integration, and custom domain bindings.

When to Use This Tool

•Building host.json configurations with correct extension bundles, logging levels, and concurrency settings for a Python Function App
•Generating application settings with managed identity references for Key Vault secrets instead of hardcoded connection strings
•Configuring Function App scaling limits, pre-warmed instances, and runtime scale monitoring for Premium plan deployments
•Creating deployment slot settings with slot-sticky application settings for staging environments that do not swap to production

Frequently Asked Questions

What is the difference between host.json and application settings?: host.json controls the Function App runtime behavior — extension bundles, function timeout, concurrency limits, retry policies, and logging configuration. It is deployed with your code and affects all functions in the app. Application settings are environment variables configured in the Azure portal, CLI, or ARM/Bicep templates. They hold connection strings, secrets, and runtime configuration that varies between environments. Application settings marked as slot-sticky remain with the deployment slot during swaps.
How do I securely manage secrets in Function App configuration?: The recommended approach is to store secrets in Azure Key Vault and reference them in application settings using Key Vault references: @Microsoft.KeyVault(SecretUri=https://myvault.vault.azure.net/secrets/mysecret). The Function App accesses the secret through a managed identity, so no credentials are stored in configuration. This requires enabling system-assigned or user-assigned managed identity and granting it Key Vault Secret User (Get) permission. Key Vault references work with Consumption, Premium, and Dedicated plans.
What are extension bundles and when should I use them?: Extension bundles automatically include all standard trigger and binding extensions (Service Bus, Storage, Cosmos DB, Event Hubs, etc.) without requiring explicit NuGet package references. They are the default for non-.NET Function Apps and simplify dependency management. You configure the bundle version range in host.json. For advanced scenarios where you need a specific extension version or a custom extension, you can disable bundles and manage extensions manually.

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