How to target Azure Functions runtime versions

A function app runs on a specific version of the Azure Functions runtime. There are three major versions: 3.x, 2.x, and 1.x. By default, function apps are created in version 3.x of the runtime. This article explains how to configure a function app in Azure to run on the version you choose. For information about how to configure a local development environment for a specific version, see Code and test Azure Functions locally.

The way that you manually target a specific version depends on whether you're running Windows or Linux.

Automatic and manual version updates

This section doesn't apply when running your function app on Linux.

Azure Functions lets you target a specific version of the runtime on Windows by using the FUNCTIONS_EXTENSION_VERSION application setting in a function app. The function app is kept on the specified major version until you explicitly choose to move to a new version. If you specify only the major version, the function app is automatically updated to new minor versions of the runtime when they become available. New minor versions shouldn't introduce breaking changes.

If you specify a minor version (for example, "2.0.12345"), the function app is pinned to that specific version until you explicitly change it. Older minor versions are regularly removed from the production environment. If your minor version gets removed, your function app goes back to running on the latest version instead of the version set in FUNCTIONS_EXTENSION_VERSION. As such, you should quickly resolve any issues with your function app that require a specific minor version. Then, you can return to targeting the major version. Minor version removals are announced in App Service announcements.

Note

If you pin to a specific major version of Azure Functions, and then try to publish to Azure using Visual Studio, a dialog window will pop up prompting you to update to the latest version or cancel the publish. To avoid this, add the <DisableFunctionExtensionVersionUpdate>true</DisableFunctionExtensionVersionUpdate> property in your .csproj file.

When a new version is publicly available, a prompt in the portal gives you the chance to move up to that version. After moving to a new version, you can always use the FUNCTIONS_EXTENSION_VERSION application setting to move back to a previous version.

The following table shows the FUNCTIONS_EXTENSION_VERSION values for each major version to enable automatic updates:

Major version FUNCTIONS_EXTENSION_VERSION value
3.x ~3
2.x ~2
1.x ~1

A change to the runtime version causes a function app to restart.

Note

.NET Function apps pinned to ~2.0 opt out of the automatic upgrade to .NET Core 3.1. To learn more, see Functions v2.x considerations.

View and update the current runtime version

This section doesn't apply when running your function app on Linux.

You can change the runtime version used by your function app. Because of the potential of breaking changes, you can only change the runtime version before you've created any functions in your function app.

Important

Although the runtime version is determined by the FUNCTIONS_EXTENSION_VERSION setting, you should only make this change in the Azure portal and not by changing the setting directly. This is because the portal validates your changes and makes other related changes as needed.

Use the following procedure to view and update the runtime version currently used by a function app.

  1. In the Azure portal, browse to your function app.

  2. Under Settings, choose Configuration. In the Function runtime settings tab, locate the Runtime version. Note the specific runtime version. In the example below, the version is set to ~3.

    View the runtime version.

  3. To pin your function app to the version 1.x runtime, choose ~1 under Runtime version. This switch is disabled when you have functions in your app.

  4. When you change the runtime version, go back to the Overview tab and choose Restart to restart the app. The function app restarts running on the version 1.x runtime, and the version 1.x templates are used when you create functions.

    Restart your function app.

Note

Using the Azure portal, you can't change the runtime version for a function app that already contains functions.

The function app restarts after the change is made to the application setting.

Manual version updates on Linux

To pin a Linux function app to a specific host version, you specify the image URL in the 'LinuxFxVersion' field in site config. For example: if we want to pin a node 10 function app to say host version 3.0.13142 -

For linux app service/elastic premium apps - Set LinuxFxVersion to DOCKER|mcr.microsoft.com/azure-functions/node:3.0.13142-node10-appservice.

For linux consumption apps - Set LinuxFxVersion to DOCKER|mcr.microsoft.com/azure-functions/mesh:3.0.13142-node10.

Viewing and modifying site config settings for function apps isn't supported in the Azure portal. Use the Azure CLI instead.

The function app restarts after the change is made to the site config.

Note

For apps running in a Consumption plan, setting LinuxFxVersion to a specific image may increase cold start times. This is because pinning to a specific image prevents Functions from using some cold start optimizations.

Next steps