Azure Functions developer guide

In Azure Functions, specific functions share a few core technical concepts and components, regardless of the language or binding you use. Before you jump into learning details specific to a given language or binding, be sure to read through this overview that applies to all of them.

This article assumes that you've already read the Azure Functions overview.

Function code

A function is the primary concept in Azure Functions. A function contains two important pieces - your code, which can be written in a variety of languages, and some config, the function.json file. For compiled languages, this config file is generated automatically from annotations in your code. For scripting languages, you must provide the config file yourself.

The function.json file defines the function's trigger, bindings, and other configuration settings. Every function has one and only one trigger. The runtime uses this config file to determine the events to monitor and how to pass data into and return data from a function execution. The following is an example function.json file.

{
    "disabled":false,
    "bindings":[
        // ... bindings here
        {
            "type": "bindingType",
            "direction": "in",
            "name": "myParamName",
            // ... more depending on binding
        }
    ]
}

For more information, see Azure Functions triggers and bindings concepts.

The bindings property is where you configure both triggers and bindings. Each binding shares a few common settings and some settings which are specific to a particular type of binding. Every binding requires the following settings:

Function app

A function app provides an execution context in Azure in which your functions run. As such, it is the unit of deployment and management for your functions. A function app is comprised of one or more individual functions that are managed, deployed, and scaled together. All of the functions in a function app share the same pricing plan, deployment method, and runtime version. Think of a function app as a way to organize and collectively manage your functions. To learn more, see How to manage a function app.

Folder structure

The code for all the functions in a specific function app is located in a root project folder that contains a host configuration file. The host.json file contains runtime-specific configurations and is in the root folder of the function app. A bin folder contains packages and other library files that the function app requires. Specific folder structures required by the function app depend on language:

• C# compiled (.csproj)
• C# script (.csx)
• F# script
• Java
• JavaScript
• Python

In version 2.x and higher of the Functions runtime, all functions in the function app must share the same language stack.

The above is the default (and recommended) folder structure for a Function app. If you wish to change the file location of a function's code, modify the scriptFile section of the function.json file. We also recommend using package deployment to deploy your project to your function app in Azure. You can also use existing tools like continuous integration and deployment and Azure DevOps.

Use local tools and publishing

Function apps can be authored and published using a variety of tools, including Visual Studio, Visual Studio Code, IntelliJ, Eclipse, and the Azure Functions Core Tools. For more information, see Code and test Azure Functions locally.

How to edit functions in the Azure portal

The Functions editor built into the Azure portal lets you update your code and your function.json file directly inline. This is recommended only for small changes or proofs of concept - best practice is to use a local development tool like VS Code.

Parallel execution

When multiple triggering events occur faster than a single-threaded function runtime can process them, the runtime may invoke the function multiple times in parallel. If a function app is using the Consumption hosting plan, the function app could scale out automatically. Each instance of the function app, whether the app runs on the Consumption hosting plan or a regular App Service hosting plan, might process concurrent function invocations in parallel using multiple threads. The maximum number of concurrent function invocations in each function app instance varies based on the type of trigger being used as well as the resources used by other functions within the function app.

Functions runtime versioning

You can configure the version of the Functions runtime using the FUNCTIONS_EXTENSION_VERSION app setting. For example, the value "~3" indicates that your function app will use 3.x as its major version. Function apps are upgraded to each new minor version as they are released. For more information, including how to view the exact version of your function app, see How to target Azure Functions runtime versions.

Repositories

The code for Azure Functions is open source and stored in GitHub repositories:

• Azure Functions
• Azure Functions host
• Azure Functions portal
• Azure Functions templates
• Azure WebJobs SDK
• Azure WebJobs SDK Extensions

Bindings

Here is a table of all supported bindings.

This table shows the bindings that are supported in the major versions of the Azure Functions runtime:

¹ Starting with the version 2.x runtime, all bindings except HTTP and Timer must be registered. See Register binding extensions.

² Triggers aren't supported in the Consumption plan. Requires runtime-driven triggers.

³ Supported only in Kubernetes, IoT Edge, and other self-hosted modes only.

Having issues with errors coming from the bindings? Review the Azure Functions Binding Error Codes documentation.

Connections

Your function project references connection information by name from its configuration provider. It does not directly accept the connection details, allowing them to be changed across environments. For example, a trigger definition might include a connection property. This might refer to a connection string, but you cannot set the connection string directly in a function.json. Instead, you would set connection to the name of an environment variable that contains the connection string.

The default configuration provider uses environment variables. These might be set by Application Settings when running in the Azure Functions service, or from the local settings file when developing locally.

Connection values

When the connection name resolves to a single exact value, the runtime identifies the value as a connection string, which typically includes a secret. The details of a connection string are defined by the service to which you wish to connect.

However, a connection name can also refer to a collection of multiple configuration items, useful for configuring identity-based connections. Environment variables can be treated as a collection by using a shared prefix that ends in double underscores __. The group can then be referenced by setting the connection name to this prefix.

For example, the connection property for a Azure Blob trigger definition might be "Storage1". As long as there is no single string value configured by an environment variable named "Storage1", an environment variable named Storage1__blobServiceUri could be used to inform the blobServiceUri property of the connection. The connection properties are different for each service. Refer to the documentation for the component that uses the connection.

Configure an identity-based connection

Some connections in Azure Functions can be configured to use an identity instead of a secret. Support depends on the extension using the connection. In some cases, a connection string may still be required in Functions even though the service to which you are connecting supports identity-based connections. For a tutorial on configuring your function apps with managed identities, see the creating a function app with identity-based connections tutorial.

Identity-based connections are supported by the following components:

When hosted in the Azure Functions service, identity-based connections use a managed identity. The system-assigned identity is used by default, although a user-assigned identity can be specified with the credential and clientID properties. When run in other contexts, such as local development, your developer identity is used instead, although this can be customized. See Local development with identity-based connections.

Grant permission to the identity

Whatever identity is being used must have permissions to perform the intended actions. You will need to assign a role in Azure RBAC, using either built-in or custom roles which provide those permissions.

Choose a tab below to learn about permissions for each component:

Common properties for identity-based connections

An identity-based connection for an Azure service accepts the following common properties, where <CONNECTION_NAME_PREFIX> is the value of your connection property in the trigger or binding definition:

Additional options may be supported for a given connection type. Please refer to the documentation for the component making the connection.

Local development with identity-based connections

When running locally, the above configuration tells the runtime to use your local developer identity. The connection will attempt to get a token from the following locations, in order:

• A local cache shared between Microsoft applications
• The current user context in Visual Studio
• The current user context in Visual Studio Code
• The current user context in the Azure CLI

If none of these options are successful, an error will occur.

Because this is using the your developer identity, you may already have some roles against development resources, but they may not provide data access. Management roles like Owner are not sufficient. Double-check what permissions are required for connections for each component, and make sure that you have them assigned to yourself.

In some cases, you may wish to specify use of a different identity. You can add configuration properties for the connection that point to the alternate identity based on a client ID and client Secret for an Azure Active Directory service principal. This configuration option is not supported when hosted in the Azure Functions service. To use an ID and secret on your local machine, define the connection with the following additional properties:

Here is an example of local.settings.json properties required for identity-based connection to Azure Blobs:

{
  "IsEncrypted": false,
  "Values": {
    "<CONNECTION_NAME_PREFIX>__blobServiceUri": "<blobServiceUri>",
    "<CONNECTION_NAME_PREFIX>__queueServiceUri": "<queueServiceUri>",
    "<CONNECTION_NAME_PREFIX>__tenantId": "<tenantId>",
    "<CONNECTION_NAME_PREFIX>__clientId": "<clientId>",
    "<CONNECTION_NAME_PREFIX>__clientSecret": "<clientSecret>"
  }
}

Connecting to host storage with an identity (Preview)

Azure Functions by default uses the "AzureWebJobsStorage" connection for core behaviors such as coordinating singleton execution of timer triggers and default app key storage. This can be configured to leverage an identity as well.

To use an identity-based connection for "AzureWebJobsStorage", configure the following app settings:

Common properties for identity-based connections may also be set as well.

If you are using a storage account that uses the default DNS suffix and service name for global Azure, following the https://<accountName>.blob/queue/file/table.core.windows.net format, you can instead set AzureWebJobsStorage__accountName to the name of your storage account. The blob and queue endpoints will be inferred for this account. This will not work if the storage account is in a sovereign cloud or has a custom DNS.

You will need to create a role assignment that provides access to the storage account for "AzureWebJobsStorage" at runtime. Management roles like Owner are not sufficient. The Storage Blob Data Owner role covers the basic needs of Functions host storage - the runtime needs both read and write access to blobs and the ability to create containers. There are some situations, as with use of the blob trigger, where Storage Queue Data Contributor may be needed as well. You may need additional permissions if you use "AzureWebJobsStorage" for any other purposes.

Reporting Issues

Next steps

For more information, see the following resources:

• Azure Functions triggers and bindings
• Code and test Azure Functions locally
• Best Practices for Azure Functions
• Azure Functions C# developer reference
• Azure Functions Node.js developer reference