Azure Functions developer guide

• 10/12/2017
• 10 minutes to read
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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 and one or more subfolders. Each subfolder contains the code for a separate function. The folder structure is shown in the following representation:

FunctionApp
 | - host.json
 | - MyFirstFunction
 | | - function.json
 | | - ...  
 | - MySecondFunction
 | | - function.json
 | | - ...  
 | - SharedCode
 | - bin

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

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. See the language-specific requirements for a function app project:

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

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. 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 with Storage1 as its name, Storage1__serviceUri would be used for the serviceUri property of the connection. The connection properties are different for each service. Refer to the documentation for the extension that uses the connection.

Configure an identity-based connection

Some connections in Azure Functions are 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.

Identity-based connections are supported by the following trigger and binding extensions:

Connection properties

An identity-based connection for an Azure service accepts the following properties:

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

When hosted in the Azure Functions service, identity-based connections use a managed identity. The system-assigned identity is used by default. When run in other contexts, such as local development, your developer identity is used instead, although this can be customized using alternative connection parameters.

Local development

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.

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.

To connect using an Azure Active Directory service principal with a client ID and secret, define the connection with the following required properties in addition to the Connection properties above:

Example of local.settings.json properties required for identity-based connection with Azure Blob:

{
  "IsEncrypted": false,
  "Values": {
    "<CONNECTION_NAME_PREFIX>__serviceUri": "<serviceUri>",
    "<CONNECTION_NAME_PREFIX>__tenantId": "<tenantId>",
    "<CONNECTION_NAME_PREFIX>__clientId": "<clientId>",
    "<CONNECTION_NAME_PREFIX>__clientSecret": "<clientSecret>"
  }
}

Grant permission to the identity

Whatever identity is being used must have permissions to perform the intended actions. This is typically done by assigning a role in Azure RBAC or specifying the identity in an access policy, depending on the service to which you are connecting. Refer to the documentation for each service on what permissions are needed and how they can be set.

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