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Triggers and bindings in Azure Functions
Learn to use triggers and bindings to connect your Azure Function to online events and cloud-based services.
conceptual
05/25/2022
devdivchpfy22

Azure Functions triggers and bindings concepts

In this article, you learn the high-level concepts surrounding functions triggers and bindings.

Triggers cause a function to run. A trigger defines how a function is invoked and a function must have exactly one trigger. Triggers have associated data, which is often provided as the payload of the function.

Binding to a function is a way of declaratively connecting another resource to the function; bindings may be connected as input bindings, output bindings, or both. Data from bindings is provided to the function as parameters.

You can mix and match different bindings to suit your needs. Bindings are optional and a function might have one or multiple input and/or output bindings.

Triggers and bindings let you avoid hardcoding access to other services. Your function receives data (for example, the content of a queue message) in function parameters. You send data (for example, to create a queue message) by using the return value of the function.

Consider the following examples of how you could implement different functions.

Example scenario Trigger Input binding Output binding
A new queue message arrives which runs a function to write to another queue. Queue* None Queue*
A scheduled job reads Blob Storage contents and creates a new Cosmos DB document. Timer Blob Storage Cosmos DB
The Event Grid is used to read an image from Blob Storage and a document from Cosmos DB to send an email. Event Grid Blob Storage and Cosmos DB SendGrid
A webhook that uses Microsoft Graph to update an Excel sheet. HTTP None Microsoft Graph

* Represents different queues

These examples aren't meant to be exhaustive, but are provided to illustrate how you can use triggers and bindings together.

Trigger and binding definitions

Triggers and bindings are defined differently depending on the development language.

Language Triggers and bindings are configured by...
C# class library      decorating methods and parameters with C# attributes
Java      decorating methods and parameters with Java annotations
JavaScript/PowerShell/Python/TypeScript      updating function.json (schema)

For languages that rely on function.json, the portal provides a UI for adding bindings in the Integration tab. You can also edit the file directly in the portal in the Code + test tab of your function. Visual Studio Code lets you easily add a binding to a function.json file by following a convenient set of prompts.

In .NET and Java, the parameter type defines the data type for input data. For instance, use string to bind to the text of a queue trigger, a byte array to read as binary, and a custom type to de-serialize to an object. Since .NET class library functions and Java functions don't rely on function.json for binding definitions, they can't be created and edited in the portal. C# portal editing is based on C# script, which uses function.json instead of attributes.

To learn more about how to add bindings to existing functions, see Connect functions to Azure services using bindings.

For languages that are dynamically typed such as JavaScript, use the dataType property in the function.json file. For example, to read the content of an HTTP request in binary format, set dataType to binary:

{
    "dataType": "binary",
    "type": "httpTrigger",
    "name": "req",
    "direction": "in"
}

Other options for dataType are stream and string.

Binding direction

All triggers and bindings have a direction property in the function.json file:

  • For triggers, the direction is always in
  • Input and output bindings use in and out
  • Some bindings support a special direction inout. If you use inout, only the Advanced editor is available via the Integrate tab in the portal.

When you use attributes in a class library to configure triggers and bindings, the direction is provided in an attribute constructor or inferred from the parameter type.

Add bindings to a function

You can connect your function to other services by using input or output bindings. Add a binding by adding its specific definitions to your function. To learn how, see Add bindings to an existing function in Azure Functions.

Supported bindings

[!INCLUDE Full bindings table]

For information about which bindings are in preview or are approved for production use, see Supported languages.

Bindings code examples

Use the following table to find examples of specific binding types that show you how to work with bindings in your functions. First, choose the language tab that corresponds to your project.

[!INCLUDE functions-bindings-code-example-chooser]

Custom bindings

You can create custom input and output bindings. Bindings must be authored in .NET, but can be consumed from any supported language. For more information about creating custom bindings, see Creating custom input and output bindings.

Resources

Next steps

[!div class="nextstepaction"] Register Azure Functions binding extensions