Azure Functions Triggers and Bindings

Last updated: October 26, 2023

Introduction

Azure Functions provides a powerful and flexible way to build serverless applications. At its core, Azure Functions relies on triggers and bindings to connect your code to other Azure services, as well as to on-premises or other cloud services.

A trigger defines how a function is executed. It's what causes your function to run. A function must have exactly one trigger.

Bindings allow you to connect your function to data sources and other services without explicitly writing client code to manage them. Bindings declare these connections, making your code cleaner and easier to manage.

Trigger Types

Triggers can be categorized based on how they initiate function execution:

• Event-driven Triggers: Functions are invoked in response to events from other Azure services or external sources. Examples include HTTP requests, new messages in a queue, or a file being added to blob storage.
• Scheduled Triggers: Functions are executed on a schedule, often defined using a CRON expression. This is useful for background tasks or periodic data processing.

Binding Types

Bindings can be classified into two main categories:

• Input Bindings: Used to read data from a service and pass it as parameters to your function.
• Output Bindings: Used to write data from your function to a service.

A single function can have multiple input and output bindings, in addition to its trigger.

Declarative Bindings

Bindings are declarative, meaning you define them in your function's configuration file (e.g., function.json for JavaScript/C#/etc., or attributes for C#/.NET). This separation of concerns makes your code more readable and maintainable.

Binding Syntax

The syntax for defining bindings varies slightly depending on the language and runtime. However, the core concept involves specifying the type of binding, the direction (input, output, or inout), and other relevant properties like path, connection, or queueName.

Example function.json

Here's a common example of a function.json file for an HTTP-triggered function that reads from a blob:

{
  "bindings": [
    {
      "authLevel": "function",
      "type": "httpTrigger",
      "direction": "in",
      "name": "req",
      "methods": [
        "get",
        "post"
      ]
    },
    {
      "type": "http",
      "direction": "out",
      "name": "res"
    },
    {
      "type": "blob",
      "direction": "in",
      "name": "myBlob",
      "path": "input-container/{name}.txt",
      "connection": "AzureWebJobsStorage"
    }
  ]
}
        

Common Bindings

Azure Functions supports a wide range of bindings for various services. Some of the most common include:

• HTTP: Trigger on HTTP requests and return HTTP responses.
• Blob Storage: Read from and write to Azure Blob Storage.
• Queue Storage: Read from and write to Azure Queue Storage.
• Cosmos DB: Interact with Azure Cosmos DB, including triggering on changes.
• Event Hubs: Process events from Azure Event Hubs.
• Service Bus: Send and receive messages from Azure Service Bus.
• Timer: Execute functions on a schedule.

Trigger Examples

HTTP Trigger

Invoked by an HTTP request.

{
  "type": "httpTrigger",
  "name": "req",
  "direction": "in",
  "methods": ["get", "post"]
}
        

Timer Trigger

Runs on a specified schedule.

{
  "type": "timerTrigger",
  "name": "myTimer",
  "direction": "in",
  "schedule": "0 */5 * * * *"
}
        

Blob Trigger

Triggered when a blob is created or updated.

{
  "type": "blobTrigger",
  "name": "myBlob",
  "direction": "in",
  "path": "samples-workitems/{name}.json"
}
        

Binding Configuration

Bindings are configured in the function.json file (or using attributes in C#). Each binding definition requires at least a type, direction, and a name, which is how the binding is referenced in your code.

Common Configuration Properties:

Property	Description
type	The type of the binding (e.g., httpTrigger, blob, queue).
direction	The direction of the binding: in, out, or inout.
name	The name of the parameter in your function code that the binding maps to.
path	For blob and file bindings, the path to the blob or file. Can include patterns.
connection	The name of the app setting that contains the connection string for the service.
queueName	For queue bindings, the name of the queue.
schedule	For timer triggers, the CRON expression defining the schedule.

Advanced Topics

Binding Expressions

Binding expressions allow you to dynamically reference properties of the trigger event or other bindings. For example, in a blob trigger, you can use {name} to refer to the blob's name in the path property of another binding.

Custom Bindings

For services not covered by built-in bindings, you can create your own custom bindings.