Azure Functions Triggers Overview

Triggers are how you invoke an Azure Function. A trigger defines how a function is initiated. Each function must have exactly one trigger. Triggers can be based on a schedule, an event from another Azure service, or an HTTP request.

Common Trigger Types

Azure Functions supports a wide variety of triggers, allowing your functions to respond to numerous events. Here are some of the most commonly used trigger types:

Trigger Type	Description	Example Use Case
HTTP Trigger	Executes when an HTTP request is received. Provides a webhook-like capability.	Building REST APIs, webhooks for external services.
Timer Trigger	Executes on a schedule defined by a CRON expression.	Scheduled tasks, recurring data processing.
Blob Storage Trigger	Executes when a blob is added, updated, or deleted in Azure Blob Storage.	Image processing upon upload, data transformation for new files.
Queue Storage Trigger	Executes when a message is added to an Azure Storage Queue.	Processing messages from a work queue, background task execution.
Service Bus Trigger	Executes when a message arrives in an Azure Service Bus Queue or Topic.	Handling critical messages, reliable message processing.
Event Grid Trigger	Executes in response to Azure Event Grid events.	Responding to resource changes, orchestrating events across services.
Cosmos DB Trigger	Executes when documents are added or updated in an Azure Cosmos DB collection.	Real-time data processing, event sourcing.

Trigger Configuration

Triggers are configured in the function.json file (or via attributes in code for languages like C#). The configuration specifies the type of trigger, the direction (in for triggers), and the connection details.

Example: HTTP Trigger Configuration

{
  "scriptFile": "../run.py",
  "bindings": [
    {
      "authLevel": "function",
      "type": "httpTrigger",
      "direction": "in",
      "name": "req",
      "methods": [
        "get",
        "post"
      ]
    },
    {
      "type": "http",
      "direction": "out",
      "name": "res"
    }
  ]
}

Example: Timer Trigger Configuration

{
  "scriptFile": "../run.py",
  "bindings": [
    {
      "type": "timerTrigger",
      "direction": "in",
      "name": "myTimer",
      "schedule": "0 */5 * * * *"
    }
  ]
}

Key Concepts

Event-driven: Functions are designed to be event-driven, meaning they react to specific events rather than running continuously.
Single Trigger per Function: Each function can only have one trigger. If you need to respond to multiple event types, you'll need multiple functions or a single function that handles different events internally.
Bindings: Triggers are a type of binding. Bindings simplify development by decoupling your function code from the underlying service integrations.
CRON Expressions: Used for scheduling timer triggers. They define a recurring schedule in a standardized format.