Azure Functions Triggers

Introduction to Triggers

Azure Functions allows you to build event-driven applications. A trigger defines how a function is invoked. It's the piece of code that listens for events and starts your function when those events occur. Triggers are a fundamental concept in Azure Functions, enabling serverless computing by automatically scaling and executing code in response to various events.

Each function must have exactly one trigger. The trigger type determines the event that starts your function and the data that is passed into it. Azure Functions supports a wide variety of triggers for different services and events within the Azure ecosystem and beyond.

Common Trigger Types

Azure Functions provides built-in triggers for many common services. Here are some of the most frequently used ones:

HTTP Trigger

The HTTP trigger allows your function to be invoked via an HTTP request. This is ideal for building web APIs, webhooks, or any scenario where you need to respond to incoming HTTP requests.

When an HTTP trigger is used, the incoming HTTP request object is passed to your function. You can configure the allowed HTTP methods (GET, POST, etc.) and authorization levels.

// Example: Node.js HTTP Trigger
module.exports = async function (context, req) {
    context.log('JavaScript HTTP trigger function processed a request.');
    const name = (req.query.name || (req.body && req.body.name));
    const responseMessage = name
        ? "Hello, " + name + ". This HTTP triggered function executed successfully."
        : "This HTTP triggered function executed successfully. Pass a name in the query string or in the request body for a personalized response.";

    context.res = {
        body: responseMessage
    };
};
            

Timer Trigger

The Timer trigger executes your function on a schedule. This is useful for tasks that need to run periodically, such as background jobs, scheduled cleanups, or data synchronization.

Timers are configured using a CRON expression, which defines the schedule for execution.

// Example: Timer Trigger configuration (function.json)
{
  "schedule": "0 */5 * * * *", // Runs every 5 minutes
  "bindings": [
    {
      "name": "myTimer",
      "type": "timerTrigger",
      "direction": "in",
      "schedule": "0 */5 * * * *"
    }
  ]
}
            

Queue Trigger

The Queue trigger invokes your function when a message is added to an Azure Storage Queue or Azure Service Bus Queue. This is a common pattern for decoupling applications and processing background tasks.

When a message arrives, the function is triggered, and the message content is passed to the function.

// Example: Queue Trigger configuration (function.json)
{
  "bindings": [
    {
      "name": "myQueueItem",
      "type": "queueTrigger",
      "direction": "in",
      "queueName": "myqueue-items",
      "connection": "AzureWebJobsStorage"
    }
  ]
}
            

Blob Trigger

The Blob trigger invokes your function when a blob is created or updated in an Azure Storage container. This is useful for processing files as they are uploaded, such as image resizing, data validation, or format conversion.

The content of the blob is passed to your function.

// Example: Blob Trigger configuration (function.json)
{
  "bindings": [
    {
      "name": "myBlob",
      "type": "blobTrigger",
      "direction": "in",
      "path": "samples-workitems/{name}",
      "connection": "AzureWebJobsStorage"
    }
  ]
}
            

Event Grid Trigger

The Event Grid trigger allows your function to be invoked in response to events published to Azure Event Grid. This is a powerful mechanism for event-driven architectures, enabling loose coupling between services.

Event Grid acts as a central event routing service, forwarding events from Azure services and custom applications to Event Handlers, including Azure Functions.

Cosmos DB Trigger

The Cosmos DB trigger invokes your function when documents are created or updated in a Cosmos DB collection. This is ideal for responding to data changes within your NoSQL database, enabling real-time processing or synchronization.

Service Bus Trigger

The Service Bus trigger allows your function to be invoked when messages arrive in an Azure Service Bus Queue or Topic. This is a robust solution for reliable messaging and asynchronous processing.

Event Hubs Trigger

The Event Hubs trigger is designed for processing high-throughput data streams. It invokes your function when data is ingested into an Azure Event Hub. This is commonly used for IoT data, application telemetry, and log streaming.

Trigger Configuration

Triggers are typically configured in the function.json file (for JavaScript, Python, and PowerShell) or through attributes/decorators (for C# and Java). This configuration specifies:

• The type of trigger (e.g., httpTrigger, timerTrigger).
• The direction of the binding (in for triggers).
• Specific properties relevant to the trigger type, such as:
  • queueName for Queue Triggers
  • path for Blob Triggers
  • schedule for Timer Triggers (CRON expression)
  • authLevel and methods for HTTP Triggers
• The connection string or reference to secrets for accessing external services (e.g., connection).

The name specified in the binding (e.g., myTimer, myQueueItem) is used to access the trigger data within your function code.

Important Considerations