Azure Functions Triggers Overview

Azure Functions provide a powerful event-driven compute experience. A trigger defines how a function is invoked. When a trigger event occurs, Azure Functions creates an instance of your function and passes data from the trigger to your function code.

Triggers can be thought of as the "entry points" for your functions. They bind to a specific event source, and when that event occurs, the function executes. Azure Functions supports a wide variety of triggers, allowing you to build serverless applications that respond to events from various Azure services and external sources.

Common Trigger Types

Here are some of the most commonly used trigger types:

HTTP Trigger

Invokes a function in response to an HTTP request. This is ideal for building APIs and webhooks.

Use cases: REST APIs, webhook handlers, serverless microservices.
Example event: An incoming HTTP GET or POST request.

Timer Trigger

Invokes a function on a schedule, defined by a CRON expression. Useful for scheduled tasks and background jobs.

Use cases: Scheduled data processing, recurring cleanup tasks, periodic reports.
Example event: A specific time according to the CRON schedule.

Blob Trigger

Invokes a function when a new or updated blob is detected in an Azure Storage container.

Use cases: Image processing upon upload, data transformation, file synchronization.
Example event: A new file added to an Azure Blob Storage container.

Queue Trigger

Invokes a function when a message is added to an Azure Storage Queue or an Azure Service Bus Queue.

Use cases: Processing queued messages, decoupling application components, background job processing.
Example event: A new message arriving in a storage queue.

Event Hubs Trigger

Invokes a function in response to events streamed by Azure Event Hubs. Excellent for real-time data streaming and IoT scenarios.

Use cases: Real-time analytics, IoT data ingestion, stream processing.
Example event: A batch of events arriving in an Event Hubs consumer group.

Cosmos DB Trigger

Invokes a function when documents are created or updated in an Azure Cosmos DB collection.

Use cases: Real-time data validation, change feed processing, data synchronization.
Example event: A new or modified document in a Cosmos DB collection.

How Triggers Work

When you define a function, you specify its trigger. The trigger binding handles the connection to the event source. When an event matching the trigger's configuration occurs:

The trigger service detects the event.
It then invokes the Azure Functions runtime.
The runtime executes your function code, passing any relevant data from the event as input.

For example, an HTTP trigger passes the request details (headers, body, query parameters) to your function. A Blob trigger might pass information about the newly created blob, such as its name and size.

Configuring Triggers

Trigger configuration is typically done in the function's function.json file (for JavaScript, C#, and other non-scripted languages) or directly in the code using attributes (for C# and F#). Here's a simplified example of a function.json for an HTTP trigger:


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

This configuration specifies that the function is triggered by HTTP requests (type: "httpTrigger") and has an HTTP output binding (type: "http"). The methods array defines which HTTP methods are allowed.

Choosing the Right Trigger

Selecting the appropriate trigger is crucial for designing efficient and responsive serverless applications. Consider the source of your events and the desired invocation pattern when making your choice.

Explore the official Azure Functions documentation for a comprehensive list of available triggers and detailed configuration options.