Azure Functions Triggers Explained

Triggers define how a function is invoked. When a trigger event occurs, Azure Functions runtime executes your function. Each trigger type has specific configuration requirements and payload structures.

What is a Trigger?

A trigger is a part of an Azure Functions binding that initiates the execution of a function. Triggers can be based on various events, such as an HTTP request, a new message in a queue, a file upload to Blob Storage, a scheduled time, or a change in a database.

Common Trigger Types

1. HTTP Trigger

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

• Event Source: HTTP request (GET, POST, etc.)
• Common Use Cases: Building RESTful APIs, processing webhook events.

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

2. Timer Trigger

The Timer trigger allows you to run a function on a schedule. You can define the schedule using a CRON expression.

• Event Source: Time-based schedule (CRON expression).
• Common Use Cases: Running scheduled tasks, data cleanup, report generation.

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

The example above schedules the function to run every 5 minutes.

3. Blob Trigger

The Blob trigger executes a function when a new or updated blob is detected in Azure Blob Storage.

• Event Source: New or updated blob in Azure Blob Storage.
• Common Use Cases: Processing uploaded images, transforming data files, reacting to data changes in storage.

{
  "path": "samples-workitems/{name}",
  "connection": "AzureWebJobsStorage",
  "bindings": [
    {
      "name": "myBlob",
      "type": "blobTrigger",
      "direction": "in",
      "path": "samples-workitems/{name}",
      "connection": "AzureWebJobsStorage"
    }
  ]
}
        

This trigger monitors the container named samples-workitems for new blobs. The {name} parameter captures the name of the blob, which can be passed to your function.

4. Queue Trigger

The Queue trigger executes a function when a new message arrives in an Azure Storage Queue.

• Event Source: New message in an Azure Storage Queue.
• Common Use Cases: Processing background tasks, handling messages from decoupled systems, event-driven processing.

{
  "queueName": "myqueue-items",
  "connection": "AzureWebJobsStorage",
  "bindings": [
    {
      "name": "myQueueItem",
      "type": "queueTrigger",
      "direction": "in",
      "queueName": "myqueue-items",
      "connection": "AzureWebJobsStorage"
    }
  ]
}
        

5. Cosmos DB Trigger

The Cosmos DB trigger executes a function when documents are created or updated in a Cosmos DB collection.

• Event Source: Changes in an Azure Cosmos DB collection.
• Common Use Cases: Real-time data processing, reacting to database changes, synchronizing data.

{
  "databaseName": "ToDoList",
  "collectionName": "Items",
  "connectionStringSetting": "CosmosDBConnectionString",
  "bindings": [
    {
      "name": "inputDocument",
      "type": "cosmosDBTrigger",
      "direction": "in",
      "databaseName": "ToDoList",
      "collectionName": "Items",
      "connectionStringSetting": "CosmosDBConnectionString",
      "createLeaseCollectionIfNotExists": true
    }
  ]
}
        

Key Considerations for Triggers

• Event Type: Understand the nature of the event that will trigger your function.
• Payload: Each trigger type provides different data to your function in its payload.
• Configuration: Triggers require specific configuration in your function.json file, including connection strings, paths, and schedules.
• Concurrency: Be mindful of how multiple trigger events might be processed concurrently.