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🟢 What is a Trigger in Azure Functions?

An Azure Function is a small piece of code that runs only when something happens.
That “something” is called a trigger.
Trigger = automatic starter for your function.

Example:
If you want a function to run every day at 9 AM, a Timer trigger starts it for you.

🧩 Key Idea

Each function must have exactly one trigger.
The trigger decides:
1. When the function runs.
2. What input data is passed to it.

🔑 Common Trigger Types (with Examples)

Below are the most used Azure Function triggers and what they do.

Trigger Type	What Starts the Function	Simple Example
HTTP Trigger	An HTTP request (like a web API call)	Someone sends a GET/POST request to a URL such as `https://myfunc.azurewebsites.net/api/hello`
Timer Trigger	A scheduled time (like a cron job)	Run cleanup every night at 2 AM
Blob Trigger	A file is added or updated in Azure Blob Storage	Automatically resize an image after it’s uploaded
Queue Storage Trigger	A new message is put in an Azure Storage Queue	Process new orders added to a queue
Service Bus Trigger	A message arrives in an Azure Service Bus queue or topic	Handle incoming payment notifications
Event Grid Trigger	An Azure event occurs (e.g., new file in a storage account, resource change)	Send a Slack message when a new file arrives
Event Hub Trigger	A batch of event-stream data arrives (IoT or telemetry)	Analyze live sensor data
Cosmos DB Trigger	A document changes in a Cosmos DB collection	Update a cache or run analytics whenever a record changes
SignalR Trigger (rare)	A SignalR event happens	Send real-time messages to connected clients

🛠️ How It Works Step by Step

Binding in Code
In the function’s code you specify the trigger type and connection details.


[FunctionName("MyHttpFunction")]
public static IActionResult Run(
    [HttpTrigger(AuthorizationLevel.Function, "get", "post")] HttpRequest req,
    ILogger log)
{
    log.LogInformation("HTTP trigger fired.");
    return new OkObjectResult("Hello World!");
}

Runtime Listens
Azure Functions runtime listens for that event (HTTP call, queue message, timer schedule, etc.).
Event Happens
When the event occurs, the runtime starts the function automatically and passes in the data (like the HTTP body, message text, or blob contents).

⚙️ Details About Each Trigger

1. HTTP Trigger

Use for: APIs or webhooks.
Input: Request body, query strings, headers.
Return: HTTP response.
Security: Set AuthorizationLevel (Anonymous, Function, Admin).

2. Timer Trigger

Use for: Scheduled tasks like cleanup, backups.
Schedule Format: CRON expression, e.g., "0 0 2 * * *" (2 AM daily).
Example: Nightly email reports.

3. Blob Trigger

Use for: Processing files.
Works with: Azure Blob Storage containers.
Example: Process a CSV when someone uploads it.

4. Queue Storage Trigger

Use for: Simple messaging between services.
Example: One system drops a message when an order is placed; the function picks it up and processes it.

5. Service Bus Trigger

Use for: Enterprise messaging (queues or topics).
Example: Payment gateway sends a Service Bus message when a payment succeeds.

6. Event Grid Trigger

Use for: Reacting to Azure events in near real-time.
Example: Notify a team when a new blob is created.

7. Event Hub Trigger

Use for: High-volume event streams like IoT telemetry.
Example: Analyze millions of device signals per minute.

8. Cosmos DB Trigger

Use for: Reacting to database changes.
Example: Automatically update a search index when a product record changes.

🧠 Best Practices

One Trigger per Function
- Each function listens to exactly one event source.
Keep Functions Short
- Do small tasks quickly to avoid timeouts.
Scale Automatically
- Triggers like Queue, Event Hub, and Blob scale the number of function instances based on event volume.
Use Bindings for Input/Output
- You can directly read or write to storage, queues, etc., with simple parameters—no extra code.

Quick Recap

Trigger = automatic starter for your code.
Common triggers: HTTP, Timer, Blob, Queue, Service Bus, Event Grid, Event Hub, Cosmos DB.
Each function must have exactly one trigger.
Azure Functions automatically listens for the event and runs your code when it happens.

In short:
Azure Function triggers let you run code automatically whenever a specific event (like a request, message, or file upload) occurs—no servers to manage, just write the function and choose the trigger.

Wednesday, 24 September 2025

Trigger in Azure Functions