Azure Functions: Blob Output Bindings

Introduction to Blob Output Bindings

Azure Functions allow you to integrate with Azure Storage Blob services seamlessly using output bindings. This enables your functions to write data directly to blob containers without explicitly writing SDK code for storage operations.

Output bindings simplify the process of sending data from your function's execution result to an Azure Blob Storage container. When your function completes successfully, the return value or a specified output parameter is automatically sent to the configured blob.

How it Works

When you define a blob output binding for your Azure Function, you specify details such as:

• The storage account connection string (usually through an App Setting).
• The container name where blobs will be stored.
• The path or name of the blob to be created or updated. This can often be dynamically generated based on function input or execution context.

The Azure Functions runtime handles the connection to the storage account and the upload of data to the specified blob.

Example: Writing to a Blob

C# Example

In this example, a function takes an input string and writes it to a blob named output-{guid}.txt.

{
  "scriptFile": "__init__.py",
  "bindings": [
    {
      "name": "req",
      "type": "httpTrigger",
      "direction": "in",
      "methods": [
        "get",
        "post"
      ]
    },
    {
      "name": "outputBlob",
      "type": "blob",
      "direction": "out",
      "path": "output-container/{sys.randguid}.txt",
      "connection": "AzureWebJobsStorage"
    }
  ]
}
            

import logging
import azure.functions as func
import uuid

def main(req: func.HttpRequest, outputBlob: func.Out[str]) -> func.HttpResponse:
    logging.info('Python HTTP trigger function processed a request.')

    name = req.params.get('name')
    if not name:
        try:
            req_body = req.get_json()
        except ValueError:
            pass
        else:
            name = req_body.get('name')

    if name:
        message = f"Hello, {name}! This is data written to Azure Blob Storage."
        logging.info(f"Writing message to blob: {message}")
        outputBlob.set(message)
        return func.HttpResponse(
             f"Data successfully sent to blob storage for {name}.",
             status_code=200
        )
    else:
        return func.HttpResponse(
             "Please pass a name on the query string or in the request body",
             status_code=400
        )
            

Node.js Example

{
  "bindings": [
    {
      "authLevel": "function",
      "type": "httpTrigger",
      "direction": "in",
      "name": "req",
      "methods": [
        "get",
        "post"
      ]
    },
    {
      "type": "http",
      "direction": "out",
      "name": "res"
    },
    {
      "name": "outputBlob",
      "type": "blob",
      "direction": "out",
      "path": "output-container/my-output-{randguid}.txt",
      "connection": "AzureWebJobsStorage"
    }
  ]
}
            

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 + ". Your input has been written to blob storage."
        : "This HTTP triggered function executed successfully. Pass a name in the query string or in the request body for a personalized response.";

    // The output binding will automatically write this to the blob
    context.bindings.outputBlob = `Data generated by function for ${name || 'anonymous user'}.`;

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

Dynamic Blob Naming

The path property in the function.json file is powerful and supports dynamic expressions. You can use system variables or input parameters to construct unique blob names.

• {randguid}: Generates a random GUID.
• {sys.randguid}: Similar to {randguid}.
• {sys.PartitionKey}: For Cosmos DB triggers, the partition key.
• {in.propertyName}: Use the value of a property from an input binding (e.g., {in.myInputBlobTrigger.name}).
• {out.propertyName}: Use the value of a property from an output binding.

For more complex dynamic naming, you can assign a value to the output binding object within your function code.

Important Considerations

Blob output bindings significantly reduce boilerplate code, allowing developers to focus on business logic while the Azure Functions runtime manages the interaction with Azure Blob Storage.

For more detailed information, refer to the official Azure Functions documentation.