Blob Input Binding

This document explains how to use the Azure Blob Storage input binding for Azure Functions.

Introduction

The blob input binding allows your function to read data from a blob in Azure Blob Storage. When the function is triggered, the binding automatically retrieves the blob's content and makes it available to your function code as a parameter.

Configuration

The blob input binding is configured in your function's function.json file. You need to specify the binding type, direction, path to the blob, and the storage connection string.

function.json Example

{
  "scriptFile": "__init__.py",
  "bindings": [
    {
      "name": "blobContent",
      "type": "blob",
      "direction": "in",
      "path": "mycontainer/{name}.txt",
      "connection": "AzureWebJobsStorage"
    },
    {
      "type": "httpTrigger",
      "direction": "in",
      "name": "req",
      "methods": [
        "get",
        "post"
      ]
    },
    {
      "type": "http",
      "direction": "out",
      "name": "res"
    }
  ]
}
            

Binding Properties

Usage in Function Code

The content of the blob is passed to your function as a parameter. The type of the parameter depends on the content of the blob and your function's language.

Python Example

Assuming the function.json above, your Python function might look like this:

import logging
import azure.functions as func

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

    if blobContent:
        logging.info(f"Blob content: {blobContent}")
        return func.HttpResponse(
             f"Successfully read blob content. Content: {blobContent[:100]}...",
             status_code=200
        )
    else:
        return func.HttpResponse(
             "Blob content could not be retrieved or was empty.",
             status_code=400
        )
            

C# Example

For C#, you might use a Stream or a strongly-typed object:

using System.IO;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Logging;

public static class BlobTriggerFunction
{
    [FunctionName("BlobInputExample")]
    public static IActionResult Run(
        [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = null)] HttpRequest req,
        [Blob("mycontainer/{Query.name}.txt", Connection = "AzureWebJobsStorage")] Stream myBlob,
        ILogger log)
    {
        log.LogInformation("C# HTTP trigger function processed a request.");

        string blobContent = "";
        if (myBlob != null)
        {
            using (var reader = new StreamReader(myBlob))
            {
                blobContent = reader.ReadToEnd();
                log.LogInformation($"Blob content: {blobContent}");
            }
            return new OkObjectResult($"Blob content read successfully. Content: {blobContent.Substring(0, Math.Min(blobContent.Length, 100))}...");
        }
        else
        {
            return new BadRequestObjectResult("Blob content could not be retrieved.");
        }
    }
}
            

Blob Path Tokens

The path property in function.json supports tokens to dynamically specify the blob to retrieve. These tokens are typically extracted from the trigger event or route parameters.

• {name}: Represents a segment of the blob name.
• {rand-guid}: Generates a new GUID.
• If your trigger is an HTTP trigger with a route like /api/process/{filename}, you can use mycontainer/{filename}.txt in the blob path.

Supported Data Types

The blob input binding can bind to various data types depending on your function's language:

• string: Reads the blob content as a string.
• byte[]: Reads the blob content as a byte array.
• Stream: Provides a stream to read the blob content.
• T (e.g., POCOs): If you have a serializer configured (like JSON), the blob can be deserialized into an object.

Troubleshooting

• Blob not found: Double-check the path in function.json, including container and blob name. Verify that the blob actually exists in your storage account.
• Authentication errors: Ensure your connection string is correct and has the necessary permissions to read from the specified container.
• Data format issues: If binding to a POCO, ensure the blob content is in the expected format (e.g., valid JSON).