Blob storage bindings for Azure Functions

Azure Functions can interact with Azure Blob Storage using input, output, and trigger bindings. These bindings let you read from or write to blobs without writing explicit storage‑client code.

Blob trigger

The blob trigger fires when a new blob is created or updated in a container.

public static void Run(
    [BlobTrigger("samples-workitems/{name}", Connection = "AzureWebJobsStorage")] Stream myBlob,
    string name,
    ILogger log)
{
    log.LogInformation($"C# Blob trigger function Processed blob\n Name:{name} \n Size: {myBlob.Length} Bytes");
}

Use {name} placeholder to capture the blob name in the function parameter.

Blob input

Bind a Stream, byte[] or POCO to read an existing blob.

public static async Task Run(
    [TimerTrigger("0 */5 * * * *")]TimerInfo myTimer,
    [Blob("samples-workitems/input.json", FileAccess.Read, Connection = "AzureWebJobsStorage")] Stream input,
    ILogger log)
{
    var json = await new StreamReader(input).ReadToEndAsync();
    // process json
}

Blob output

Write data back to a blob by binding a out parameter.

public static void Run(
    [HttpTrigger(AuthorizationLevel.Function, "post")] HttpRequest req,
    [Blob("samples-workitems/{rand-guid}.txt", FileAccess.Write, Connection = "AzureWebJobsStorage")] out string outputBlob,
    ILogger log)
{
    var body = new StreamReader(req.Body).ReadToEnd();
    outputBlob = $"Received: {DateTime.UtcNow}\n{body}";
}

Full sample (C#)

using System.IO;
using Microsoft.Azure.WebJobs;
using Microsoft.Extensions.Logging;

public static class BlobTriggerCSharp
{
    [FunctionName("BlobTriggerCSharp")]
    public static void Run(
        [BlobTrigger("input/{name}", Connection = "AzureWebJobsStorage")] Stream inputBlob,
        string name,
        [Blob("output/{name}", FileAccess.Write, Connection = "AzureWebJobsStorage")] Stream outputBlob,
        ILogger log)
    {
        // simple copy
        inputBlob.CopyTo(outputBlob);
        log.LogInformation($"Blob {name} copied to output container.");
    }
}

module.exports = async function (context, myBlob) {
    const blobService = require('azure-storage').createBlobService(process.env.AzureWebJobsStorage);
    const outputContainer = 'output';
    const blobName = context.bindingData.name;

    await new Promise((resolve, reject) => {
        blobService.createBlockBlobFromStream(outputContainer, blobName, myBlob, myBlob.length, err => {
            if (err) reject(err); else resolve();
        });
    });

    context.log(`Blob ${blobName} copied to ${outputContainer} container.`);
};

Key concepts

Connection string: Store in AzureWebJobsStorage app setting.
Path pattern: Use binding expressions ({name}, {queueTrigger}) to capture values.
Access mode: FileAccess.Read, FileAccess.Write, or default for trigger.
Scaling: Blob triggers scale out automatically based on the number of blobs.

Limitations & considerations

Aspect	Details
Blob size limit for trigger	Up to 4 GB (Azure Functions runtime limitation).
Binding concurrency	Parallelism controlled by `functions.workerProcessCount` and host scaling.
Eventual consistency	Blob trigger may have a slight delay (seconds) after upload.
Authentication	Managed identity supported via `IdentityConnection` attribute.

For detailed guidance, see the official documentation.