Azure Functions Blob storage bindings

This article explains how to configure and work with Azure Blob storage bindings in Azure Functions. Azure Functions supports trigger, input, and output bindings for Azure Blob storage. For features that are available in all bindings, see Azure Functions triggers and bindings concepts.

This is reference information for Azure Functions developers. If you're new to Azure Functions, start with the following resources:


A blob only storage account is not supported. Blob storage triggers and bindings require a general-purpose storage account.

Blob storage triggers and bindings

Using the Azure Blob storage trigger, your function code is called when a new or updated blob is detected. The blob contents are provided as input to the function.

Define a blob storage trigger using the Integrate tab in the Functions portal. The portal creates the following definition in the bindings section of function.json:

    "name": "<The name used to identify the trigger data in your code>",
    "type": "blobTrigger",
    "direction": "in",
    "path": "<container to monitor, and optionally a blob name pattern - see below>",
    "connection": "<Name of app setting - see below>"

Blob input and output bindings are defined using blob as the binding type:

  "name": "<The name used to identify the blob input in your code>",
  "type": "blob",
  "direction": "in", // other supported directions are "inout" and "out"
  "path": "<Path of input blob - see below>",
  "connection":"<Name of app setting - see below>"
  • The path property supports binding expressions and filter parameters. See Name patterns.
  • The connection property must contain the name of an app setting that contains a storage connection string. In the Azure portal, the standard editor in the Integrate tab configures this app setting for you when you select a storage account.

When you're using a blob trigger on a Consumption plan, there can be up to a 10-minute delay in processing new blobs after a function app has gone idle. After the function app is running, blobs are processed immediately. To avoid this initial delay, consider one of the following options:

  • Use an App Service plan with Always On enabled.
  • Use another mechanism to trigger the blob processing, such as a queue message that contains the blob name. For an example, see Queue trigger with blob input binding.

Name patterns

You can specify a blob name pattern in the path property, which can be a filter or binding expression. See Binding expressions and patterns.

For example, to filter to blobs that start with the string "original," use the following definition. This path finds a blob named original-Blob1.txt in the input container, and the value of the name variable in function code is Blob1.

"path": "input/original-{name}",

To bind to the blob file name and extension separately, use two patterns. This path also finds a blob named original-Blob1.txt, and the value of the blobname and blobextension variables in function code are original-Blob1 and txt.

"path": "input/{blobname}.{blobextension}",

You can restrict the file type of blobs by using a fixed value for the file extension. For instance, to trigger only on .png files, use the following pattern:

"path": "samples/{name}.png",

Curly braces are special characters in name patterns. To specify blob names that have curly braces in the name, you can escape the braces using two braces. The following example finds a blob named {20140101}-soundfile.mp3 in the images container, and the name variable value in the function code is soundfile.mp3.

"path": "images/{{20140101}}-{name}",

Trigger metadata

The blob trigger provides several metadata properties. These properties can be used as part of bindings expressions in other bindings or as parameters in your code. These values have the same semantics as Cloud​Blob.

  • BlobTrigger. Type string. The triggering blob path
  • Uri. Type System.Uri. The blob's URI for the primary location.
  • Properties. Type Microsoft.WindowsAzure.Storage.Blob.BlobProperties. The blob's system properties.
  • Metadata. Type IDictionary<string,string>. The user-defined metadata for the blob.

Blob receipts

The Azure Functions runtime ensures that no blob trigger function gets called more than once for the same new or updated blob. To determine if a given blob version has been processed, it maintains blob receipts.

Azure Functions stores blob receipts in a container named azure-webjobs-hosts in the Azure storage account for your function app (defined by the app setting AzureWebJobsStorage). A blob receipt has the following information:

  • The triggered function ("<function app name>.Functions.<function name>", for example: "MyFunctionApp.Functions.CopyBlob")
  • The container name
  • The blob type ("BlockBlob" or "PageBlob")
  • The blob name
  • The ETag (a blob version identifier, for example: "0x8D1DC6E70A277EF")

To force reprocessing of a blob, delete the blob receipt for that blob from the azure-webjobs-hosts container manually.

Handling poison blobs

When a blob trigger function fails for a given blob, Azure Functions retries that function a total of 5 times by default.

If all 5 tries fail, Azure Functions adds a message to a Storage queue named webjobs-blobtrigger-poison. The queue message for poison blobs is a JSON object that contains the following properties:

  • FunctionId (in the format <function app name>.Functions.<function name>)
  • BlobType ("BlockBlob" or "PageBlob")
  • ContainerName
  • BlobName
  • ETag (a blob version identifier, for example: "0x8D1DC6E70A277EF")

Blob polling for large containers

If the blob container being monitored contains more than 10,000 blobs, the Functions runtime scans log files to watch for new or changed blobs. This process is not real time. A function might not get triggered until several minutes or longer after the blob is created. In addition, storage logs are created on a "best effort" basis. There is no guarantee that all events are captured. Under some conditions, logs may be missed. If you require faster or more reliable blob processing, consider creating a queue message when you create the blob. Then, use a queue trigger instead of a blob trigger to process the blob.

Using a blob trigger and input binding

In .NET functions, access the blob data using a method parameter such as Stream paramName. Here, paramName is the value you specified in the trigger configuration. In Node.js functions, access the input blob data using context.bindings.<name>.

In .NET, you can bind to any of the types in the list below. If used as an input binding, some of these types require an inout binding direction in function.json. This direction is not supported by the standard editor, so you must use the advanced editor.

  • TextReader
  • Stream
  • ICloudBlob (requires "inout" binding direction)
  • CloudBlockBlob (requires "inout" binding direction)
  • CloudPageBlob (requires "inout" binding direction)
  • CloudAppendBlob (requires "inout" binding direction)

If text blobs are expected, you can also bind to a .NET string type. This is only recommended if the blob size is small, as the entire blob contents are loaded into memory. Generally, it is preferable to use a Stream or CloudBlockBlob type.

Trigger sample

Suppose you have the following function.json that defines a blob storage trigger:

    "disabled": false,
    "bindings": [
            "name": "myBlob",
            "type": "blobTrigger",
            "direction": "in",
            "path": "samples-workitems",

See the language-specific sample that logs the contents of each blob that is added to the monitored container.

Blob trigger examples in C#

// Blob trigger sample using a Stream binding
public static void Run(Stream myBlob, TraceWriter log)
   log.Info($"C# Blob trigger function Processed blob\n Name:{name} \n Size: {myBlob.Length} Bytes");
// Blob trigger binding to a CloudBlockBlob
#r "Microsoft.WindowsAzure.Storage"

using Microsoft.WindowsAzure.Storage.Blob;

public static void Run(CloudBlockBlob myBlob, string name, TraceWriter log)
    log.Info($"C# Blob trigger function Processed blob\n Name:{name}\nURI:{myBlob.StorageUri}");

Trigger example in Node.js

module.exports = function(context) {
    context.log('Node.js Blob trigger function processed', context.bindings.myBlob);

<a name=storage-blob-output-binding">

Using a blob output binding

In .NET functions, you should either use a out string parameter in your function signature or use one of the types in the following list. In Node.js functions, you access the output blob using context.bindings.<name>.

In .NET functions you can output to any of the following types:

  • out string
  • TextWriter
  • Stream
  • CloudBlobStream
  • ICloudBlob
  • CloudBlockBlob
  • CloudPageBlob

Queue trigger with blob input and output sample

Suppose you have the following function.json, that defines a Queue Storage trigger, a blob storage input, and a blob storage output. Notice the use of the queueTrigger metadata property. in the blob input and output path properties:

  "bindings": [
      "queueName": "myqueue-items",
      "connection": "MyStorageConnection",
      "name": "myQueueItem",
      "type": "queueTrigger",
      "direction": "in"
      "name": "myInputBlob",
      "type": "blob",
      "path": "samples-workitems/{queueTrigger}",
      "connection": "MyStorageConnection",
      "direction": "in"
      "name": "myOutputBlob",
      "type": "blob",
      "path": "samples-workitems/{queueTrigger}-Copy",
      "connection": "MyStorageConnection",
      "direction": "out"
  "disabled": false

See the language-specific sample that copies the input blob to the output blob.

Blob binding example in C#

// Copy blob from input to output, based on a queue trigger
public static void Run(string myQueueItem, Stream myInputBlob, out string myOutputBlob, TraceWriter log)
    log.Info($"C# Queue trigger function processed: {myQueueItem}");
    myOutputBlob = myInputBlob;

Blob binding example in Node.js

// Copy blob from input to output, based on a queue trigger
module.exports = function(context) {
    context.log('Node.js Queue trigger function processed', context.bindings.myQueueItem);
    context.bindings.myOutputBlob = context.bindings.myInputBlob;

Next steps

For information about other bindings and triggers for Azure Functions, see Azure Functions triggers and bindings developer reference.