title	description	ms.topic	ms.date	ms.devlang	ms.custom	zone_pivot_groups
Azure Blob storage trigger for Azure Functions	Learn how to use Azure Function to run your custom code based on changes in an Azure Blob storage container.	reference	05/14/2024	csharp	devx-track-csharp, devx-track-python, devx-track-extended-java, devx-track-js, devx-track-ts	programming-languages-set-functions

Azure Blob storage trigger for Azure Functions

The Blob storage trigger starts a function when a new or updated blob is detected. The blob contents are provided as input to the function.

Tip

There are several ways to execute your function code based on changes to blobs in a storage container. If you choose to use the Blob storage trigger, note that there are two implementations offered: a polling-based one (referenced in this article) and an event-based one. It is recommended that you use the event-based implementation as it has lower latency than the other. Also, the Flex Consumption plan supports only the event-based Blob storage trigger.

For details about differences between the two implementations of the Blob storage trigger, as well as other triggering options, see Working with blobs.

For information on setup and configuration details, see the overview.

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[!INCLUDE functions-nodejs-model-tabs-description]
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[!INCLUDE functions-python-model-tabs-description]
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Example

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[!INCLUDE functions-bindings-csharp-intro]

[!INCLUDE functions-in-process-model-retirement-note]

Isolated worker model

The following example is a C# function that runs in an isolated worker process and uses a blob trigger with both blob input and blob output blob bindings. The function is triggered by the creation of a blob in the test-samples-trigger container. It reads a text file from the test-samples-input container and creates a new text file in an output container based on the name of the triggered file.

:::code language="csharp" source="~/azure-functions-dotnet-worker/samples/Extensions/Blob/BlobFunction.cs" range="9-25":::

In-process model

The following example shows a C# function that writes a log when a blob is added or updated in the samples-workitems container.

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

The string {name} in the blob trigger path samples-workitems/{name} creates a binding expression that you can use in function code to access the file name of the triggering blob. For more information, see Blob name patterns later in this article.

For more information about the BlobTrigger attribute, see Attributes.

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This function writes a log when a blob is added or updated in the myblob container.

@FunctionName("blobprocessor")
public void run(
  @BlobTrigger(name = "file",
               dataType = "binary",
               path = "myblob/{name}",
               connection = "MyStorageAccountAppSetting") byte[] content,
  @BindingName("name") String filename,
  final ExecutionContext context
) {
  context.getLogger().info("Name: " + filename + " Size: " + content.length + " bytes");
}

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::: zone pivot="programming-language-typescript"

Model v4

The following example shows a blob trigger TypeScript code. The function writes a log when a blob is added or updated in the samples-workitems container.

The string {name} in the blob trigger path samples-workitems/{name} creates a binding expression that you can use in function code to access the file name of the triggering blob. For more information, see Blob name patterns later in this article.

:::code language="typescript" source="~/azure-functions-nodejs-v4/ts/src/functions/storageBlobTrigger1.ts" :::

Model v3

TypeScript samples are not documented for model v3.

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Model v4

The following example shows a blob trigger JavaScript code. The function writes a log when a blob is added or updated in the samples-workitems container.

The string {name} in the blob trigger path samples-workitems/{name} creates a binding expression that you can use in function code to access the file name of the triggering blob. For more information, see Blob name patterns later in this article.

:::code language="javascript" source="~/azure-functions-nodejs-v4/js/src/functions/storageBlobTrigger1.js" :::

Model v3

The following example shows a blob trigger binding in a function.json file and JavaScript code that uses the binding. The function writes a log when a blob is added or updated in the samples-workitems container.

Here's the function.json file:

{
    "disabled": false,
    "bindings": [
        {
            "name": "myBlob",
            "type": "blobTrigger",
            "direction": "in",
            "path": "samples-workitems/{name}",
            "connection":"MyStorageAccountAppSetting"
        }
    ]
}

The string {name} in the blob trigger path samples-workitems/{name} creates a binding expression that you can use in function code to access the file name of the triggering blob. For more information, see Blob name patterns later in this article.

For more information about function.json file properties, see the Configuration section explains these properties.

Here's the JavaScript code:

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

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The following example demonstrates how to create a function that runs when a file is added to source blob storage container.

The function configuration file (function.json) includes a binding with the type of blobTrigger and direction set to in.

{
  "bindings": [
    {
      "name": "InputBlob",
      "type": "blobTrigger",
      "direction": "in",
      "path": "source/{name}",
      "connection": "MyStorageAccountConnectionString"
    }
  ]
}

Here's the associated code for the run.ps1 file.

param([byte[]] $InputBlob, $TriggerMetadata)

Write-Host "PowerShell Blob trigger: Name: $($TriggerMetadata.Name) Size: $($InputBlob.Length) bytes"

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v2

This example uses SDK types to directly access the underlying BlobClient object provided by the Blob storage trigger:

:::code language="python" source="~/functions-python-extensions/azurefunctions-extensions-bindings-blob/samples/blob_samples_blobclient/function_app.py" range="9-14,31-39":::

For examples of using other SDK types, see the ContainerClient and StorageStreamDownloader samples.

To learn more, including how to enable SDK type bindings in your project, see SDK type bindings.

This example logs information from the incoming blob metadata.

import logging
import azure.functions as func

app = func.FunctionApp()

@app.function_name(name="BlobTrigger1")
@app.blob_trigger(arg_name="myblob", 
                  path="PATH/TO/BLOB",
                  connection="CONNECTION_SETTING")
def test_function(myblob: func.InputStream):
   logging.info(f"Python blob trigger function processed blob \n"
                f"Name: {myblob.name}\n"
                f"Blob Size: {myblob.length} bytes")

v1

The function writes a log when a blob is added or updated in the samples-workitems container. Here's the function.json file:

{
    "scriptFile": "__init__.py",
    "disabled": false,
    "bindings": [
        {
            "name": "myblob",
            "type": "blobTrigger",
            "direction": "in",
            "path": "samples-workitems/{name}",
            "connection":"MyStorageAccountAppSetting"
        }
    ]
}

The string {name} in the blob trigger path samples-workitems/{name} creates a binding expression that you can use in function code to access the file name of the triggering blob. For more information, see Blob name patterns later in this article.

For more information about function.json file properties, see the Configuration section explains these properties.

Here's the Python code:

import logging
import azure.functions as func


def main(myblob: func.InputStream):
    logging.info('Python Blob trigger function processed %s', myblob.name)

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Attributes

Both in-process and isolated worker process C# libraries use the BlobAttribute attribute to define the function. C# script instead uses a function.json configuration file as described in the C# scripting guide.

The attribute's constructor takes the following parameters:

Parameter	Description
BlobPath	The path to the blob.
Connection	The name of an app setting or setting collection that specifies how to connect to Azure Blobs. See Connections.
Access	Indicates whether you will be reading or writing.
Source	Sets the source of the triggering event. Use `BlobTriggerSource.EventGrid` for an Event Grid-based blob trigger, which provides much lower latency. The default is `BlobTriggerSource.LogsAndContainerScan`, which uses the standard polling mechanism to detect changes in the container.

Isolated worker model

Here's an BlobTrigger attribute in a method signature:

:::code language="csharp" source="~/azure-functions-dotnet-worker/samples/Extensions/Blob/BlobFunction.cs" range="11-16":::

In-process model

In C# class libraries, the attribute's constructor takes a path string that indicates the container to watch and optionally a blob name pattern. Here's an example:

[FunctionName("ResizeImage")]
public static void Run(
  [BlobTrigger("sample-images/{name}")] Stream image,
  [Blob("sample-images-md/{name}", FileAccess.Write)] Stream imageSmall)
{
  ....
}

[!INCLUDE functions-bindings-storage-attribute]

[!INCLUDE app settings to local.settings.json]

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Decorators

Applies only to the Python v2 programming model.

For Python v2 functions defined using decorators, the following properties on the blob_trigger decorator define the Blob Storage trigger:

Property	Description
`arg_name`	Declares the parameter name in the function signature. When the function is triggered, this parameter's value has the contents of the queue message.
`path`	The container to monitor. May be a blob name pattern.
`connection`	The storage account connection string.
`source`	Sets the source of the triggering event. Use `EventGrid` for an Event Grid-based blob trigger, which provides much lower latency. The default is `LogsAndContainerScan`, which uses the standard polling mechanism to detect changes in the container.

For Python functions defined by using function.json, see the Configuration section. ::: zone-end ::: zone pivot="programming-language-java"

Annotations

The @BlobTrigger attribute is used to give you access to the blob that triggered the function. Refer to the trigger example for details. Use the source property to set the source of the triggering event. Use EventGrid for an Event Grid-based blob trigger, which provides much lower latency. The default is LogsAndContainerScan, which uses the standard polling mechanism to detect changes in the container. | ::: zone-end
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Configuration

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::: zone pivot="programming-language-python" Applies only to the Python v1 programming model.

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Model v4

The following table explains the properties that you can set on the options object passed to the app.storageBlob() method.

Property	Description
path	The container to monitor. May be a blob name pattern.
connection	The name of an app setting or setting collection that specifies how to connect to Azure Blobs. See Connections.
source	Sets the source of the triggering event. Use `EventGrid` for an Event Grid-based blob trigger, which provides much lower latency. The default is `LogsAndContainerScan`, which uses the standard polling mechanism to detect changes in the container.

Model v3

The following table explains the binding configuration properties that you set in the function.json file.

Property	Description
type	Must be set to `blobTrigger`. This property is set automatically when you create the trigger in the Azure portal.
direction	Must be set to `in`. This property is set automatically when you create the trigger in the Azure portal. Exceptions are noted in the usage section.
name	The name of the variable that represents the blob in function code.
path	The container to monitor. May be a blob name pattern.
connection	The name of an app setting or setting collection that specifies how to connect to Azure Blobs. See Connections.
source	Sets the source of the triggering event. Use `EventGrid` for an Event Grid-based blob trigger, which provides much lower latency. The default is `LogsAndContainerScan`, which uses the standard polling mechanism to detect changes in the container.

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::: zone pivot="programming-language-powershell,programming-language-python"
The following table explains the binding configuration properties that you set in the function.json file.

function.json property	Description
type	Must be set to `blobTrigger`. This property is set automatically when you create the trigger in the Azure portal.
direction	Must be set to `in`. This property is set automatically when you create the trigger in the Azure portal. Exceptions are noted in the usage section.
name	The name of the variable that represents the blob in function code.
path	The container to monitor. May be a blob name pattern.
connection	The name of an app setting or setting collection that specifies how to connect to Azure Blobs. See Connections.
source	Sets the source of the triggering event. Use `EventGrid` for an Event Grid-based blob trigger, which provides much lower latency. The default is `LogsAndContainerScan`, which uses the standard polling mechanism to detect changes in the container.

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See the Example section for complete examples.

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Metadata

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

Property	Type	Description
`BlobTrigger`	`string`	The path to the triggering blob.
`Uri`	`System.Uri`	The blob's URI for the primary location.
`Properties`	BlobProperties	The blob's system properties.
`Metadata`	`IDictionary<string,string>`	The user-defined metadata for the blob.

The following example logs the path to the triggering blob, including the container:

public static void Run(string myBlob, string blobTrigger, ILogger log)
{
    log.LogInformation($"Full blob path: {blobTrigger}");
}

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Metadata

The blob trigger provides several metadata properties. These properties can be used as part of binding expressions in other bindings or as parameters in your code.

Property	Description
`blobTrigger`	The path to the triggering blob.
`uri`	The blob's URI for the primary location.
`properties`	The blob's system properties.
`metadata`	The user-defined metadata for the blob.

Model v4

Metadata can be obtained from the triggerMetadata property of the supplied context object, as shown in the following example, which logs the path to the triggering blob (blobTrigger), including the container:

context.log(`Full blob path: ${context.triggerMetadata.blobTrigger}`);

Model v3

Metadata can be obtained from the bindingData property of the supplied context object, as shown in the following example, which logs the path to the triggering blob (blobTrigger), including the container:

module.exports = async function (context, myBlob) {
    context.log("Full blob path:", context.bindingData.blobTrigger);
};

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Metadata

Metadata is available through the $TriggerMetadata parameter. ::: zone-end

Usage

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The binding types supported by Blob trigger depend on the extension package version and the C# modality used in your function app.

Isolated worker model

[!INCLUDE functions-bindings-storage-blob-trigger-dotnet-isolated-types]

In-process model

See Binding types for a list of supported types.

Binding to string, or Byte[] is only recommended when the blob size is small. This is recommended because the entire blob contents are loaded into memory. For most blobs, use a Stream or BlobClient type. For more information, see Concurrency and memory usage.

If you get an error message when trying to bind to one of the Storage SDK types, make sure that you have a reference to the correct Storage SDK version.

[!INCLUDE functions-bindings-blob-storage-attribute] ::: zone-end

::: zone pivot="programming-language-java" The @BlobTrigger attribute is used to give you access to the blob that triggered the function. Refer to the trigger example for details. ::: zone-end
::: zone pivot="programming-language-javascript,programming-language-typescript"

Model v4

Access the blob data as the first argument to your function.

Model v3

Access blob data using context.bindings.<NAME> where <NAME> matches the value defined in function.json.

::: zone-end
::: zone pivot="programming-language-powershell"
Access the blob data via a parameter that matches the name designated by binding's name parameter in the function.json file. ::: zone-end
::: zone pivot="programming-language-python" Access blob data via the parameter typed as InputStream. Refer to the trigger example for details.

Functions also supports Python SDK type bindings for Azure Blob storage, which lets you work with blob data using these underlying SDK types:

BlobClient
ContainerClient
StorageStreamDownloader

Important

SDK types support for Python is currently in preview and is only supported for the Python v2 programming model. For more information, see SDK types in Python.

::: zone-end

[!INCLUDE functions-storage-blob-connections]

Blob name patterns

You can specify a blob name pattern in the path property in function.json or in the BlobTrigger attribute constructor. The name pattern can be a filter or binding expression. The following sections provide examples.

Tip

A container name can't contain a resolver in the name pattern.

Get file name and extension

The following example shows how to bind to the blob file name and extension separately:

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

If the blob is named original-Blob1.txt, the values of the blobname and blobextension variables in function code are original-Blob1 and txt.

Filter on blob name

The following example triggers only on blobs in the input container that start with the string "original-":

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

If the blob name is original-Blob1.txt, the value of the name variable in function code is Blob1.txt.

Filter on file type

The following example triggers only on .png files:

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

Filter on curly braces in file names

To look for curly braces in file names, escape the braces by using two braces. The following example filters for blobs that have curly braces in the name:

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

If the blob is named {20140101}-soundfile.mp3, the name variable value in the function code is soundfile.mp3.

Polling and latency

Polling works as a hybrid between inspecting logs and running periodic container scans. Blobs are scanned in groups of 10,000 at a time with a continuation token used between intervals. If your function app is on the Consumption plan, there can be up to a 10-minute delay in processing new blobs if a function app has gone idle.

Warning

Storage logs are created on a "best effort" basis. There's no guarantee that all events are captured. Under some conditions, logs may be missed.

If you require faster or more reliable blob processing, you should consider switching your hosting to use an App Service plan with Always On enabled, which may result in increased costs. You might also consider using a trigger other than the classic polling blob trigger. For more information and a comparison of the various triggering options for blob storage containers, see Trigger on a blob container.

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. While reprocessing might not occur immediately, it's guaranteed to occur at a later point in time. To reprocess immediately, the scaninfo blob in azure-webjobs-hosts/blobscaninfo can be updated. Any blobs with a last modified timestamp after the LatestScan property will be scanned again.

Poison blobs

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

If all 5 tries fail, Azure Functions adds a message to a Storage queue named webjobs-blobtrigger-poison. The maximum number of retries is configurable. The same MaxDequeueCount setting is used for poison blob handling and poison queue message handling. 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)

Memory usage and concurrency

::: zone pivot="programming-language-csharp" When you bind to an output type that doesn't support streaming, such as string, or Byte[], the runtime must load the entire blob into memory more than one time during processing. This can result in higher-than expected memory usage when processing blobs. When possible, use a stream-supporting type. Type support depends on the C# mode and extension version. For more information, see Binding types. ::: zone-end
::: zone pivot="programming-language-javascript,programming-language-typescript,programming-language-python,programming-language-powershell,programming-language-java" At this time, the runtime must load the entire blob into memory more than one time during processing. This can result in higher-than expected memory usage when processing blobs. ::: zone-end
Memory usage can be further impacted when multiple function instances are concurrently processing blob data. If you are having memory issues using a Blob trigger, consider reducing the number of concurrent executions permitted. Of course, reducing the concurrency can have the side effect of increasing the backlog of blobs waiting to be processed. The memory limits of your function app depends on the plan. For more information, see Service limits.

The way that you can control the number of concurrent executions depends on the version of the Storage extension you are using.

Extension 5.x and higher

When using version 5.0.0 of the Storage extension or a later version, you control trigger concurrency by using the maxDegreeOfParallelism setting in the blobs configuration in host.json.

Pre-extension 5.x

Because the blob trigger uses a queue internally, the maximum number of concurrent function invocations is controlled by the queues configuration in host.json.

Limits apply separately to each function that uses a blob trigger.

host.json properties

The host.json file contains settings that control blob trigger behavior. See the host.json settings section for details regarding available settings.

Next steps

Read blob storage data when a function runs
Write blob storage data from a function

Files

functions-bindings-storage-blob-trigger.md

Latest commit

History

functions-bindings-storage-blob-trigger.md

File metadata and controls

Azure Blob storage trigger for Azure Functions

Example

Attributes

Decorators

Annotations

Configuration

Metadata

Metadata

Metadata

Usage

Blob name patterns

Get file name and extension

Filter on blob name

Filter on file type

Filter on curly braces in file names

Polling and latency

Blob receipts

Poison blobs

Memory usage and concurrency

host.json properties

Next steps