Getting Started with Azure Function Blob Trigger

To create an Azure Function with a blob trigger, you need to have an Azure Storage account with a blob container. This account will be used to store the files that trigger the Azure Function.

First, you need to create a new Azure Function. In the Azure portal, go to the "Functions" section and click on the "New function" button. Then, select "Blob trigger" as the trigger type.

The blob trigger is triggered when a new file is added to the specified blob container. This can be a great way to automate tasks that need to be performed when a new file is uploaded.

To configure the blob trigger, you need to specify the blob container and the file types that should trigger the function.

Additional reading: Terraform Azure Blob Container

To get started with Azure Function Blob trigger, you'll need to create a new Azure Function project in Visual Studio. This will set up the basic structure for your function.

First, make sure you have the Azure Storage account and Azure Function Core Tools installed on your machine. You can download the Azure Function Core Tools from the official Microsoft documentation.

Next, create a new Azure Function project using the "Azure Functions v3" template in Visual Studio. This will give you a starting point for your function.

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Creating an app is a straightforward process. You can create a new function app in the Azure portal by selecting "Create a resource" from the menu or Home page.

To start, you'll need to select Compute > Function App. Make sure to choose the Consumption plan, which offers serverless hosting where you only pay for the time your functions run.

You'll also need to create a new resource group, as creating a new function app in an existing group can lead to known limitations. Name your resource group something like "myResourceGroup".

Additional reading: Azure Function C

Your function app name should be globally unique and follow a specific naming convention. You can choose a runtime that supports your preferred programming language, such as JavaScript, PowerShell, Python, or C# script.

Here's a table summarizing the suggested values for the Function App settings:

After filling in the settings, review and create your function app. You can also choose to use an existing storage account or Application Insights instance. Once you've created your function app, you can view it by selecting the "Go to resource" button.

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Support for in-process models will end on November 10, 2026, so it's essential to migrate to the isolated worker model for full support.

The isolated worker model is a great choice for Azure Functions, and it's easy to get started with. You can create a C# function that runs in an isolated worker process and uses a blob trigger with both blob input and blob output blob bindings.

Here's an example of a C# function that 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.

You can also use a blob trigger to write a log when a blob is added or updated in a container. For example, you can create a function that 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.

Here's a list of examples that demonstrate how to create a function that runs when a file is added to source blob storage container:

These examples show you how to create a function that runs when a file is added to source blob storage container, and how to use different programming languages and configuration files to achieve this.

The configuration of an Azure Function with a Blob trigger is a crucial step in setting up your function. You can set properties on the options object passed to the app.storageBlob() method, such as path, connection, and source.

To set up the binding configuration, you need to specify the type as blobTrigger, direction as in, and name of the variable that represents the blob in function code. The path property should be set to the container to monitor, which may be a blob name pattern. The connection property should be set to the name of an app setting or setting collection that specifies how to connect to Azure Blobs.

Here is a summary of the required properties:

To create a function in your new function app, you'll need to navigate to the Functions section of your app. From there, select the + Create under Functions option to start the process.

Discover more: Create Azure Blob Storage

You can choose from a variety of templates, but for this example, we'll create a new function from scratch. Select the "Create from Blank" option to begin.

In the Azure portal, you can select the "Create a resource" option from the menu or the Home page. From there, select "Compute" and then "Function App" to create a new function app.

To create a function, you'll need to specify a few details such as the function name, path, and storage account connection. These details can be found in the "Create an Azure Blob storage triggered function" section of the Azure documentation.

Here's a summary of the settings you'll need to configure:

Once you've configured these settings, you can create the function and start monitoring your Blob storage for new files.

The configuration of an Azure Function is a crucial aspect of its setup. You can configure the options object passed to the app.storageBlob() method with various properties.

The path property specifies the container to monitor, which can be a blob name pattern. The connection property is a reference to environment configuration that specifies how the app should connect to Azure Blobs.

You can also set the source property to 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.

To set up the binding configuration properties, you can use the function.json file. The type property must be set to blobTrigger, and the direction property must be set to in.

The name property is the name of the variable that represents the blob in function code, while the path property is the container to monitor, which can be a blob name pattern. The connection property is a reference to environment configuration that specifies how the app should connect to Azure Blobs.

Here is a table summarizing the properties that can be set in the function.json file:

The connection property can be a reference to an application setting containing a connection string, or a shared prefix for multiple application settings that define an identity-based connection. If the configured value is both an exact match for a single setting and a prefix match for other settings, the exact match is used.

To obtain a connection string, you can follow the steps shown at Manage storage account access keys. The connection string must be for a general-purpose storage account, not a Blob storage account.

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To grant access to Azure Functions, you need to give the correct permissions to the identity being used. This means assigning a role in Azure RBAC that provides the necessary permissions for the intended actions.

The principle of least privilege is essential here, where you grant only the required privileges to avoid excessive permissions. For example, if your app only needs to read from a data source, use a role that only has permission to read.

You need to create a role assignment that provides access to your blob container at runtime. Management roles like Owner aren't sufficient. Recommended built-in roles for the Blob Storage extension in normal operation include Storage Blob Data Owner, Storage Queue Data Contributor, Storage Blob Data Reader, Storage Blob Data Owner, and Storage Account Contributor.

The AzureWebJobsStorage connection requires extra permissions beyond the default requirement if it's configured to use an identity-based connection. These required permissions are covered by the Storage Blob Data Owner, Storage Queue Data Contributor, and Storage Account Contributor roles.

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To prepare the Azure Storage account, you need to create a general-purpose v2 storage account using the Azure Storage extension for Visual Studio Code. This involves providing a globally unique name, selecting a location, and creating a blob container.

You can create an event subscription by providing the URL of the specific endpoint to report Blob Storage events. This blob extension URL is composed of several parts, including the base function app URL, blob-specific path, function query string, and blob extension access key.

The blob extension access key is designed to make it more difficult for others to access your blob extension endpoint. To determine your blob extension access key, you need to copy the key value from the System keys section in the portal.

Here's a summary of the recommended built-in roles for the Blob Storage extension:

This list shows the recommended built-in roles for different binding types in the Blob Storage extension.

To create a Blob Storage triggered function in Azure, you'll need to use the @BlobTrigger attribute. This attribute gives you access to the blob that triggered the function.

The @BlobTrigger attribute can be used to set the source of the triggering event. You can 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.

Here are some key settings for the @BlobTrigger attribute:

In Azure Blob Trigger Attributes, attributes play a crucial role in defining how your functions interact with Azure Blob storage.

The attribute's constructor takes the following parameters: BlobPath, Connection, Access, and Source.

The BlobPath parameter indicates the path to the blob, while the Connection parameter specifies how to connect to Azure Blobs, which can be an app setting or setting collection.

The Access parameter indicates whether you will be reading or writing, giving you control over the direction of data flow.

Additional reading: Python Access Azure Blob Storage

The Source parameter sets the source of the triggering event, with options including EventGrid and LogsAndContainerScan.

You can use the StorageAccountAttribute to specify a storage account connection when you need to use a different storage account than other functions in the library.

The constructor of the StorageAccountAttribute takes the name of an app setting that contains a storage connection string.

The storage account to use is determined in the following order:

When developing locally, add your application settings in the local.settings.json file in the Values collection.

See what others are reading: Azure Function Local Settings Json

Annotations are a crucial part of setting up a blob trigger.

The @BlobTrigger attribute is used to give you access to the blob that triggered the function, which is essential for processing the uploaded file.

You can use the source property to set the source of the triggering event, allowing you to customize how your function responds to different types of blob uploads.

EventGrid is a great option for an Event Grid-based blob trigger, providing much lower latency compared to the standard polling mechanism.

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The default setting is LogsAndContainerScan, which uses the standard polling mechanism to detect changes in the container, but keep in mind that this may not be the most efficient option for large-scale applications.

To take full advantage of EventGrid, you'll need to use it explicitly, as it's not enabled by default.

When creating a Blob Storage triggered function, you'll need to configure the trigger settings. To do this, you'll select the "Blob trigger" template when creating a new function.

The "Job type" setting is only visible for Python v2 apps, and it defaults to "Append to app". This setting determines how the blob trigger will be used in your function app.

To name your new function, enter a unique name in the "New Function" field. This name will be used to identify the function in your function app.

The "Path" setting determines the location in Blob storage being monitored. You can specify a path like "samples-workitems/{name}", where the file name of the blob is passed in the binding as the name parameter.

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You can use the storage account connection already being used by your function app, or create a new one, for the "Storage account connection" setting.

Here's a summary of the trigger settings:

In addition to trigger settings, you'll also need to configure host.json settings for your function app. These settings only apply when using extension version 5.0.0 and higher.

The "maxDegreeOfParallelism" setting determines the number of concurrent invocations allowed for all blob-triggered functions in a given function app. The default value is 8 times the number of available cores, with a minimum allowed value of 1.

The "poisonBlobThreshold" setting determines the number of times to try processing a message before moving it to the poison queue. The default value is 5, with a minimum allowed value of 1.

You can filter blob triggers to only run on specific blobs by using a filter in the path property of function.json or in the BlobTrigger attribute constructor.

The filter can be a simple string, like "original-" to match blobs that start with the string "original-".

The blob name is then truncated to the part after the filter string, so if the blob is named "original-Blob1.txt", the name variable value in the function code is "Blob1.txt".

To look for curly braces in file names, you need to escape them by using two braces. This is because curly braces have a special meaning in Azure Functions, so you need to double them up to match them literally.

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

You can also specify a blob name pattern in the path property or in the BlobTrigger attribute constructor. This allows you to filter blobs based on their names, and can be a filter or binding expression.

A unique perspective: Azure Code