Azure Functions Storage Bindings

Storage bindings in Azure Functions enable your functions to interact with various Azure storage services like Blob Storage, Table Storage, Queue Storage, and Cosmos DB in a declarative way. This simplifies data access by abstracting away the complexities of the storage SDKs.

Supported Storage Services

Azure Functions provide built-in support for the following storage services:

Azure Blob Storage: For storing and retrieving large amounts of unstructured data, such as documents or media files.
Azure Table Storage: A NoSQL key-attribute store for storing semi-structured NoSQL data.
Azure Queue Storage: For reliable messaging between application components.
Azure Cosmos DB: A globally distributed, multi-model database service for managing data at scale.

Types of Bindings

Storage bindings can be used in two primary ways:

Input Bindings

Input bindings allow you to read data from a storage service and pass it as an argument to your function. The runtime handles fetching the data before your function code is executed.

Example: Reading a blob file

In your function.json:


{
  "bindings": [
    {
      "name": "myBlob",
      "type": "blobTrigger",
      "direction": "in",
      "path": "samples-workitems/{name}.txt",
      "connection": "AzureWebJobsStorage"
    },
    {
      "name": "fileContent",
      "type": "blob",
      "direction": "in",
      "path": "samples-workitems/{name}.txt",
      "connection": "AzureWebJobsStorage"
    }
  ]
}

In your C# function:


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

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

Output Bindings

Output bindings allow your function to write data to a storage service. The runtime handles sending the data after your function has successfully executed.

Example: Writing to a blob

In your function.json:


{
  "bindings": [
    {
      "name": "inputQueueItem",
      "type": "queueTrigger",
      "direction": "in",
      "queueName": "myqueue-items",
      "connection": "AzureWebJobsStorage"
    },
    {
      "name": "outputBlob",
      "type": "blob",
      "direction": "out",
      "path": "output-blobs/{rand-guid}.txt",
      "connection": "AzureWebJobsStorage"
    }
  ]
}

In your C# function:


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

public static class BlobOutputExample
{
    public static void Run(
        string inputQueueItem,
        [Blob("output-blobs/{rand-guid}.txt", Connection = "AzureWebJobsStorage")] out string outputBlob,
        ILogger log)
    {
        log.LogInformation($"C# Queue trigger function processed: {inputQueueItem}");
        outputBlob = $"Processed: {inputQueueItem}";
    }
}

Configuration

Storage bindings are configured in the function.json file (or via attributes in compiled languages). Key properties include:

name: The name of the parameter in your function code.
type: The type of the binding (e.g., blob, queue, table, cosmosDB).
direction: in for input, out for output, inout for bidirectional.
path: The resource identifier (e.g., container/blob name, table name). Placeholders are supported.
connection: The name of the application setting that contains the storage connection string.

Connection Strings

Connection strings for Azure Storage services are typically stored in your function app's application settings. The default connection name is AzureWebJobsStorage, which often points to your primary Azure Storage account.

Common Connection Settings:

AzureWebJobsStorage
MyCustomStorageConnection

Advanced Scenarios

Storage bindings offer flexibility for various use cases, including:

Batching: Reading multiple queue messages or blob items at once.
Item-level processing: Processing individual items within a collection.
Specific Partition/Row Keys for Table Storage.
Document IDs and collections for Cosmos DB.

For detailed examples and specific configurations for each storage service, please refer to the official Azure Functions documentation for Blob Storage Bindings, Queue Storage Bindings, and Cosmos DB Bindings.