Manage blob properties and metadata with Go
In addition to the data they contain, blobs support system properties and user-defined metadata. This article shows how to manage system properties and user-defined metadata using the Azure Storage client module for Go.
Prerequisites
- Azure subscription - create one for free
- Azure storage account - create a storage account
- Go 1.18+
Set up your environment
If you don't have an existing project, this section shows how to set up a project to work with the Azure Blob Storage client module for Go. The steps include module installation, adding import
paths, and creating an authorized client object. For details, see Get started with Azure Blob Storage and Go.
Install modules
Install the azblob module using the following command:
go get github.com/Azure/azure-sdk-for-go/sdk/storage/azblob
To authenticate with Microsoft Entra ID (recommended), install the azidentity
module using the following command:
go get github.com/Azure/azure-sdk-for-go/sdk/azidentity
Add import paths
In your code file, add the following import paths:
import (
"github.com/Azure/azure-sdk-for-go/sdk/azidentity"
"github.com/Azure/azure-sdk-for-go/sdk/storage/azblob"
)
These import paths represent the minimum needed to get started. Some code examples in this article might require additional import paths. For specific details and example usage, see Code samples.
Create a client object
To connect an app to Blob Storage, create a client object using azblob.NewClient. The following example shows how to create a client object using DefaultAzureCredential
for authorization:
func getServiceClientTokenCredential(accountURL string) *azblob.Client {
// Create a new service client with token credential
credential, err := azidentity.NewDefaultAzureCredential(nil)
handleError(err)
client, err := azblob.NewClient(accountURL, credential, nil)
handleError(err)
return client
}
Authorization
The authorization mechanism must have the necessary permissions to work with container properties or metadata. For authorization with Microsoft Entra ID (recommended), you need Azure RBAC built-in role Storage Blob Data Reader or higher for the get operations, and Storage Blob Data Contributor or higher for the set operations. To learn more, see the authorization guidance for Set Blob Properties (REST API), Get Blob Properties (REST API), Set Blob Metadata (REST API), or Get Blob Metadata (REST API).
About properties and metadata
System properties: System properties exist on each Blob storage resource. Some of them can be read or set, while others are read-only. Under the covers, some system properties correspond to certain standard HTTP headers. The Azure Storage client library for Go maintains these properties for you.
User-defined metadata: User-defined metadata consists of one or more name-value pairs that you specify for a Blob storage resource. You can use metadata to store additional values with the resource. Metadata values are for your own purposes only, and don't affect how the resource behaves.
Metadata name/value pairs are valid HTTP headers and should adhere to all restrictions governing HTTP headers. For more information about metadata naming requirements, see Metadata names.
Note
Blob index tags also provide the ability to store arbitrary user-defined key/value attributes alongside an Azure Blob storage resource. While similar to metadata, only blob index tags are automatically indexed and made searchable by the native blob service. Metadata cannot be indexed and queried unless you utilize a separate service such as Azure Search.
To learn more about this feature, see Manage and find data on Azure Blob storage with blob index (preview).
Set and retrieve properties
To set properties on a blob, call the following method from a blob client object:
Any properties not explicitly set are cleared. To preserve any existing properties, you can first retrieve the blob properties, then use them to populate the headers that aren't being updated.
The following code example sets the BlobContentType
and BlobContentLanguage
system properties on a blob, while preserving the existing properties:
func setBlobProperties(client *azblob.Client, containerName string, blobName string) {
// Reference the blob as a client object
blobClient := client.ServiceClient().NewContainerClient(containerName).NewBlobClient(blobName)
// Get the existing blob properties
resp, err := blobClient.GetProperties(context.TODO(), nil)
handleError(err)
// Set the new blob properties and include existing properties
_, err = blobClient.SetHTTPHeaders(context.TODO(), blob.HTTPHeaders{
BlobContentType: to.Ptr("text/plain"),
BlobContentLanguage: to.Ptr("en-us"),
BlobContentEncoding: resp.ContentEncoding,
BlobContentDisposition: resp.ContentDisposition,
BlobCacheControl: resp.CacheControl,
}, nil)
handleError(err)
}
To retrieve properties on a blob, call the following method from a blob client object:
The following code example gets a blob's system properties and displays some of the values:
func getBlobProperties(client *azblob.Client, containerName string, blobName string) {
// Reference the blob as a client object
blobClient := client.ServiceClient().NewContainerClient(containerName).NewBlobClient(blobName)
// Get the blob properties
resp, err := blobClient.GetProperties(context.TODO(), nil)
handleError(err)
// Print the blob properties
fmt.Printf("Content type: %v\n", *resp.ContentType)
fmt.Printf("Content language: %v\n", *resp.ContentLanguage)
}
Set and retrieve metadata
You can specify metadata as one or more name-value pairs on a blob or container resource. To set metadata, send a map containing name-value pairs using the following method from a blob client object:
The following code example sets metadata on a blob:
func setBlobMetadata(client *azblob.Client, containerName string, blobName string) {
// Reference the blob as a client object
blobClient := client.ServiceClient().NewContainerClient(containerName).NewBlobClient(blobName)
// Set the blob metadata
var metadata = make(map[string]*string)
metadata["key1"] = to.Ptr("value1")
metadata["key2"] = to.Ptr("value2")
_, err := blobClient.SetMetadata(context.TODO(), metadata, nil)
handleError(err)
}
To retrieve metadata, call the GetProperties method from a blob client object, and access the Metadata
field in the response. The GetProperties
method retrieves blob properties and metadata by calling both the Get Blob Properties operation and the Get Blob Metadata operation.
The following code example reads metadata on a blob and prints each key/value pair:
func getBlobMetadata(client *azblob.Client, containerName string, blobName string) {
// Reference the blob as a client object
blobClient := client.ServiceClient().NewContainerClient(containerName).NewBlobClient(blobName)
// Get the blob properties, which includes metadata
resp, err := blobClient.GetProperties(context.TODO(), nil)
handleError(err)
// Print the blob metadata
for k, v := range resp.Metadata {
fmt.Printf("%v: %v\n", k, *v)
}
}
Note
The code samples in this guide are intended to help you get started with Azure Blob Storage and Go. You should modify error handling and Context
values to meet the needs of your application.
Resources
To learn more about how to manage system properties and user-defined metadata using the Azure Blob Storage client module for Go, see the following resources.
Code samples
- View code samples from this article (GitHub)
REST API operations
The Azure SDK for Go contains libraries that build on top of the Azure REST API, allowing you to interact with REST API operations through familiar Go paradigms. The client library methods for managing system properties and user-defined metadata use the following REST API operations:
- Set Blob Properties (REST API)
- Get Blob Properties (REST API)
- Set Blob Metadata (REST API)
- Get Blob Metadata (REST API)