Azure Storage Blobs

Introduction to Blob Snapshots and Leases

Azure Blob Storage offers powerful features for managing your data, including snapshots and leases. Snapshots provide read-only, point-in-time copies of a blob, while leases offer a concurrency control mechanism for managing exclusive write access to a blob.

What are Blob Snapshots?

A blob snapshot is a read-only version of a blob that is taken at a specific point in time. When you create a snapshot, it's stored as a separate entity from the original blob. The snapshot retains the data and metadata of the blob at the time it was created. Snapshots are useful for:

• Creating backups or restore points for your data.
• Tracking changes to a blob over time.
• Providing a consistent view of data for applications.

Snapshots are billed separately from the original blob and are stored at the same redundancy level. You can list and retrieve snapshots, but you cannot modify them.

What are Blob Leases?

A blob lease grants exclusive write and delete access to a blob for a specified period. A lease is acquired by a client application, and other applications cannot modify or delete the blob until the lease expires or is released. Leases are essential for:

• Preventing multiple clients from simultaneously writing to the same blob.
• Ensuring data integrity in distributed systems.
• Implementing locking mechanisms for critical data.

Leases can be acquired with fixed durations (20-60 seconds) or infinite durations. You can also renew a lease if you still need exclusive access.

Creating and Managing Blob Snapshots

You can create a snapshot of a blob using the Azure portal, Azure CLI, PowerShell, or the Azure Storage SDKs.

Using Azure CLI to Create a Snapshot

To create a snapshot of a blob named myblob.txt in a container named mycontainer:

az storage blob snapshot --container-name mycontainer --name myblob.txt --output table

This command will return information about the snapshot, including its URI.

Listing Snapshots

You can list all snapshots for a specific blob:

az storage blob list --container-name mycontainer --prefix myblob.txt --snapshot --output table

Restoring from a Snapshot

To restore a blob to a previous state from a snapshot, you typically copy the snapshot back to the original blob name or a new blob name.

az storage blob copy --destination-blob myblob.txt --destination-container mycontainer --source-uri "https://mystorageaccount.blob.core.windows.net/mycontainer/myblob.txt?snapshot=2023-10-27T10:00:00Z"

Replace the snapshot URI with the actual URI of your snapshot.

Acquiring and Managing Blob Leases

Leases are managed through the Lease Blob operation. You specify the lease action, duration, and optionally a lease ID.

Acquiring a Lease

Acquiring an infinite lease on a blob:

az storage blob lease acquire --container-name mycontainer --blob-name myblob.txt --lease-duration infinite --lease-id "YOUR-UNIQUE-LEASE-ID"

YOUR-UNIQUE-LEASE-ID should be a GUID or other unique identifier. The response will include the lease ID if successful.

Renewing a Lease

If the lease is about to expire and you still require exclusive access:

az storage blob lease renew --container-name mycontainer --blob-name myblob.txt --lease-id "YOUR-UNIQUE-LEASE-ID"

Releasing a Lease

When you no longer need exclusive access:

az storage blob lease release --container-name mycontainer --blob-name myblob.txt --lease-id "YOUR-UNIQUE-LEASE-ID"

Breaking a Lease

If a lease holder becomes unavailable, you can break the lease:

az storage blob lease break --container-name mycontainer --blob-name myblob.txt

Breaking a lease makes it available for other clients to acquire after a short grace period.

Best Practices and Considerations

• Snapshot Management: Implement a strategy for managing snapshot retention to avoid excessive storage costs.
• Lease Duration: Choose appropriate lease durations. For long-running operations, consider renewing leases periodically.
• Error Handling: Your application should handle potential errors, such as trying to modify a blob with an active lease or acquiring a lease on a blob that already has one.
• Lease ID: Always use a lease ID when acquiring or renewing a lease. This helps prevent accidental release or renewal by other clients.
• Concurrency: Use leases judiciously. Overuse can lead to performance bottlenecks.

Further Reading

• Lease Blob REST API
• Snapshot Blob REST API
• Understanding and Using Blob Leases

Code Examples (Conceptual)

Here's a conceptual example of how you might use a lease in Python to ensure exclusive write access.

from azure.storage.blob import BlobServiceClient, LeaseClient
import uuid

# Replace with your actual connection string and container/blob names
connection_string = "YOUR_AZURE_STORAGE_CONNECTION_STRING"
container_name = "mycontainer"
blob_name = "mydata.txt"
lease_duration_seconds = 60 # Or -1 for infinite

blob_service_client = BlobServiceClient.from_connection_string(connection_string)
blob_client = blob_service_client.get_blob_client(container_name, blob_name)

lease_client = LeaseClient(blob_client)
lease_id = str(uuid.uuid4())

try:
    # Acquire the lease
    lease_client.acquire(lease_duration=lease_duration_seconds, lease_id=lease_id)
    print(f"Lease acquired successfully with ID: {lease_id}")

    # Now you have exclusive write access
    data = "This data can only be written while the lease is active."
    blob_client.upload_blob(data, overwrite=True)
    print("Blob data updated.")

    # You can renew the lease if needed
    # lease_client.renew(lease_id=lease_id)
    # print("Lease renewed.")

except Exception as e:
    print(f"An error occurred: {e}")
finally:
    # Release the lease when done
    if lease_client.lease_id:
        try:
            lease_client.release(lease_id=lease_id)
            print(f"Lease {lease_id} released.")
        except Exception as e:
            print(f"Failed to release lease {lease_id}: {e}")