Mutex With GCP Bucket

Here’s a conceptual approach to implementing a mutex using Python and Google Cloud Storage:

1. Enable Object Versioning on your GCS Bucket: This step is optional but recommended. It ensures that every change to an object, including the creation and deletion of the lock file, is tracked with a unique version. This can help prevent race conditions.

2. Check for the Existence of the Lock File: Before performing the operation that requires mutual exclusion, the application should check if a lock file exists in the GCS bucket.

3. Create the Lock File if it Doesn’t Exist: If the lock file does not exist, the application should attempt to create it. Use the ifGenerationMatch precondition set to 0 to ensure that the operation only succeeds if the file does not exist.

4. Perform the Operation: If the application successfully creates the lock file, it can proceed with the operation that requires mutual exclusion.

5. Delete the Lock File: After the operation is complete, the application should delete the lock file, allowing other instances to proceed.

Code:

from google.cloud import storage
from google.cloud.exceptions import NotFound, PreconditionFailed
 
def acquire_lock(bucket_name, lock_file_name):
    storage_client = storage.Client()
    bucket = storage_client.bucket(bucket_name)
    blob = bucket.blob(lock_file_name)
    
    try:
        # Attempt to create the lock file. This operation will fail if the file already exists.
        blob.upload_from_string('lock', if_generation_match=0)
        return True
    except PreconditionFailed:
        # Lock file already exists
        return False
 
def release_lock(bucket_name, lock_file_name):
    storage_client = storage.Client()
    bucket = storage_client.bucket(bucket_name)
    blob = bucket.blob(lock_file_name)
    
    try:
        # Delete the lock file
        blob.delete()
    except NotFound:
        # Lock file was already deleted, or never existed
        pass
 
# Usage
bucket_name = 'your-bucket-name'
lock_file_name = 'your-lock-file-name'
 
if acquire_lock(bucket_name, lock_file_name):
    try:
        # Perform your operation here
        print("Lock acquired, performing operation.")
    finally:
        release_lock(bucket_name, lock_file_name)
else:
    print("Lock could not be acquired, operation skipped.")

Possible context manager:

from google.cloud import storage
from google.cloud.exceptions import NotFound, PreconditionFailed
from contextlib import contextmanager
 
# Assuming acquire_lock and release_lock functions are defined as above
 
@contextmanager
def gcs_lock(bucket_name, lock_file_name):
    storage_client = storage.Client()
    bucket = storage_client.bucket(bucket_name)
    blob = bucket.blob(lock_file_name)
    
    try:
        # Try to acquire the lock
        blob.upload_from_string('lock', if_generation_match=0)
        acquired = True
    except PreconditionFailed:
        # Lock file already exists
        acquired = False
 
    if acquired:
        try:
            yield
        finally:
            # Ensure the lock is released after the operation
            try:
                blob.delete()
            except NotFound:
                # Lock file was already deleted, or never existed
                pass
    else:
        raise RuntimeError("Failed to acquire lock")
 
# Usage example
bucket_name = 'your-bucket-name'
lock_file_name = 'your-lock-file-name'
 
def my_sensitive_operation():
    print("Performing a sensitive operation with mutual exclusion.")
 
with gcs_lock(bucket_name, lock_file_name):
    my_sensitive_operation()