gcslock 0.0.2

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gcs-atomic-lock

Google Cloud Storage (GCS) object generations and metagenerations are used to implement a lightweight distributed lock with strong, atomic semantics. This library wraps the key conditional operations and provides a simple Python API (context manager) to acquire and release a lock stored as a GCS object.

• Language: Python 3.10
• Package manager: uv
• Status: Library + unit tests
• License: See LICENSE

Why use this?

• Atomic lock acquisition via GCS conditional writes (ifGenerationMatch=0)
• Lock extension/update using metageneration preconditions (ifMetagenerationMatch)
• Simple Pythonic API (with-context)
• Pluggable access layer for HTTP/credential handling
• Helpful exceptions and logging
• Optional in-memory TTL map to minimize redundant work

How it works (high level)

1. Check lock object

   • GET the lock object in the bucket.
   • If not found → lock appears free (or expired; see metadata).
   • If found → read generation, metageneration, metadata.

2. Try to acquire (create) the lock

   • PUT (or upload) with ifGenerationMatch=0.
   • If the object already exists, GCS returns 412 Precondition Failed → lock acquisition fails.
   • If created (200 OK), you hold the lock.

3. Update metadata (optional/renew)

   • PATCH the object with ifMetagenerationMatch=<current>.
   • If someone else updated it in between, 412 is returned → treat as contention.
   • On success (200 OK) → your lock metadata (e.g., owner, expiry) is updated.

4. Release the lock

   • DELETE the object to unlock (204 No Content on success).
   • Optionally validate ownership based on stored metadata before deleting.

Flow

flowchart
    A[Start] --> B{GET Bucket} 
    B -- Not Found --> Z[Bucket Not Found Error] 
    B -- Found --> C{GET Object} 
    C -- Not Found --> D[PUT with ifGenerationMatch=0] 
    D -- 412 --> E[LOCK FAILED] 
    D -- 200 --> F[LOCK SUCCESS] 
    C -- Found --> G[PATCH with ifMetagenerationMatch=CurrentValue] 
    G -- 412 --> E 
    G -- 200 --> F 
    F --> H[DELETE Object to Unlock] 
    H -- 204 --> I[LOCK RELEASED]

Installation

pip install gcslock

Example

from gcslock import GcsLock, LockState
from gcslock.exception import GcsLockError

BUCKET = "your-bucket-name"
OBJECT = "locks/my-resource.lock"
OWNER = "your-owner-id"
LOCK_EXPIRES_SEC = 60

lock = GcsLock(BUCKET, OBJECT, lock_owner=OWNER)

try:
    with lock.acquire(lock_id=OBJECT, expires_seconds=LOCK_EXPIRES_SEC) as lock:
        # Critical section
        ...
except GcsLockError as e:
    print(f"Lock error: {e}")