Practical Guide to Preventing Duplicate Processing from Concurrent Webhook Deliveries Using Redis Distributed Locks

1. System Architecture and Concurrency Control Design

To prevent duplicate processing of webhook deliveries, you need a mechanism that guarantees a given event is processed within a single execution context—even when multiple worker processes or server instances receive concurrent requests simultaneously.

While relational database transaction isolation levels and unique constraints can enforce idempotency, high-throughput webhook endpoints typically rely on Redis-based distributed locks (mutual exclusion). Redis provides fast, in-memory, atomic operations that minimize lock contention overhead.

The following sequence diagram illustrates the flow of acquiring an exclusive lock using Redis's SET command with the NX (set only if the key does not exist) and EX (set an expiration time in seconds) options:

sequenceDiagram
    participant Client as "External Webhook Sender"
    participant API as "Webhook Server (Python)"
    participant Redis as "Redis Server"

    Client->>API: Webhook Delivery (event_id: "evt_9988")
    Note over API: 1. Payload Validation (Pydantic)
    API->>Redis: SET webhook:lock:evt_9988 "locked" NX EX 30
    Redis-->>API: OK (Lock Acquired)
    Note over API: 2. Execute Business Logic (DB write, billing, etc.)
    API-->>Client: HTTP 200 OK

    par Concurrent or Delayed Duplicate Delivery
        Client->>API: Identical Webhook Delivery (event_id: "evt_9988")
        Note over API: 1. Payload Validation (Pydantic)
        API->>Redis: SET webhook:lock:evt_9988 "locked" NX EX 30
        Redis-->>API: Nil (Lock Acquisition Failed)
        Note over API: 2. Duplicate Delivery Detected
        API-->>Client: HTTP 409 Conflict
    end

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2. Implementation (High-Performance Processing with Pydantic v2 and orjson)

Below is the baseline Python implementation featuring strict event payload validation with Pydantic v2, high-speed JSON parsing/serialization via orjson, and atomic lock acquisition using Redis SET (NX + EX).

import sys
import time
from typing import Any, Dict, Optional
import redis
import orjson
from pydantic import BaseModel, Field, field_validator


class WebhookPayload(BaseModel):
    event_id: str = Field(..., description="Unique event ID sent by the external service")
    event_type: str = Field(..., description="Event type (e.g., payment.succeeded)")
    timestamp: int = Field(..., description="Event occurrence timestamp")
    data: Dict[str, Any] = Field(default_factory=dict, description="Detailed event payload")

    @field_validator("event_id")
    @classmethod
    def validate_event_id(cls, v: str) -> str:
        if not v or len(v.strip()) == 0:
            raise ValueError("event_id must not be empty.")
        return v.strip()


class WebhookResponse(BaseModel):
    status_code: int
    message: str
    event_id: str


class WebhookLockManager:
    """Atomic webhook exclusive lock manager using Redis SET NX and TTL."""

    def __init__(
        self,
        redis_client: redis.Redis,
        lock_ttl_seconds: int = 30,
        key_prefix: str = "webhook:lock:",
    ):
        self.redis = redis_client
        self.lock_ttl_seconds = lock_ttl_seconds
        self.key_prefix = key_prefix

    def _get_lock_key(self, event_id: str) -> str:
        return f"{self.key_prefix}{event_id}"

    def acquire_lock(self, event_id: str) -> bool:
        lock_key = self._get_lock_key(event_id)
        # NX=True sets the key only if it does not exist; EX sets the expiration time
        result = self.redis.set(
            name=lock_key, value="locked", ex=self.lock_ttl_seconds, nx=True
        )
        return bool(result)

    def release_lock(self, event_id: str) -> None:
        lock_key = self._get_lock_key(event_id)
        self.redis.delete(lock_key)


def process_webhook(
    payload_json: bytes, lock_manager: WebhookLockManager
) -> WebhookResponse:
    try:
        raw_data = orjson.loads(payload_json)
        payload = WebhookPayload(**raw_data)
    except (orjson.JSONDecodeError, ValueError) as e:
        return WebhookResponse(
            status_code=400,
            message=f"Invalid payload format or validation error: {str(e)}",
            event_id="unknown",
        )

    event_id = payload.event_id

    acquired = lock_manager.acquire_lock(event_id)
    if not acquired:
        return WebhookResponse(
            status_code=409,
            message="Conflict: Duplicate webhook delivery detected. Processing aborted.",
            event_id=event_id,
        )

    try:
        # Simulate business logic execution
        time.sleep(0.1)
        return WebhookResponse(
            status_code=200,
            message="Success: Webhook processed successfully.",
            event_id=event_id,
        )
    except Exception as e:
        lock_manager.release_lock(event_id)
        return WebhookResponse(
            status_code=500,
            message=f"Internal Server Error: {str(e)}",
            event_id=event_id,
        )


def main() -> None:
    try:
        client = redis.Redis(host="localhost", port=6379, db=0, socket_timeout=2.0)
        client.ping()
    except redis.ConnectionError:
        error_output = orjson.dumps(
            {
                "status_code": 503,
                "message": "Redis connection failed. Ensure Redis is running and dependencies are installed via 'pip install -r requirements.txt'.",
            }
        ).decode("utf-8")
        print(error_output, file=sys.stderr)
        sys.exit(1)

    lock_manager = WebhookLockManager(redis_client=client, lock_ttl_seconds=10)

    test_payload = WebhookPayload(
        event_id="evt_tx_9988776654",
        event_type="payment.succeeded",
        timestamp=int(time.time()),
        data={"amount": 5000, "currency": "JPY"},
    )
    payload_bytes = orjson.dumps(test_payload.model_dump())

    print("--- 1st Webhook Delivery (Initial Request) ---")
    response_1 = process_webhook(payload_bytes, lock_manager)
    print(orjson.dumps(response_1.model_dump()).decode("utf-8"))

    print("\n--- 2nd Webhook Delivery (Duplicate Event due to Network Delay, etc.) ---")
    response_2 = process_webhook(payload_bytes, lock_manager)
    print(orjson.dumps(response_2.model_dump()).decode("utf-8"))

    assert response_1.status_code == 200, "The initial request should succeed (200)"
    assert response_2.status_code == 409, "The duplicate request should be detected as a conflict (409)"
    print("\n[Verification Succeeded] Duplicate processing prevention via Redis distributed lock is working properly.")


if __name__ == "__main__":
    main()

---

3. Post-Mortem: Failure Analysis During Implementation and Verification

During the implementation and QA testing phases of this project, despite the core logic being theoretically sound, the final execution phase failed with the following fatal error:

Error Encountered

Traceback (most recent call last):
  File "/home/phenox/gemini-sandbox/TOAI_Workspace/V2_SandBox/V2_PROD_20261113_073754_test.py", line 16, in <module>
    import redis
ModuleNotFoundError: No module named 'redis'

Root Cause Analysis

1. Discrepancy Between Execution Environment and Dependencies: Although the script's comments and documentation explicitly instructed running pip install -r requirements.txt beforehand, the automated test runner and isolated process space did not execute the package installation step within the target Python virtual environment.
2. Missing Dependency Guard in Script Design: Because the Python script statically imported redis and pydantic at the top level, executing it in an environment lacking external dependencies caused an immediate interpreter-level crash before reaching any initialization logic or connection checks—a classic anti-pattern for standalone scripts.

Architectural Lessons Learned

When designing CLI scripts or batch processors that depend on external libraries, relying solely on documentation or comments to instruct users to run pip install leads to brittle failures in automated tests, CI/CD pipelines, and distributed execution environments.

To build a resilient system, one of the following safeguards should have been integrated into the codebase:

• Dynamic Dependency Check and Auto-Installation: At script startup (before top-level third-party imports), use importlib.util to verify the presence of required packages. If any are missing, either attempt a dynamic pip install via subprocess or exit cleanly with a structured JSON error payload and a clear exit code.
• Strict Containerization: Eliminate environment drift entirely by encapsulating both the Python runtime and the Redis server lifecycle within a Dockerfile or DevContainer.

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