"MEDIUM: FCM topic subscription blocks synchronously in request loop, causing API latency"

[pyrahermesagent]

Severity: MEDIUM

File: ApiApp/views.py
Lines: 64-69

Description

FCMTokenUpdateView calls messaging.subscribe_to_topic() synchronously inside a request loop. Each FCM topic subscription is an HTTP call to Google Cloud, and these calls are made sequentially without any timeout or error recovery.

Current code:

topics = ["global", device.type]
for topic in topics:
    try:
        messaging.subscribe_to_topic([device.registration_id], topic)
    except Exception as e:
        logger.debug(f"Failed to subscribe device {device.id} to topic {topic}: {e}")

Problems:

1. Blocking I/O: Each subscribe_to_topic() call makes an HTTP request to Google Cloud FCM. If the network is slow or Google Cloud is under load, the request hangs.
2. No timeout: The HTTP call has no explicit timeout, so a slow response can block the request indefinitely.
3. Request latency: The user experience is degraded because the API response waits for all topic subscriptions to complete before returning.
4. Error swallowing: Failures are only logged at debug level — no visibility into subscription failures in production.

Impact

• API latency: Every FCM token update request is delayed by network round-trips to Google Cloud
• User experience: Mobile apps wait longer for token registration confirmation
• Silent failures: Topic subscription errors are invisible in production logging

Root Cause

Synchronous FCM API calls inside the request handler without timeout, async offloading, or error monitoring.

Fix

Move topic subscriptions to a background task (Celery/Django Q) with proper timeout and error handling:

# Offload to background task
from django_q.tasks import async_task

async_task("ApiApp.tasks.subscribe_device_to_topics", device.id, topics)

# Or use a simple async approach with timeout
import asyncio

async def subscribe_topics(device, topics):
    for topic in topics:
        try:
            await asyncio.wait_for(
                messaging.subscribe_to_topic([device.registration_id], topic),
                timeout=5.0
            )
        except Exception as e:
            logger.error(f"Failed to subscribe device {device.id} to topic {topic}: {e}")

@valentynhol please review.