As Django developers, we often encounter performance bottlenecks, especially when dealing with session management. Database-backed sessions can slow down your application significantly. While Memcached offers a solution, it comes with its own challenge: losing session data during server restarts or cache refreshes. In this post, I’ll show you how to overcome this issue by using two separate Memcached instances - one for regular Python objects and another dedicated to session data.

The Problem with Single Memcached Instance

When using a single Memcached instance for both general caching and sessions, restarting the server or clearing the cache results in all users being logged out. This creates a poor user experience and can be particularly problematic for high-traffic sites.

The Solution: Dual Memcached Setup

By implementing a separate Memcached instance for sessions, we can maintain user sessions even when clearing the main cache. Here’s how to set it up:

1. Create two new files in your project directory:

File 1: session_backend.py

This file is a modified version of Django’s contrib/sessions/backends/cache.py:

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from django.contrib.sessions.backends.base import SessionBase, CreateError
from yourproject.session_cache import cache

class SessionStore(SessionBase):
    # ... [rest of the code remains the same as in the original post]

File 2: session_cache.py

This file initializes the separate cache for sessions:

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from django.core.cache.backends.memcached import CacheClass
from django.conf import settings

scheme, rest = settings.SESSION_CACHE.split(':', 1)
host = rest[2:-1]
cache = CacheClass(host, {})

2. Update your settings.py:

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SESSION_ENGINE = "yourproject.session_backend"
SESSION_CACHE = 'memcached://127.0.0.1:11200/'

3. Start a new Memcached instance on port 11200.

Benefits of This Approach

1. Improved Performance: Sessions are now managed in memory, significantly faster than database queries.
2. User Session Persistence: Users remain logged in even when clearing the main application cache.
3. Scalability: Separating session storage allows for easier scaling of your caching infrastructure.

Implementation Tips

• Ensure your Memcached instances are properly secured, especially if running on a separate server.
• Monitor the memory usage of your session Memcached instance to prevent overflow.
• Consider implementing session expiration policies to manage long-inactive sessions.

By implementing this dual Memcached setup, you’ll notice a substantial improvement in your Django application’s performance and user experience. The separation of concerns allows for more flexible cache management without disrupting user sessions.

Have you implemented this solution or have questions? Feel free to reach out to me at [email protected]. I’m always excited to discuss Django optimizations and help fellow developers enhance their applications.

Happy coding, and may your Django apps be ever swift and stable!