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Using another memcached for sessions

Author:
dipankarsarkar
Posted:
November 15, 2008
Language:
Python
Version:
1.0
Tags:
django python memcached sessions
Score:
0 (after 0 ratings)

This solves the problem of losing sessions data when you restart memcached. So you use a different memcached instance for sessions which you rarely restart.

Use the above code and add the following to you settings.py

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

(Above assumes that your session's memcached is running on port 11200)

Feel free to contact me in case you need help. By Dipankar sarkar [email protected]

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<project>/session_backend.py

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

class SessionStore(SessionBase):
    """
    A cache-based session store.
    """
    def __init__(self, session_key=None):
        self._cache = cache
        super(SessionStore, self).__init__(session_key)

    def load(self):
        session_data = self._cache.get(self.session_key)
        if session_data is not None:
            return session_data
        self.create()
        return {}

    def create(self):
        # Because a cache can fail silently (e.g. memcache), we don't know if
        # we are failing to create a new session because of a key collision or
        # because the cache is missing. So we try for a (large) number of times
        # and then raise an exception. That's the risk you shoulder if using
        # cache backing.
        for i in xrange(10000):
            self.session_key = self._get_new_session_key()
            try:
                self.save(must_create=True)
            except CreateError:
                continue
            self.modified = True
            return
        raise RuntimeError("Unable to create a new session key.")

    def save(self, must_create=False):
        if must_create:
            func = self._cache.add
        else:
            func = self._cache.set
        result = func(self.session_key, self._get_session(no_load=must_create),
                self.get_expiry_age())
        if must_create and not result:
            raise CreateError

    def exists(self, session_key):
        if self._cache.get(session_key):
            return True
        return False

    def delete(self, session_key=None):
        if session_key is None:
            if self._session_key is None:
                return
            session_key = self._session_key
        self._cache.delete(session_key)

----------------------
<project>/session_cache.py

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,{})

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