Advanced Topics

Session Stores

The molten.contrib.sessions module adds support for user sessions. It works by providing a Session component – a standard dictionary that’s guaranteed to contain a unique “id” key – and a way to serialize and deserialize Session objects via the SessionStore protocol.

The only session store implementation that’s provided out of the box is CookieStore, which stores all the session information in browser cookies.

Session Lifecycle

When a handler requests the Session component for the first time, SessionStore.load is called to create the Session object based on the incoming request data. After the handler completes, SessionStore.dump is called to serialize the Session and return a Cookie representing it.

In pseudocode this process might look something like this:

session = session_store.load()
    response = handler(session)

Custom Session Stores

To implement a custom session store, all you have to do is provide a class that implements the SessionStore protocol. Here’s what a redis-backed session store might look like, assuming you have a Redis component in the system that can communicate with a redis server:

class RedisStore:
    def load(self, cookies: Cookies, redis: Redis) -> Session:
        session_id = cookies.get("session-id")
        if not session_id:
            return Session.empty()

        session_data = redis.hgetall(session_id)
        if not session_data:
            return Session.empty()

        return Session(**session_data)

    def dump(self, session: Session, redis: Redis) -> Cookie:
        redis.hmset(, session)
        return Cookie("session-id",

On load, this store tries to get the current session id from the request (via the “session-id” cookie) and then load the session data from redis. On dump, it writes the session data to redis under the session id key and it returns a Cookie containing the session id so that subsequent calls to load can retrieve the session from Redis.