How to share a redis session across multiple express services

I'm currently working on an application built on microservices that looks roughly like this :

          Client
            |
         Gateway (express)
    ________|_________
   |        |         |
Express   Express    Other
Service   Service   Service

I would like to share a session between the gateway and all the services using redis as a session storage.

I tried using express-session and connect-redis and configure it like this on every service :

app.use(
    session({
        store: new redisStore({
            host: "redis",
            port: 6379,
            prefix: "sess",
            db: 4,
        }),
        name: "shared_session",
        secret: "mysecret",
        saveUninitialized: false,
        resave: false,
    }),
);

But when I try to set a variable in a service, I can't retrieve it in the other services as the session object seems different each time.

How to correctly configure express-session and connect-redis to share a session between my services and how to retrieve the correct session ?

via C. Gelnä