Remember user login state
I want to take  `koa-session` to remember  user  login state. And after reading docs and code ,I got  blocked by the concept `store session  in cookie`.

I take this code  to show my understanding 
 route --> `/signin`
```
router.post('signin', async ctx => { 
   /*
    *  parse  http  request  body    param-->(userName, password)
    */
    ctx.session.userList = ctx.session.userList || []    // userList to register user who have logged 
    if (ctx.session.userList.indexOf(userName) === -1) {
      ctx.throw (400, 'user  is online')
    }  else {
     //  check userName and  password  in db
     ctx.session.userList.push(userName)
     ctx.status=200  // response to browser and  create a cookie  (key as config.key, value as encode(session))
   }
})
```

In this way, anyOne  who visits route `signin`  will recieve a cookie.  And as more users log in, `userList` in session  and  `cookie(key.value)` both will grow.

Is this a  correct understaning?I used to store session id in db to implement this situation so i'm really not certain about this. Please show me something if it's wrong,  thanks a lot.