# Sessions

*NaifJs* is a multi-user dialog manager. 
The dialog system act as a "server" that manages dialogues with multiple users, in parallel.
Even if used as an embedded library, the engine manages different conversational sessions at once.

## User session thread

A user session, or *session thread* 
is a sequence of back-and-forth between a final user and the dialog system.

A session start when a dialog unit state is activated by a `start` DSL statement
and it terminates:
- normally when the dialog flow encounter an `end` statement. 
- when the user abandons the dialog and a prompt timeout expires.

## Session id

Each inbound `request`, the input sentence coming from a user, 
is identified by an *session id* (any unique identifier string defined by the caller application).

> Note that *user id*, *session id*, *sender_id*, *chat id* or simply *id* are synonyms in NaifJs parlance.

```javascript
  const user = 'Giorgio Robino'
  const sentence = 'I love you, Naif!' 

  naif.request(user, sentence)
```

## Session data storage

To manage multi users at the same time, 
for each *sessionid* the engine store some dialog data 
(the *stateid* conversation state and more attributes) into a *session* data structure:

- dialog *stateid*
- time of last interaction
- last user request
- last response to user
- flag indicating if the dialog is currently active 
- [dialog unit variables](variables.md)

The sessions database is currently managed as in-memory object, 
disk-persistent (with a loaded/saved JSON file). 

When the engine bootstrap (`up`), it checks if the session data storage JSON file does exist and load it in memory.
When the engine stop working (`down`) the in-memory sessions are saved again on the JSON file.
By default, if not specified by developer, the engine load or create a `sessions.json` in the application directory. 

> If *NaifJs* is used for a single-user application (by example, an embedded system/IoT device), nothing change. 
> In this specific case the session storage will manage just a single entry.


### Session file example

Here below a session file example, 
containing entries for two users (session ids are in this case: `34824744` and `103034393`). 
```
$ cat examples/story_it/sessions.js
```
```json
{
  "34824744": {

    "unit": "secondUnit",
    "state": "restart",

    "time": 1602775249799,
    "active": false,

    "variables": {
      "firstUnit": {
        "firstName": "Giorgio"
      }
    },

    "response": {
      "text": "Arrivederci Giorgio!",
      "speech": false,
      "time": 1602775251566,
      "stateid": "secondUnit.restart"
    },

    "request": {
      "text": "no",
      "speech": false,
      "time": 1602775251565
    }

  },

  "103034393": {

    "unit": "ascoltaeripeti",
    "state": "restart",

    "time": 1561539134,
    "active": false,

    "response": {
      "text": "Arrivederci Giorgio!",
      "speech": false,
      "time": 1602775251577,
      "stateid": "ascoltaeripeti.restart"
    },

    "request": {
      "text": "no",
      "speech": false,
      "time": 1602775251000
    },

    "variables": {
      "leggieripeti": {
        "word": "TASCHE",
        "retryNum": 5,
        "score": 0
      },

      "ascoltaescrivi": {
        "word": "MERCATI",
        "retryNum": 5,
        "score": 0
      }
    }
  }
}  
```

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