How to identify unique users with Diagflow

I am trying to make an assistant app and was using the cloud firestore service of firebase to send the response back to my app using webhook as fulfilment. I have used 'session' parameter in request JSON according to this documentation and sending fulfilmentText as response to the user. But whenever user launches the app, a new session is created which I don't want. I simply want, just a single entry for each user in my database so how to achieve that using dialogflow.

In Alexa Skill, we have deviceId as parameter by which we can uniquely identify the user irrespective of the session id but is there any parameter in the dialogflow request JSON. If not, then how to achieve this task without it.

The request JSON I am getting from Dialogflow has a userID in it, so can I use the userId or should I go with userStorage provided the userStorage parameter is not available in the request JSON.

request.body.originalDetectIntentRequest { source: 'google',   version: '2',   payload:     { surface: { capabilities: [Object] },
     inputs: [ [Object] ],
     user: 
      { locale: 'en-US',
        userId: 'ABwppHG5OfRf2qquWWjI-Uy-MwfiE1DQlCCeoDrGhG8b0fHVg7GsPmaKehtxAcP-_ycf_9IQVtUISgfKhZzawL7spA' },
     conversation: 
      { conversationId: '1528790005269',
        type: 'ACTIVE',
        conversationToken: '["generate-number-followup"]' },
     availableSurfaces: [ [Object] ] } }

EDIT : Thank You @Prisoner for the answer but I am unable to send the random ID generated in the response and set in in the payload. Below is the code where I am generating the uuid and storing it in firestore. What I am doing wrong in the below code due to which new uuid is generated for returning user and therefore response is shown as No document found in the database. I suppose I am not sending uuid appropriately. Please help.

exports.webhook = functions.https.onRequest((request, response) => {


    console.log("request.body.queryResult.parameters", request.body.queryResult.parameters);
    console.log("request.body.originalDetectIntentRequest.payload", request.body.originalDetectIntentRequest.payload);

    let userStorage = request.body.originalDetectIntentRequest.payload.user.userStorage || {};
    let userId;
    console.log("userStorage", userStorage);

    if (userId in userStorage) {
      userId = userStorage.userId;
    } else {
      var uuid = require('uuid/v4');
      userId = uuid();
      userStorage.userId = userId
    }

    console.log("userID", userId);

    switch (request.body.queryResult.action) {
      case 'FeedbackAction': {

            let params = request.body.queryResult.parameters;

            firestore.collection('users').doc(userId).set(params)
              .then(() => {

              response.send({
                'fulfillmentText' : `Thank You for visiting our ${params.resortLocation} hotel branch and giving us ${params.rating} and your comment as ${params.comments}.` ,
                'payload': {
                  'google': {
                    'userStorage': userStorage
                  }
                }

                });
                return console.log("resort location", params.resortLocation);
            })
            .catch((e => {

              console.log('error: ', e);

              response.send({
             'fulfillmentText' : `something went wrong when writing to database`,
             'payload': {
               'google': {
                 'userStorage': userStorage
               }
             }
                });
            }))

        break;
      }
        case 'countFeedbacks':{

          var docRef = firestore.collection('users').doc(userId);

          docRef.get().then(doc => {
              if (doc.exists) {
                  // console.log("Document data:", doc.data());
                  var dat = doc.data();
                  response.send({
                    'fulfillmentText' : `You have given feedback for ${dat.resortLocation} and rating as ${dat.rating}`,
                    'payload': {
                      'google': {
                        'userStorage': userStorage
                      }
                    }
                  });

              } else {
                  // doc.data() will be undefined in this case
                  console.log("No such document!");

                  response.send({
                    'fulfillmentText' : `No feedback found in our database`,
                    'payload': {
                      'google': {
                        'userStorage': userStorage
                      }
                    }
                  });

              }
              return console.log("userStorage_then_wala", userStorage);
          }).catch((e => {
              console.log("Error getting document:", error);
              response.send({
                'fulfillmentText' : `something went wrong while reading from the database`,
                'payload': {
                  'google': {
                    'userStorage': userStorage
                  }
                }
              })
          }));

          break;
        }

You have a couple of options, depending on your exact needs.

Simple: userStorage

Google provides a userStorage object which is persisted across conversations when it can identify a user. This lets you store your own identifier when you need to track when a user returns.

The easiest way to do this is to check the userStorage object for the identifier when your webhook is called. If it doesn't exist, create one using something like a v4 UUID and save it in the userStorage object.

If you are using the actions-on-google library, the code might look something like this:

let userId;
// if a value for userID exists un user storage, it's a returning user so we can
// just read the value and use it. If a value for userId does not exist in user storage,
// it's a new user, so we need to generate a new ID and save it in user storage.
if (userId in conv.user.storage) {
  userId = conv.user.storage.userId;
} else {
  // Uses the "uuid" package. You can get this with "npm install --save uuid"
  var uuid = require('uuid/v4');
  userId = uuid();
  conv.user.storage.userId = userId
}

If you are using the dialogflow library, you can use the above, but you'll need this line first:

let conv = agent.conv();

If you're using the multivocal library, it does all of the above for you and will provide a UserID in the environment under the path User/Id.

If you're handling the JSON directly, and you are using the Dialogflow v2 protocol, you can get the userStorage object by examining originalDetectIntentRequest.payload.user.userStorage in the JSON request object. You'll set the payload.google.userStorage object in the JSON response. The code is similar to the above and might look something like this:

let userStorage = body.originalDetectIntentRequest.payload.user.userStorage || {};
let userId;
// if a value for userID exists un user storage, it's a returning user so we can
// just read the value and use it. If a value for userId does not exist in user storage,
// it's a new user, so we need to generate a new ID and save it in user storage.
if (userId in userStorage) {
  userId = userStorage.userId;
} else {
  // Uses the "uuid" package. You can get this with "npm install --save uuid"
  var uuid = require('uuid/v4');
  userId = uuid();
  userStorage.userId = userId
}

// ... Do stuff with the userID

// Make sure you include the userStorage as part of the response
var responseBody = {
  payload: {
    google: {
      userStorage: JSON.stringify(userStorage),
      // ...
    }
  }
};

Note the first line of the code - if userStorage doesn't exist, use an empty object. It won't exist until you send a response that includes storing something in it for the first time, which will happen in the last few lines of this code.

Advanced: Account Linking

You can request users to sign in to your Action using Google Sign In. This can be done just using voice for the simplest of cases and would only interrupt the flow the first time.

After this, your Action is given a JWT which contains their Google ID which you can use as their identifier.

If you're using the actions-on-google library, you can get the ID from the decoded JWT with a line such as:

const userId = conv.user.profile.payload.sub;

In the multivocal library, the ID from the decoded JWT is available in the environment under the path User/Profile/sub

Deprecated: Anonymous User ID

You'll see some answers here on StackOverflow that reference an Anonymous User ID. Google has deprecated this identifier, which was not always a reliable way to verify returning users, and will be removing it 1 Jun 2019.

This code is currently still being sent, but this will be removed starting 1 Jun 2019.