Best way to index faces with AWS Rekognition and associate them to the userId in MySQL

I'm including facial recognition in my Electron app using Node.js and I would like to know if I'm doing the process correctly, as I was a little confused by some points in the documentation.

I'm using the aws-sdk library and I want that, when taking a picture of the user, my system searches for him in the database of faces registered in Rekognition and returns me who this user is - that is, his userId in my MySQL database.

I've already created a Collection called "Users" and I would like to know how to inform the userId to the indexFaces method. Initially, I intend to allow the inclusion of only one photo per user and I'm currently doing it like this:

const params = {
    CollectionId: 'Users',
    Image: { Bytes }, // The image in base64
    ExternalImageId: '1', // The userId in MySQL
    MaxFaces: 1,
    QualityFilter: 'HIGH'
};

Rekognition.indexFaces(params, (err, data) => {
    if (err) {
        console.log(err, err.stack);
        return;
    }

    console.log(data);
});

The entire face registration process works normally and the code I use to search for the faces already registered is:

const params = {
    CollectionId: 'Users',
    Image: { Bytes }, // The image in base64
    MaxFaces: 1,
    FaceMatchThreshold: 99
};

Rekognition.searchFacesByImage(params, (err, data) => {
    if (err) {
        console.log(err, err.stack);
        return;
    }

    for (let i = 0; i < data.FaceMatches.length; i++) {
        console.log(data.FaceMatches[i].Face);
    }
});

If the consulted face is registered and found in the Collection, the value returned is:

{
  FaceId: 'b7506fc1-e8...',
  BoundingBox: {...},
  ImageId: 'e11d94c1-831...',
  ExternalImageId: '1', // `userId` in my MySQL as desired
  Confidence: 99.999...,
  IndexFacesModelVersion: '6.0'
}

My question is: am I associating the userId with the face correctly through the ExternalImageId property or should I do it in a more appropriate way? If yes, how?

Yes, that is the purpose of the ExternalImageId field -- it is for you to associate your own reference to the face. You are doing it correctly (assuming that the code works).