Google Chat User Authorization App

This sample demonstrates how to create a Google Chat app that requests authorization from the user to make calls to Chat API on their behalf. The first time the user interacts with the app, it requests offline OAuth tokens for the user and saves them to a Firestore database. If the user interacts with the app again, the saved tokens are used so the app can call Chat API on behalf of the user without asking for authorization again. Once saved, the OAuth tokens could even be used to call Chat API without the user being present.

This app is built using Node.js on Google App Engine (Standard Environment) and leverages Google's OAuth2 for authorization and Firestore for data storage.

Key Features:

User Authorization: Securely requests user consent to call Chat API with their credentials.
Chat API Integration: Calls Chat API to post messages on behalf of the user.
Google Chat Integration: Responds to DMs or @mentions in Google Chat. If necessary, request configuration to start an OAuth authorization flow.
App Engine Deployment: Provides step-by-step instructions for deploying to App Engine.
Cloud Firestore: Stores user tokens in a Firestore database.

Prerequisites

Node.js: Download
Google Cloud SDK: Install
Google Cloud Project: Create

Deployment Steps

Enable APIs:
- Enable the Cloud Firestore and Google Chat APIs using the console or gcloud:
```
gcloud services enable firestore.googleapis.com chat.googleapis.com
```
Initiate Deployment to App Engine:
- Go to App Engine and initialize an application.
- Deploy the User Authorization app to App Engine:
```
gcloud app deploy
```
Create and Use OAuth Client ID:
- Get the app hostname:
```
gcloud app describe | grep defaultHostname
```
- In your Google Cloud project, go to APIs & Services > Credentials.
- Click Create Credentials > OAuth client ID.
- Select Web application as the application type.
- Add <hostname from the previous step>/oauth2 to Authorized redirect URIs.
- Download the JSON file and rename it to client_secrets.json in your project directory.
- Redeploy the app with the file client_secrets.json:
```
gcloud app deploy
```
Create a Firestore Database:
- Create a Firestore database in native mode named auth-data using the console or gcloud:
```
gcloud firestore databases create \
--database=auth-data \
--location=REGION \
--type=firestore-native
```
  Replace REGION with a Firestore location such as nam5 or eur3.

Create the Google Chat app

Go to Google Chat API and click Configuration.
In App name, enter User Auth App.
In Avatar URL, enter https://developers.google.com/chat/images/quickstart-app-avatar.png.
In Description, enter Quickstart app.
Under Functionality, select Receive 1:1 messages and Join spaces and group conversations.
Under Connection settings, select HTTP endpoint URL and enter your App Engine app's URL (obtained in the previous deployment steps).
In Authentication Audience, select HTTP endpoint URL.
Under Visibility, select Make this Google Chat app available to specific people and groups in your domain and enter your email address.
Click Save.

The Chat app is ready to receive and respond to messages on Chat.

Interact with the App

Add the app to a Google Chat space.
@mention the app.
Follow the authorization link to grant the app access to your account.
Once authorization is complete, the app will post a message to the space using your credentials.
If you @mention the app again, it will post a new message to the space with your credentials using the saved tokens, without asking for authorization again.

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