How to configure rewrite rules inside firebase-hosting to route certain requests to cloud functions?

Question

I've a PWA built using polymer 2.0 and polymerfire and is my web application. I've an express app acting as a cloud function (microservice). Example: exports.register=functions.https.onRequest(app);

How to add the rewrite rules to map say /fns/register and /fns/verify to the above app register.

I've updated my firebase.json file in the cloudfunction microservice project, but when I run firebase deploy --only functions:register it says there is no public folder for deploying the hosting configuration!

{
    "hosting": {
        "rewrites": [{
            "source": "/fns/**", "function": "register"
        }]
    }    
}

Maintaining the rewrite rules in the original web applicaiton could be one option, but still, is not ideal IMHO. If I've to do it in my original web application, I tried that as well, but couldn't make it. Following is my updated firebase.json in my original web application:

{
  "database": {
    "rules": "database.rules.json"
  },
  "hosting": {
    "public": "build/default/public",
    "rewrites": [
      {
        "source": "/fns/**",
        "function": "register"
      },
      {
        "source": "**",
        "destination": "/index.html"
      }
    ]
  }
}

Answer 1

Maintaining just one project for all resources (Hosting, Functions and Database) is the ideal, I think is the right way to manage Firebase projects.

You are trying to change just one parameter (rewrites) of the hosting service, and it's not the way it works. When you deploy the firebase.json, all the others configurations are overwritten. So, the error you got is because Firebase don't look the last configuration file and check what's different to update, it just tries to overwrite all the last configuration file and get an error because "public" is a required parameter for hosting.

That explained, now you are expecting that Firebase rewrites /fns/register to just /register, but it'll not occur. Your function gonna receive the "full" url /fns/register.

The best way, I think, is to create a root route:

var functions = require('firebase-functions');
var express = require('express');

var app = express();
var router = express.Router();

router.post('/register', registerFunction);
router.post('/verify', verifyFunction);

app.use('/fns', router);

exports.fns = functions.https.onRequest(app);

And rewrites all functions to fns function:

{
  "database": {
    "rules": "database.rules.json"
  },
  "hosting": {
    "public": "build/default/public",
    "rewrites": [
      {
        "source": "/fns/**",
        "function": "fns"
      },
      {
        "source": "**",
        "destination": "/index.html"
      }
    ]
  }
}

Now you can use https://<your-project-id>.firebaseapp.com/fns/register to reach your register function and https://<your-project-id>.firebaseapp.com/fns/verify to reach your verify function.