Authentication and Authorization

Our todo app is nearly functionally complete, but it still doesn't fulfill a very basic requirement - that users should log in before they can view, create or modify tasks.

Remult provides a flexible mechanism that enables placing code-based authorization rules at various levels of the application's API. To maintain high code cohesion, entity and field-level authorization code should be placed in entity classes.

Remult is completely unopinionated when it comes to user authentication. You are free to use any kind of authentication mechanism, and only required to provide Remult with an object which implements the Remult UserInfo interface.

In this tutorial, we'll use NextAuth.js for authentication.

Tasks CRUD Requires Sign-in

This rule is implemented within the Task @Entity decorator, by modifying the value of the allowApiCrud property. This property can be set to a function that accepts a Remult argument and returns a boolean value. Let's use the Allow.authenticated function from Remult.

// src/app/shared/Task.ts

@Entity("tasks", {
    allowApiCrud: Allow.authenticated
})

Import Allow

This code requires adding an import of Allow from remult.

After the browser refreshes, the list of tasks disappeared and the user can no longer create new tasks.

Inspect the HTTP error returned by the API using cURL

curl -i http://localhost:3000/api/tasks

Authorized server-side code can still modify tasks

Although client CRUD requests to tasks API endpoints now require a signed-in user, the API endpoint created for our setAllCompleted server function remains available to unauthenticated requests. Since the allowApiCrud rule we implemented does not affect the server-side code's ability to use the Task entity class for performing database CRUD operations, the setAllCompleted function still works as before.

To fix this, let's implement the same rule using the @BackendMethod decorator of the setAllCompleted method of TasksController.

// src/shared/TasksController.ts

@BackendMethod({ allowed: Allow.authenticated })

This code requires adding an import of Allow from remult.

User Authentication

Let's set-up NextAuth.js to authenticate users to our app.

Backend setup

1. Install next-auth:

   npm i next-auth

2. NextAuth requires a "secret" used to encrypt the NextAuth.js JWT, see Options | NextAuth.js for more info.

   Create a file called .env.local and set the NEXTAUTH_SECRET to a random string.

   // .env.local
   
   NEXTAUTH_SECRET=something-secret

   TIP

   you can use an online UUID generator to generate a completely random string

3. Inside the src folder, create an auth.ts file with the following code.

   // src/auth.ts
   
   import NextAuth, { getServerSession } from 'next-auth/next'
   import Credentials from 'next-auth/providers/credentials'
   import { UserInfo } from 'remult'
   
   const validUsers: UserInfo[] = [
     { id: '1', name: 'Jane' },
     { id: '2', name: 'Steve' },
   ]
   function findUser(name?: string | null) {
     return validUsers.find((user) => user.name === name)
   }
   
   export const auth = NextAuth({
     providers: [
       Credentials({
         credentials: {
           name: {
             placeholder: 'Try Steve or Jane',
           },
         },
         authorize: (credentials) => findUser(credentials?.name) || null,
       }),
     ],
     callbacks: {
       session: ({ session }) => ({
         ...session,
         user: findUser(session.user?.name),
       }),
     },
   })
   
   export async function getUserOnServer() {
     const session = await getServerSession()
     return findUser(session?.user?.name)
   }

   This (very) simplistic NextAuth.js Credentials authorizes users by looking up the user's name in a predefined list of valid users.

   We've configured the session callback to include the user info as part of the session info, so that remult on the frontend will have the authorization info.

4. Create an auth folder within the 'api' folder, and inside it, create a [...nextauth] subdirectory. Inside the src/app/api/auth/[...nextauth] directory, craft a route.ts file with the following code.

   // src/app/api/auth/[...nextauth]/route.ts
   
   import { auth } from '../../../../auth'
   
   export { auth as GET, auth as POST }

Frontend setup

1. Create a src/components/auth.tsx file, and place the following code to it:

   // src/components/auth.tsx
   
   import { signIn, signOut, useSession } from 'next-auth/react'
   import { useEffect } from 'react'
   import { UserInfo, remult } from 'remult'
   import Todo from './todo'
   
   export default function Auth() {
     const session = useSession()
     remult.user = session.data?.user as UserInfo
   
     useEffect(() => {
       if (session.status === 'unauthenticated') signIn()
     }, [session])
     if (session.status !== 'authenticated') return <></>
     return (
       <div>
         Hello {remult.user?.name}{' '}
         <button onClick={() => signOut()}>Sign Out</button>
         <Todo />
       </div>
     )
   }

2. Update the src/app/page.tsx with the highlighted changes:

   // src/app/page.tsx
   
   "use client"
   import { SessionProvider } from "next-auth/react"
   import Auth from "../components/auth"
   
   export default function Home() {
     return (
       <SessionProvider>
         <Auth />
       </SessionProvider>
     )
   }

Connect Remult-Next On the Backend

Once an authentication flow is established, integrating it with Remult in the backend is as simple as providing Remult with a getUser function that uses the getUserOnServer function we've created in src/auth.ts

// src/api.ts

import { getUserOnServer } from "./auth"

const api = remultNextApp({
  //...
  getUser: getUserOnServer,
})
//...

The todo app now supports signing in and out, with all access restricted to signed in users only.

Role-based Authorization

Usually, not all application users have the same privileges. Let's define an admin role for our todo app, and enforce the following authorization rules:

• All signed in users can see the list of tasks.
• All signed in users can set specific tasks as completed.
• Only users belonging to the admin role can create, delete or edit the titles of tasks.

1. Modify the highlighted lines in the Task entity class to reflect the top three authorization rules.

// src/shared/Task.ts

import { Allow, Entity, Fields } from "remult"

@Entity<Task>("tasks", {
  allowApiCrud: Allow.authenticated,
  allowApiInsert: "admin",
  allowApiDelete: "admin"
})
export class Task {
  @Fields.uuid()
  id!: string

  @Fields.string({
    validate: (task) => {
      if (task.title.length < 3) throw "Too Short"
    }
    allowApiUpdate: "admin"
  })
  title = ""

  @Fields.boolean()
  completed = false
}

2. Let's give the user "Jane" the admin role by modifying the roles array of her validUsers entry.

// src/auth.ts

const validUsers = [
  { id: "1", name: "Jane", roles: ["admin"] },
  { id: "2", name: "Steve" }
]

Sign in to the app as "Steve" to test that the actions restricted to admin users are not allowed. 🔒

Role-based Authorization on the Frontend

From a user experience perspective it only makes sense that users that can't add or delete, would not see these buttons.

Let's reuse the same definitions on the Frontend.

We'll use the entity's metadata to only show the form if the user is allowed to insert

// src/components/todo.tsx

<main>
  {taskRepo.metadata.apiInsertAllowed() && (
    <form onSubmit={addTask}>
      <input
        value={newTaskTitle}
        placeholder="What needs to be done?"
        onChange={e => setNewTaskTitle(e.target.value)}
      />
      <button>Add</button>
    </form>
  )}
  ...
</main>

And let's do the same for the delete button:

// src/components/todo.tsx

return (
  <div key={task.id}>
    <input
      type="checkbox"
      checked={task.completed}
      onChange={e => setCompleted(e.target.checked)}
    />
    <input value={task.title} onChange={e => setTitle(e.target.value)} />
    <button onClick={saveTask}>Save</button>
    {taskRepo.metadata.apiDeleteAllowed(task) && (
      <button onClick={deleteTask}>Delete</button>
    )}
  </div>
)

This way we can keep the frontend consistent with the api's Authorization rules

• Note We send the task to the apiDeleteAllowed method, because the apiDeleteAllowed option, can be sophisticated and can also be based on the specific item's values.