a GraphQL-serverb React and GraphQLc Database and user administration
e Fragments and subscriptions

d

Login and updating the cache

The frontend of our application shows the phone directory just fine with the updated server. However, if we want to add new persons, we have to add login functionality to the frontend.

User login

Let's add the variable token to the application's state. When a user is logged in, it will contain a user token. If token is undefined, we render the LoginForm component responsible for user login. The component receives an error handler and the setToken function as parameters:

const App = () => {
  const [token, setToken] = useState(null)
  // ...

  if (!token) {
    return (
      <div>
        <Notify errorMessage={errorMessage} />
        <h2>Login</h2>
        <LoginForm
          setToken={setToken}
          setError={notify}
        />
      </div>
    )
  }

  return (
    // ...
  )
}

Next, we define a mutation for logging in:

export const LOGIN = gql`
  mutation login($username: String!, $password: String!) {
    login(username: $username, password: $password)  {
      value
    }
  }
`

The LoginForm component works pretty much just like all the other components doing mutations that we have previously created. Interesting lines in the code have been highlighted:

import { useState, useEffect } from 'react'
import { useMutation } from '@apollo/client'
import { LOGIN } from '../queries'

const LoginForm = ({ setError, setToken }) => {
  const [username, setUsername] = useState('')
  const [password, setPassword] = useState('')

  const [ login, result ] = useMutation(LOGIN, {    onError: (error) => {
      setError(error.graphQLErrors[0].message)
    }
  })

  useEffect(() => {    if ( result.data ) {      const token = result.data.login.value      setToken(token)      localStorage.setItem('phonenumbers-user-token', token)    }  }, [result.data])
  const submit = async (event) => {
    event.preventDefault()

    login({ variables: { username, password } })
  }

  return (
    <div>
      <form onSubmit={submit}>
        <div>
          username <input
            value={username}
            onChange={({ target }) => setUsername(target.value)}
          />
        </div>
        <div>
          password <input
            type='password'
            value={password}
            onChange={({ target }) => setPassword(target.value)}
          />
        </div>
        <button type='submit'>login</button>
      </form>
    </div>
  )
}

export default LoginForm

We are using an effect hook to save the token's value to the state of the App component and the local storage after the server has responded to the mutation. Use of the effect hook is necessary to avoid an endless rendering loop.

Let's also add a button which enables a logged-in user to log out. The button's onClick handler sets the token state to null, removes the token from local storage and resets the cache of the Apollo client. The last step is important, because some queries might have fetched data to cache, which only logged-in users should have access to.

We can reset the cache using the resetStore method of an Apollo client object. The client can be accessed with the useApolloClient hook:

const App = () => {
  const [token, setToken] = useState(null)
  const [errorMessage, setErrorMessage] = useState(null)
  const result = useQuery(ALL_PERSONS)
  const client = useApolloClient()
  if (result.loading)  {
    return <div>loading...</div>
  }

  const logout = () => {    setToken(null)    localStorage.clear()    client.resetStore()  }
  if (!token) {    return (      <>        <Notify errorMessage={errorMessage} />        <LoginForm setToken={setToken} setError={notify} />      </>    )  }
  return (
    <>
      <Notify errorMessage={errorMessage} />
      <button onClick={logout}>logout</button>      <Persons persons={result.data.allPersons} />
      <PersonForm setError={notify} />
      <PhoneForm setError={notify} />
    </>
  )
}

Adding a token to a header

After the backend changes, creating new persons requires that a valid user token is sent with the request. In order to send the token, we have to change the way we define the ApolloClient object in main.jsx a little.

import { ApolloClient, InMemoryCache, ApolloProvider, createHttpLink } from '@apollo/client'import { setContext } from '@apollo/client/link/context'
const authLink = setContext((_, { headers }) => {  const token = localStorage.getItem('phonenumbers-user-token')  return {    headers: {      ...headers,      authorization: token ? `Bearer ${token}` : null,    }  }})
const httpLink = createHttpLink({
  uri: 'http://localhost:4000',
})

const client = new ApolloClient({
  cache: new InMemoryCache(),
  link: authLink.concat(httpLink)})

The field uri that was previously used when creating the client object has been replaced by the field link, which defines in a more complicated case how Apollo is connected to the server. The server url is now wrapped using the function createHttpLink into a suitable httpLink object. The link is modified by the context defined by the authLink object so that a possible token in localStorage is set to header authorization for each request to the server.

Creating new persons and changing numbers works again. There is however one remaining problem. If we try to add a person without a phone number, it is not possible.

browser showing person validation failed

Validation fails, because frontend sends an empty string as the value of phone.

Let's change the function creating new persons so that it sets phone to undefined if user has not given a value.

const PersonForm = ({ setError }) => {
  // ...
  const submit = async (event) => {
    event.preventDefault()
    createPerson({
      variables: { 
        name, street, city,        phone: phone.length > 0 ? phone : undefined      }
    })

  // ...
  }

  // ...
}

Updating cache, revisited

We have to update the cache of the Apollo client on creating new persons. We can update it using the mutation's refetchQueries option to define that the ALL_PERSONS query is done again.

const PersonForm = ({ setError }) => {
  // ...

  const [ createPerson ] = useMutation(CREATE_PERSON, {
    refetchQueries: [  {query: ALL_PERSONS} ],    onError: (error) => {
      const messages = error.graphQLErrors.map(e => e.message).join('\n')
      setError(messages)
    }
  })

This approach is pretty good, the drawback being that the query is always rerun with any updates.

It is possible to optimize the solution by handling updating the cache ourselves. This is done by defining a suitable update callback for the mutation, which Apollo runs after the mutation:

const PersonForm = ({ setError }) => {
  // ...

  const [ createPerson ] = useMutation(CREATE_PERSON, {
    onError: (error) => {
      const messages = error.graphQLErrors.map(e => e.message).join('\n')
      setError(messages)
    },
    update: (cache, response) => {      cache.updateQuery({ query: ALL_PERSONS }, ({ allPersons }) => {        return {          allPersons: allPersons.concat(response.data.addPerson),        }      })    },  })
 
  // ..
}

The callback function is given a reference to the cache and the data returned by the mutation as parameters. For example, in our case, this would be the created person.

Using the function updateQuery the code updates the query ALLPERSONS in the cache by adding the new person to the cached data.

In some situations, the only sensible way to keep the cache up to date is using the update callback.

When necessary, it is possible to disable cache for the whole application or single queries by setting the field managing the use of cache, fetchPolicy as no-cache.

Be diligent with the cache. Old data in the cache can cause hard-to-find bugs. As we know, keeping the cache up to date is very challenging. According to a coder proverb:

There are only two hard things in Computer Science: cache invalidation and naming things. Read more here.

The current code of the application can be found on Github, branch part8-5.

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