debian-mirror-gitlab/doc/development/fe_guide/graphql.md

GraphQL

Our GraphQL API can be explored via GraphiQL at your instance's /-/graphql-explorer or at GitLab.com.

You can check all existing queries and mutations on the right side of GraphiQL in its Documentation explorer. It's also possible to write queries and mutations directly on the left tab and check their execution by clicking Execute query button on the top left:

We use Apollo and Vue Apollo for working with GraphQL on the frontend.

Apollo Client

To save duplicated clients getting created in different apps, we have a default client that should be used. This setups the Apollo client with the correct URL and also sets the CSRF headers.

Default client accepts two parameters: resolvers and config.

• resolvers parameter is created to accept an object of resolvers for local state management queries and mutations
• config parameter takes an object of configuration settings:
  • cacheConfig field accepts an optional object of settings to customize Apollo cache
  • baseUrl allows us to pass a URL for GraphQL endpoint different from our main endpoint (i.e.${gon.relative_url_root}/api/graphql)
  • assumeImmutableResults (set to false by default) - this setting, when set to true, will assume that every single operation on updating Apollo Cache is immutable. It also sets freezeResults to true, so any attempt on mutating Apollo Cache will throw a console warning in development environment. Please ensure you're following the immutability pattern on cache update operations before setting this option to true.

GraphQL Queries

To save query compilation at runtime, webpack can directly import .graphql files. This allows webpack to preprocess the query at compile time instead of the client doing compilation of queries.

To distinguish queries from mutations and fragments, the following naming convention is recommended:

• allUsers.query.graphql for queries;
• addUser.mutation.graphql for mutations;
• basicUser.fragment.graphql for fragments.

Fragments

Fragments are a way to make your complex GraphQL queries more readable and re-usable. Here is an example of GraphQL fragment:

fragment DesignListItem on Design {
  id
  image
  event
  filename
  notesCount
}

Fragments can be stored in separate files, imported and used in queries, mutations, or other fragments.

#import "./designList.fragment.graphql"
#import "./diffRefs.fragment.graphql"

fragment DesignItem on Design {
  ...DesignListItem
  fullPath
  diffRefs {
    ...DesignDiffRefs
  }
}

More about fragments: GraphQL Docs

Usage in Vue

To use Vue Apollo, import the Vue Apollo plugin as well as the default client. This should be created at the same point the Vue application is mounted.

import Vue from 'vue';
import VueApollo from 'vue-apollo';
import createDefaultClient from '~/lib/graphql';
Vue.use(VueApollo);

const apolloProvider = new VueApollo({
  defaultClient: createDefaultClient(),
});

new Vue({
  ...,
  apolloProvider,
  ...
});

Read more about Vue Apollo in the Vue Apollo documentation.

Local state with Apollo

It is possible to manage an application state with Apollo by passing in a resolvers object when creating the default client. The default state can be set by writing to the cache after setting up the default client.

import Vue from 'vue';
import VueApollo from 'vue-apollo';
import createDefaultClient from '~/lib/graphql';
Vue.use(VueApollo);

const defaultClient = createDefaultClient({
  resolvers: {}
});

defaultClient.cache.writeData({
  data: {
    user: {
      name: 'John',
      surname: 'Doe',
      age: 30
    },
  },
});

const apolloProvider = new VueApollo({
  defaultClient,
});

We can query local data with @client Apollo directive:

// user.query.graphql

query User {
  user @client {
    name
    surname
    age
  }
}

Along with creating local data, we can also extend existing GraphQL types with @client fields. This is extremely useful when we need to mock an API responses for fields not yet added to our GraphQL API.

Mocking API response with local Apollo cache

Using local Apollo Cache is handy when we have a need to mock some GraphQL API responses, queries or mutations locally (e.g. when they're still not added to our actual API).

For example, we have a fragment on DesignVersion used in our queries:

fragment VersionListItem on DesignVersion {
  id
  sha
}

We need to fetch also version author and the 'created at' property to display them in the versions dropdown but these changes are still not implemented in our API. We can change the existing fragment to get a mocked response for these new fields:

fragment VersionListItem on DesignVersion {
  id
  sha
  author @client {
    avatarUrl
    name
  }
  createdAt @client
}

Now Apollo will try to find a resolver for every field marked with @client directive. Let's create a resolver for DesignVersion type (why DesignVersion? because our fragment was created on this type).

// resolvers.js

const resolvers = {
  DesignVersion: {
    author: () => ({
      avatarUrl:
        'https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80&d=identicon',
      name: 'Administrator',
      __typename: 'User',
    }),
    createdAt: () => '2019-11-13T16:08:11Z',
  },
};

export default resolvers;

We need to pass resolvers object to our existing Apollo Client:

// graphql.js

import createDefaultClient from '~/lib/graphql';
import resolvers from './graphql/resolvers';

const defaultClient = createDefaultClient(
  {},
  resolvers,
);

Now every single time on attempt to fetch a version, our client will fetch id and sha from the remote API endpoint and will assign our hardcoded values to author and createdAt version properties. With this data, frontend developers are able to work on UI part without being blocked by backend. When actual response is added to the API, a custom local resolver can be removed fast and the only change to query/fragment is @client directive removal.

Read more about local state management with Apollo in the Vue Apollo documentation.

Using with Vuex

When Apollo Client is used within Vuex and fetched data is stored in the Vuex store, there is no need in keeping Apollo Client cache enabled. Otherwise we would have data from the API stored in two places - Vuex store and Apollo Client cache. More to say, with Apollo default settings, a subsequent fetch from the GraphQL API could result in fetching data from Apollo cache (in the case where we have the same query and variables). To prevent this behavior, we need to disable Apollo Client cache passing a valid fetchPolicy option to its constructor:

import fetchPolicies from '~/graphql_shared/fetch_policy_constants';

export const gqClient = createGqClient(
  {},
  {
    fetchPolicy: fetchPolicies.NO_CACHE,
  },
);

Feature flags in queries

Sometimes it may be useful to have an entity in the GraphQL query behind a feature flag. For example, when working on a feature where the backend has already been merged but the frontend hasn't you might want to put the GraphQL entity behind a feature flag to allow for smaller merge requests to be created and merged.

To do this we can use the @include directive to exclude an entity if the if statement passes.

query getAuthorData($authorNameEnabled: Boolean = false) {
  username
  name @include(if: $authorNameEnabled)
}

Then in the Vue (or JavaScript) call to the query we can pass in our feature flag. This feature flag will need to be already setup correctly. See the feature flag documentation for the correct way to do this.

export default {
  apollo: {
    user: {
      query: QUERY_IMPORT,
      variables() {
        return {
          authorNameEnabled: gon?.features?.authorNameEnabled,
        };
      },
    }
  },
};

Working with pagination

GitLab's GraphQL API uses Relay-style cursor pagination for connection types. This means a "cursor" is used to keep track of where in the data set the next items should be fetched from.

Every connection type (for example, DesignConnection and DiscussionConnection) has a field pageInfo that contains an information required for pagination:

pageInfo {
  endCursor
  hasNextPage
  hasPreviousPage
  startCursor
}

Here:

• startCursor and endCursor display the cursor of the first and last items respectively.
• hasPreviousPage and hasNextPage allow us to check if there are more pages available before or after the current page.

When we fetch data with a connection type, we can pass cursor as after or before parameter, indicating a starting or ending point of our pagination. They should be followed with first or last parameter respectively to indicate how many items we want to fetch after or before a given endpoint.

For example, here we're fetching 10 designs after a cursor:

query {
  project(fullPath: "root/my-project") {
    id
    issue(iid: "42") {
      designCollection {
        designs(atVersion: null, after: "Ihwffmde0i", first: 10) {
          edges {
            node {
              id
            }
          }
        }
      }
    }
  }
}

Using fetchMore method in components

When making an initial fetch, we usually want to start a pagination from the beginning. In this case, we can either:

• Skip passing a cursor.
• Pass null explicitly to after.

After data is fetched, we should save a pageInfo object. Let's assume we're storing it to Vue component data:

data() {
  return {
    pageInfo: null,
  }
},
apollo: {
  designs: {
    query: projectQuery,
    variables() {
      return {
        // rest of design variables
        ...
        first: 10,
      };
    },
    result(res) {
      this.pageInfo = res.data?.project?.issue?.designCollection?.designs?.pageInfo;
    },
  },
},

When we want to move to the next page, we use an Apollo fetchMore method, passing a new cursor (and, optionally, new variables) there. In the updateQuery hook, we have to return a result we want to see in the Apollo cache after fetching the next page.

fetchNextPage() {
  // as a first step, we're checking if we have more pages to move forward
  if (this.pageInfo?.hasNextPage) {
    this.$apollo.queries.designs.fetchMore({
      variables: {
        // rest of design variables
        ...
        first: 10,
        after: this.pageInfo?.endCursor,
      },
      updateQuery(previousResult, { fetchMoreResult }) {
        // here we can implement the logic of adding new designs to fetched one (for example, if we use infinite scroll)
        // or replacing old result with the new one if we use numbered pages

        const newDesigns = fetchMoreResult.project.issue.designCollection.designs;
        previousResult.project.issue.designCollection.designs.push(...newDesigns)

        return previousResult;
      },
    });
  }
}

Please note we don't have to save pageInfo one more time; fetchMore triggers a query result hook as well.

Limitations

Currently, bidirectional pagination doesn't work:

• hasNextPage returns a correct value only when we paginate forward using endCursor and first parameters.
• hasPreviousPage returns a correct value only when we paginate backward using startCursor and last parameters.

This should be resolved in the scope of the issue Bi-directional Pagination in GraphQL doesn't work as expected.

Testing

Mocking response as component data

With Vue test utils it is easy to quickly test components that fetch GraphQL queries. The simplest way is to use shallowMount and then set the data on the component

it('tests apollo component', () => {
  const vm = shallowMount(App);

  vm.setData({
    ...mock data
  });
});

Testing loading state

If we need to test how our component renders when results from the GraphQL API are still loading, we can mock a loading state into respective Apollo queries/mutations:

  function createComponent({
    loading = false,
  } = {}) {
    const $apollo = {
      queries: {
        designs: {
          loading,
        },
    };

    wrapper = shallowMount(Index, {
      sync: false,
      mocks: { $apollo }
    });
  }

  it('renders loading icon', () => {
  createComponent({ loading: true });

  expect(wrapper.element).toMatchSnapshot();
})

Testing Apollo components

If we use ApolloQuery or ApolloMutation in our components, in order to test their functionality we need to add a stub first:

import { ApolloMutation } from 'vue-apollo';

function createComponent(props = {}) {
  wrapper = shallowMount(MyComponent, {
    sync: false,
    propsData: {
      ...props,
    },
    stubs: {
      ApolloMutation,
    },
  });
}

ApolloMutation component exposes mutate method via scoped slot. If we want to test this method, we need to add it to mocks:

const mutate = jest.fn().mockResolvedValue();
const $apollo = {
  mutate,
};

function createComponent(props = {}) {
  wrapper = shallowMount(MyComponent, {
    sync: false,
    propsData: {
      ...props,
    },
    stubs: {
      ApolloMutation,
    },
    mocks: {
      $apollo:
    }
  });
}

Then we can check if mutate is called with correct variables:

const mutationVariables = {
  mutation: createNoteMutation,
  update: expect.anything(),
  variables: {
    input: {
      noteableId: 'noteable-id',
      body: 'test',
      discussionId: '0',
    },
  },
};

it('calls mutation on submitting form ', () => {
  createComponent()
  findReplyForm().vm.$emit('submitForm');

  expect(mutate).toHaveBeenCalledWith(mutationVariables);
});

Handling errors

GitLab's GraphQL mutations currently have two distinct error modes: Top-level and errors-as-data.

When utilising a GraphQL mutation, we must consider handling both of these error modes to ensure that the user receives the appropriate feedback when an error occurs.

Top-level errors

These errors are located at the "top level" of a GraphQL response. These are non-recoverable errors including argument errors and syntax errors, and should not be presented directly to the user.

Handling top-level errors

Apollo is aware of top-level errors, so we are able to leverage Apollo's various error-handling mechanisms to handle these errors (e.g. handling Promise rejections after invoking the mutate method, or handling the error event emitted from the ApolloMutation component).

Because these errors are not intended for users, error messages for top-level errors should be defined client-side.

Errors-as-data

These errors are nested within the data object of a GraphQL response. These are recoverable errors that, ideally, can be presented directly to the user.

Handling errors-as-data

First, we must add errors to our mutation object:

mutation createNoteMutation($input: String!) {
  createNoteMutation(input: $input) {
    note {
      id
+     errors
    }
  }

Now, when we commit this mutation and errors occur, the response will include errors for us to handle:

{
  data: {
    mutationName: {
      errors: ["Sorry, we were not able to update the note."]
    }
  }
}

When handling errors-as-data, use your best judgement to determine whether to present the error message in the response, or another message defined client-side, to the user.

Usage outside of Vue

It is also possible to use GraphQL outside of Vue by directly importing and using the default client with queries.

import createDefaultClient from '~/lib/graphql';
import query from './query.graphql';

const defaultClient = createDefaultClient();

defaultClient.query({ query })
  .then(result => console.log(result));

Read more about the Apollo client in the Apollo documentation.