Default client accepts two parameters: `resolvers` and `config`.
-`resolvers` parameter is created to accept an object of resolvers for [local state management](#local-state-with-apollo) queries and mutations
-`config` parameter takes an object of configuration settings:
-`cacheConfig` field accepts an optional object of settings to [customize Apollo cache](https://github.com/apollographql/apollo-client/tree/master/packages/apollo-cache-inmemory#configuration)
-`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`.
We can query local data with `@client` Apollo directive:
```javascript
// 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](#fragments) 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).
We need to pass resolvers object to our existing Apollo Client:
```javascript
// 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](https://vue-apollo.netlify.com/guide/local-state.html#local-state).
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:
```js
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.
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:
```javascript
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:
```javascript
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:
GitLab's GraphQL mutations currently have two distinct error modes: [Top-level](#top-level-errors) and [errors-as-data](#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`](https://www.apollographql.com/docs/react/api/apollo-client/#ApolloClient.mutate) method, or handling the `error` event emitted from the [`ApolloMutation`](https://apollo.vuejs.org/api/apollo-mutation.html#events) 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:
```diff
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:
```javascript
{
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.
