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+---
+title: Relay
+category: React
+layout: 2017/sheet
+updated: 2024-02-25
+---
+
+## Intro
+Relay is a JavaScript framework for building data-driven React applications powered by GraphQL. It enables declarative data fetching and efficient data management.
+
+## Queries
+{: .-three-column}
+
+### Basic Query
+Relay uses GraphQL queries to fetch data. Components specify their data requirements using fragments.
+
+### Fragments
+```js
+fragment UserProfile on User {
+  name
+  age
+}
+```
+Fragments allow components to declare their data needs.
+
+### Query Renderer
+```js
+<QueryRenderer
+  environment={environment}
+  query={graphql`
+    query AppQuery {
+      user(id: "4") {
+        ...UserProfile
+      }
+    }
+  `}
+  render={({error, props}) => {
+    if (props) {
+      return <UserProfile user={props.user} />;
+    } else {
+      return <div>Loading</div>;
+    }
+  }}
+/>
+```
+QueryRenderer fetches query data and renders the UI based on the data state.
+
+## Mutations
+{: .-three-column}
+
+### Basic Mutation
+```js
+mutation CreateUserMutation($input: CreateUserInput!) {
+  createUser(input: $input) {
+    user {
+      id
+      name
+    }
+  }
+}
+```
+Mutations modify data. Relay handles optimistic updates and error handling.
+
+## Subscriptions
+Subscriptions allow real-time updates to data.
+
+### Example
+```js
+// Subscription query
+const CommentSubscription = graphql`
+  subscription CommentSubscription($input: CommentSubscriptionInput!) {
+    commentAdded(input: $input) {
+      comment {
+        id
+        text
+      }
+    }
+  }
+`;
+
+// Using the useSubscription hook
+function CommentAddedSubscription({ postId }) {
+  const [commit, isInFlight] = useSubscription(
+    {
+      subscription: CommentSubscription,
+      variables: { input: { postId } },
+      onNext: (data) => {
+        console.log('Subscription data received: ', data);
+      },
+      onError: (error) => console.error(`An error occurred: ${error.message}`),
+    }
+  );
+
+  return <div>Subscribing to new comments for post {postId}</div>;
+}
+```
+
+## Relay Hooks
+Relay provides a powerful set of React hooks that facilitate working with GraphQL data in a React application.
+
+### useQuery
+Fetches GraphQL queries with a hook.
+```js
+const data = useQuery(graphql`
+  query UserQuery {
+    user(id: "4") {
+      name
+      age
+    }
+  }
+`);
+```
+
+### useMutation
+Allows mutations to be executed from components.
+```js
+const [commit, isInFlight] = useMutation(graphql`
+  mutation CreateUserMutation($input: CreateUserInput!) {
+    createUser(input: $input) {
+      user {
+        id
+        name
+      }
+    }
+  }
+`);
+```
+
+### useSubscription
+Subscribes to data changes.
+```js
+useSubscription(
+  graphql`
+    subscription UserSubscription {
+      userUpdated(id: "4") {
+        user {
+          name
+          age
+        }
+      }
+    }
+  `,
+  {
+    onNext: (data) => console.log(data),
+  }
+);
+```
+
+### useFragment
+Allows components to specify their data requirements.
+```js
+const data = useFragment(fragmentNode, fragmentRef);
+```
+
+### useLazyLoadQuery
+Fetches a query at component render.
+```js
+const data = useLazyLoadQuery(graphql`
+  query UserQuery($id: ID!) {
+    user(id: $id) {
+      ...UserProfile
+    }
+  }
+`, {id: '4'});
+```
+
+### usePaginationFragment
+Handles data fetching for paginated data.
+```js
+const {data, loadNext, hasNext} = usePaginationFragment(fragmentNode, fragmentRef);
+```
+
+## Relay Entrypoint APIs
+Entrypoint APIs provide a way to define entry points to Relay data for components.
+
+### loadQuery
+Used for loading a query outside of React components.
+```js
+const preloadedQuery = loadQuery(RelayEnvironment, query, variables);
+```
+
+## Relay Runtime API Reference
+The Relay Runtime offers a set of APIs for low-level manipulation of the Relay store and environment.
+
+### fetchQuery
+Executes a query against the store.
+```js
+fetchQuery(RelayEnvironment, query, variables).then(data => {
+  console.log(data);
+});
+```
+
+### commitMutation
+Commits a mutation.
+```js
+commitMutation(RelayEnvironment, {
+  mutation,
+  variables,
+  onCompleted: (response, errors) => {
+    console.log('Response received from server.');
+  },
+  onError: err => console.error(err),
+});
+```
+
+## Advanced Topics
+- **Pagination**: Relay handles list data and pagination efficiently.
+- **Refetching**: Components can refetch their data in response to specific events.
+
+## Best Practices
+- **Data Masking**: Use fragments for component-specific data requirements.
+- **Optimistic Updates**: Provide immediate feedback for user actions with optimistic updates.
+
+## References
+- Official Relay [Documentation](https://relay.dev/docs/)