Advanced Topics on Caching

Here are some common situations where you would need to access the cache directly. If you’re manipulating the cache in an interesting way and would like your example to be featured, please send in a pull request!

Bypassing the cache

Sometimes it makes sense to not use the cache for a specific operation. This can be done using the no-cache fetchPolicy. The no-cache policy does not write to the cache with the response. This may be useful for sensitive data like passwords that you don’t want to keep in the cache.

Updating after a mutation

In some cases, just using dataIdFromObject is not enough for your application UI to update correctly. For example, if you want to add something to a list of objects without refetching the entire list, or if there are some objects that to which you can’t assign an object identifier, Apollo Client cannot update existing queries for you. Read on to learn about the other tools at your disposal.

refetchQueries is the simplest way of updating the cache. With refetchQueries you can specify one or more queries that you want to run after a mutation is completed in order to refetch the parts of the store that may have been affected by the mutation:

apollo.mutate({
  // ... insert comment mutation
  refetchQueries: [
    {
      query: gql`
        query UpdateCache($repoName: String!) {
          entry(repoFullName: $repoName) {
            id
            comments {
              postedBy {
                login
                html_url
              }
              createdAt
              content
            }
          }
        }
      `,
      variables: { repoName: 'apollographql/apollo-client' },
    },
  ],
});

Please note that if you call refetchQueries with an array of strings, then Apollo Client will look for any previously called queries that have the same names as the provided strings. It will then refetch those queries with their current variables.

A very common way of using refetchQueries is to import queries defined for other components to make sure that those components will be updated:

import RepoCommentsQuery from '../queries/RepoCommentsQuery';
 
apollo.mutate({
  //... insert comment mutation
  refetchQueries: [
    {
      query: RepoCommentsQuery,
      variables: { repoFullName: 'apollographql/apollo-client' },
    },
  ],
});

Using update gives you full control over the cache, allowing you to make changes to your data model in response to a mutation in any way you like. update is the recommended way of updating the cache after a query. It is explained in full here.

import CommentAppQuery from '../queries/CommentAppQuery';
 
const SUBMIT_COMMENT_MUTATION = gql`
  mutation SubmitComment($repoFullName: String!, $commentContent: String!) {
    submitComment(
      repoFullName: $repoFullName
      commentContent: $commentContent
    ) {
      postedBy {
        login
        html_url
      }
      createdAt
      content
    }
  }
`;
 
@Component({
  // ...
})
class CommentsPageComponent {
  constructor(private apollo: Apollo) {}
 
  submit({repoFullName, commentContent}) {
    this.apollo.mutate({
        mutation: SUBMIT_COMMENT_MUTATION,
variables: { repoFullName, commentContent },
            update: (store, { data: { submitComment } }) => {
              // Read the data from our cache for this query.
              const data = store.readQuery({ query: CommentAppQuery });
              // Add our comment from the mutation to the end.
              const comments = [...data.comments, submitComment];
              // Write our data back to the cache.
              store.writeQuery({ query: CommentAppQuery, { comments }  });
            }
    })
  }
}

Incremental loading: fetchMore

fetchMore can be used to update the result of a query based on the data returned by another query. Most often, it is used to handle infinite-scroll pagination or other situations where you are loading more data when you already have some.

In our GitHunt example, we have a paginated feed that displays a list of GitHub repositories. When we hit the “Load More” button, we don’t want Apollo Client to throw away the repository information it has already loaded. Instead, it should just append the newly loaded repositories to the list that Apollo Client already has in the store. With this update, our UI component should re-render and show us all of the available repositories.

Let’s see how to do that with the fetchMore method on a query:

const FEED_QUERY = gql`
  query Feed($offset: Int, $limit: Int) {
    feed(offset: $offset, limit: $limit) {
      id
      # ...
    }
  }
`;
 
@Component({
  // ...
})
class FeedComponent {
  feedQuery: QueryRef<any>;
  feed: Observable<any>;
  offset: number;
 
  constructor(private apollo: Apollo) {
    this.feedQuery = this.apollo.watchQuery({
      query: FEED_QUERY,
      variables: {
        offset: 0,
        limit: 10,
      },
      fetchPolicy: 'cache-and-network',
    });
 
    this.feed = this.feedQuery.valueChanges.pipe(
      tap(result => {
        this.offset = result.data.feed.length;
      }),
      map(result => result.data.feed),
    );
  }
 
  loadMore() {
    this.feedQuery.fetchMore({
      variables: {
        offset: this.offset,
      },
      updateQuery: (prev, { fetchMoreResult }) => {
        if (!fetchMoreResult) return prev;
 
        return Object.assign({}, prev, {
          feed: [...prev.feed, ...fetchMoreResult.feed],
        });
      },
    });
  }
}

The fetchMore method takes a map of variables to be sent with the new query. Here, we’re setting the offset to feed.length so that we fetch items that aren’t already displayed on the feed. This variable map is merged with the one that’s been specified for the query associated with the component. This means that other variables, e.g. the limit variable, will have the same value as they do within the component query.

It can also take a query named argument, which can be a GraphQL document containing a query that will be fetched in order to fetch more information; we refer to this as the fetchMore query. By default, the fetchMore query is the query associated with the container, in this case the FEED_QUERY.

When we call fetchMore, Apollo Client will fire the fetchMore query and use the logic in the updateQuery option to incorporate that into the original result. The named argument updateQuery should be a function that takes the previous result of the query associated with your component (i.e. FEED_QUERY in this case) and the information returned by the fetchMore query and return a combination of the two.

Here, the fetchMore query is the same as the query associated with the component. Our updateQuery takes the new feed items returned and just appends them onto the feed items that we’d asked for previously. With this, the UI will update and the feed will contain the next page of items!

Although fetchMore is often used for pagination, there are many other cases in which it is applicable. For example, suppose you have a list of items (say, a collaborative todo list) and you have a way to fetch items that have been updated after a certain time. Then, you don’t have to refetch the whole todo list to get updates: you can just incorporate the newly added items with fetchMore, as long as your updateQuery function correctly merges the new results.

The @connection directive

Fundamentally, paginated queries are the same as any other query with the exception that calls to fetchMore update the same cache key. Since these queries are cached by both the initial query and their parameters, a problem arises when later retrieving or updating paginated queries in the cache. We don’t care about pagination arguments such as limits, offsets, or cursors outside of the need to fetchMore, nor do we want to provide them simply for accessing cached data.

To solve this Apollo Client 1.6 introduced the @connection directive to specify a custom store key for results. A connection allows us to set the cache key for a field and to filter which arguments actually alter the query.

To use the @connection directive, simply add the directive to the segment of the query you want a custom store key for and provide the key parameter to specify the store key. In addition to the key parameter, you can also include the optional filter parameter, which takes an array of query argument names to include in the generated custom store key.

const query = gql`
  query Feed($type: FeedType!, $offset: Int, $limit: Int) {
    feed(type: $type, offset: $offset, limit: $limit) @connection(key: "feed", filter: ["type"]) {
      ...FeedEntry
    }
  }
`;

With the above query, even with multiple fetchMores, the results of each feed update will always result in the feed key in the store being updated with the latest accumulated values. In this example, we also use the @connection directive’s optional filter argument to include the type query argument in the store key, which results in multiple store values that accumulate queries from each type of feed.

Now that we have a stable store key, we can easily use writeQuery to perform a store update, in this case clearing out the feed.

cache.writeQuery({
  query: gql`
    query Feed($type: FeedType!) {
      feed(type: $type) @connection(key: "feed", filter: ["type"]) {
        id
      }
    }
  `,
  variables: {
    type: 'top',
  },
  data: {
    feed: [],
  },
});

Note that because we are only using the type argument in the store key, we don’t have to provide offset or limit.

Cache redirects using field policy read functions

In some cases, a query requests data that already exists in the cache under a different reference. A very common example of this is when your UI has a list view and a detail view that both use the same data. The list view might run the following query:

query Books {
  books {
    id
    title
    abstract
  }
}

When a specific book is selected, the detail view displays an individual item using this query:

query Book($id: ID!) {
  book(id: $id) {
    id
    title
    abstract
  }
}

We know that the data is most likely already in the client cache, but because it was requested with a different query, Apollo Client doesn’t know that. To tell Apollo Client where to look for the existing book data, we can define a field policy read function for the book field:

import { InMemoryCache } from '@apollo/client/core';
 
const cache = new InMemoryCache({
  typePolicies: {
    Query: {
      fields: {
        book(_, { args, toReference }) {
          return toReference({
            __typename: 'Book',
            id: args.id,
          });
        },
      },
    },
  },
});

Now whenever a query is run that includes a book field, the read function above will be executed, and return a reference that points to the book entity that was already created in the cache when the Books list view query ran. Apollo Client will use the reference returned by the read function to look up the item in its cache. toReference is a helper utility that is passed into read functions as part of the second parameter options object, and is used to generate an entity reference based on its __typename and id.

Resetting the store

Sometimes, you may want to reset the store entirely, such as when a user logs out. To accomplish this, use client.resetStore to clear out your Apollo cache. Since client.resetStore also refetches any of your active queries for you, it is asynchronous.

To register a callback function to be executed after the store has been reset, call client.onResetStore and pass in your callback. If you would like to register multiple callbacks, simply call client.onResetStore again. All of your callbacks will be pushed into an array and executed concurrently.

In this example, we’re using client.onResetStore to write default values to the cache. This is useful when using Apollo Client’s local state management features and calling client.resetStore anywhere in your application.

You can also call client.onResetStore from your React components. This can be useful if you would like to force your UI to rerender after the store has been reset.

If you want to clear the store but don’t want to refetch active queries, use client.clearStore() instead of client.resetStore().

Cache persistence

If you would like to persist and rehydrate your Apollo Cache from a storage provider like localStorage, you can use apollo3-cache-persist. apollo3-cache-persist works with all Apollo caches, including InMemoryCache & Hermes, and a variety of different storage providers.

To get started, simply pass your Apollo Cache and a storage provider to persistCacheSync. By default, the contents of your Apollo Cache will be immediately restored, and persisted upon every write to the cache with a short configurable debounce interval.

import { LocalStorageWrapper, persistCacheSync } from 'apollo3-cache-persist';
import { InMemoryCache } from '@apollo/client/core';
 
const cache = new InMemoryCache();
 
persistCacheSync({
  cache,
  storage: new LocalStorageWrapper(window.LocalStorage),
});

For more advanced usage, such as persisting the cache when the app is in the background, and additional configuration options, please check the README of apollo3-cache-persist.