Configuring the cache

Apollo Client stores the results of its GraphQL queries in a normalized, in-memory cache. This enables your client to respond to future queries for the same data without sending unnecessary network requests.

This article describes cache setup and configuration. To learn how to interact with cached data, see Reading and writing data to the cache.

Installation

As of Apollo Client 3.0, the InMemoryCache class is provided by the @apollo/client package. No additional libraries are required.

Initialization

Create an InMemoryCache object and provide it to the ApolloClient constructor, like so:

import { InMemoryCache, ApolloClient } from '@apollo/client';

const client = new ApolloClient({
  // ...other arguments...
  cache: new InMemoryCache(options)
});

The InMemoryCache constructor accepts a variety of configuration options.

Configuration options

Although the cache's default behavior is suitable for a wide variety of applications, you can configure its behavior to better suit your particular use case. In particular, you can:

• Specify custom primary key fields
• Customize the storage and retrieval of individual fields
• Customize the interpretation of field arguments
• Define supertype-subtype relationships for fragment matching
• Define patterns for pagination
• Manage client-side local state

To customize cache behavior, provide an options object to the InMemoryCache constructor. This object supports the following fields:

Data normalization

The InMemoryCache normalizes query response objects before it saves them to its internal data store. Normalization involves the following steps:

1. The cache generates a unique ID for every identifiable object included in the response.
2. The cache stores the objects by ID in a flat lookup table.

3. Whenever an incoming object is stored with the same ID as an existing object, the fields of those objects are merged.

   • If the incoming object and the existing object share any fields, the incoming object overwrites the cached values for those fields.
   • Fields that appear in only the existing object or only the incoming object are preserved.

Normalization constructs a partial copy of your data graph on your client, in a format that's optimized for reading and updating the graph as your application changes state.

Generating unique identifiers

In Apollo Client 3 and later, the InMemoryCache never creates a fallback, "fake" identifier for an object when identifier generation fails or is disabled.

Default identifier generation

By default, the InMemoryCache generates a unique identifier for any object that includes a __typename field. To do so, it combines the object's __typename with its id or _id field (whichever is defined). These two values are separated by a colon (:).

For example, an object with a __typename of Task and an id of 14 is assigned a default identifier of Task:14.

Customizing identifier generation by type

If one of your types defines its primary key with a field besides id or _id, you can customize how the InMemoryCache generates unique identifiers for that type. To do so, you define TypePolicy for the type. You specify all of your cache's typePolicies in the options object you provide to the InMemoryCache constructor.

Include a keyFields field in relevant TypePolicy objects, like so:

const cache = new InMemoryCache({
  typePolicies: {
    Product: {
      // In most inventory management systems, a single UPC code uniquely
      // identifies any product.
      keyFields: ["upc"],
    },
    Person: {
      // In some user account systems, names or emails alone do not have to
      // be unique, but the combination of a person's name and email is
      // uniquely identifying.
      keyFields: ["name", "email"],
    },
    Book: {
      // If one of the keyFields is an object with fields of its own, you can
      // include those nested keyFields by using a nested array of strings:
      keyFields: ["title", "author", ["name"]],
    },
  },
});

This example shows three typePolicies: one for a Product type, one for a Person type, and one for a Book type. Each TypePolicy's keyFields array defines which fields on the type together represent the type's primary key.

The Book type above uses a subfield as part of its primary key. The ["name"] item indicates that the name field of the previous field in the array (author) is part of the primary key. The Book's author field must be an object that includes a name field for this to be valid.

In the example above, the resulting identifier string for a Book object has the following structure:

Book:{"title":"Fahrenheit 451","author":{"name":"Ray Bradbury"}}

An object's primary key fields are always listed in the same order to ensure uniqueness.

Note that these keyFields strings always refer to the actual field names as defined in your schema, meaning the ID computation is not sensitive to field aliases.

Calculating an object's identifier

If you define a custom identifier that uses multiple fields, it can be challenging to calculate and provide that identifier to methods that require it (such as cache.readFragment).

To help with this, you can use the cache.identify method to calculate the identifier for any normalized object you fetch from your cache. See Obtaining an object's custom ID.

Customizing identifier generation globally

If you need to define a single fallback keyFields function that isn't specific to any particular __typename, you can use the dataIdFromObject function that was introduced in Apollo Client 2.x:

import { defaultDataIdFromObject } from '@apollo/client';

const cache = new InMemoryCache({
  dataIdFromObject(responseObject) {
    switch (responseObject.__typename) {
      case 'Product': return `Product:${responseObject.upc}`;
      case 'Person': return `Person:${responseObject.name}:${responseObject.email}`;
      default: return defaultDataIdFromObject(responseObject);
    }
  }
});

The dataIdFromObject API is included in Apollo Client 3.0 to ease the transition from Apollo Client 2.x. The API might be removed in a future version of @apollo/client.

Notice that the above function still uses different logic to generate keys based on an object's __typename. In the above case, you might as well define keyFields arrays for the Product and Person types via typePolicies. Also, this code is sensitive to aliasing mistakes, it does nothing to protect against undefined object properties, and accidentally using different key fields at different times can cause inconsistencies in the cache.

Disabling normalization

You can instruct the InMemoryCache not to normalize objects of a certain type. This can be useful for metrics and other transient data that's identified by a timestamp and never receives updates.

To disable normalization for a type, define a TypePolicy for the type (as shown in Customizing identifier generation by type) and set the policy's keyFields field to false.

Objects that are not normalized are instead embedded within their parent object in the cache. You can't access these objects directly, but you can access them via their parent.

TypePolicy fields

To customize how the cache interacts with specific types in your schema, you can provide an object mapping __typename strings to TypePolicy objects when you create a new InMemoryCache object.

A TypePolicy object can include the following fields:

type TypePolicy = {
  // Allows defining the primary key fields for this type, either using an
  // array of field names, a function that returns an arbitrary string, or
  // false to disable normalization for objects of this type.
  keyFields?: KeySpecifier | KeyFieldsFunction | false;

  // If your schema uses a custom __typename for any of the root Query,
  // Mutation, and/or Subscription types (rare), set the corresponding
  // field below to true to indicate that this type serves as that type.
  queryType?: true,
  mutationType?: true,
  subscriptionType?: true,

  fields?: {
    [fieldName: string]:
      | FieldPolicy<StoreValue>
      | FieldReadFunction<StoreValue>;
  }
};

// Recursive type aliases are coming in TypeScript 3.7, so this isn't the
// actual type we use, but it's what it should be:
type KeySpecifier = (string | KeySpecifier)[];

type KeyFieldsFunction = (
  object: Readonly<StoreObject>,
  context: {
    typename: string;
    selectionSet?: SelectionSetNode;
    fragmentMap?: FragmentMap;
  },
) => string | null | void;

Overriding root operation types (uncommon)

In addition to keyFields, a TypePolicy can indicate that it represents the root query, mutation, or subscription type by setting queryType, mutationType, or subscriptionType as true:

const cache = new InMemoryCache({
  typePolicies: {
    UnconventionalRootQuery: {
      // The RootQueryFragment can only match if the cache knows the __typename
      // of the root query object.
      queryType: true,
    },
  },
});

const result = cache.readQuery({
  query: gql`
    query MyQuery {
      ...RootQueryFragment
    }
    fragment RootQueryFragment on UnconventionalRootQuery {
      field1
      field2 {
        subfield
      }
    }
  `,
});

const equivalentResult = cache.readQuery({
  query: gql`
    query MyQuery {
      field1
      field2 {
        subfield
      }
    }
  `,
});

The cache normally obtains __typename information by adding the __typename field to every query selection set it sends to the server. It could technically use the same trick for the outermost selection set of every operation, but the __typename of the root query or mutation is almost always simply "Query" or "Mutation", so the cache assumes those common defaults unless instructed otherwise in a TypePolicy.

Compared to the __typenames of entity objects like Books or Persons, which are absolutely vital to proper identification and normalization, the __typename of the root query or mutation type is not nearly as useful or important, because those types are singletons with only one instance per client.

The fields property

The final property within TypePolicy is the fields property, which is a map from string field names to FieldPolicy objects. For more information on this field, see Customizing the behavior of cached fields.