Designing Response Types

Response types for mutations

This response type pattern is useful for mutations because they can result in many valid failure states (such as attempting to delete an object that doesn't exist).

For example, let's say we have an e-commerce graph. When executing a checkout mutation, it's valid for that mutation to fail if a purchased item is out of stock or the buyer has insufficient funds.

If we define a set of ErrorResponse types in our schema, frontend developers can provide customized experiences based on the type of failure that occurred:

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interface MutationResponse {
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  status: ResponseStatus!
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}
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type CheckoutSuccess implements MutationResponse {
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  status: ResponseStatus!
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  cart: [Product!]!
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  invoice: Invoice!
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}
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interface ErrorResponse {
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  status: ResponseStatus!
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  message: String!
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}
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type CheckoutError implements ErrorResponse {
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  status: ResponseStatus!
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  message: String!
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}
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union CheckoutResponse = CheckoutSuccess | CheckoutError
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type Mutation {
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  checkout(cart: ID!): CheckoutResponse!
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}

Response types for queries

Let's say we have a basic GraphQL API that defines the following Query type:

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type Query {
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  users: [User!]!
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}

This Query.users field makes intuitive sense: if you query it, you get back a list of User objects. However, this return type doesn't provide any insight into the result:

• If the list is empty, is that because there are zero users, or did an error occur?
• Even if the list is populated, did the API return all users or just a subset?
• Are there multiple pages of results?

To answer questions like these, it may be helpful for top-level fields of Query to return "wrapper" objects that can include both the operation result and metadata about the operation's execution. For example, in cases where you need to paginate your results or implement Relay-style connections this pattern can be helpful.

Example: UsersResponse

The following example defines a UsersResponse type for our Query.users field:

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type User {
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  id: ID!
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  firstName: String!
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  lastName: String!
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}
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type UsersResponse {
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  offset: Int!
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  limit: Int!
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  totalResults: Int!
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  data: [User!]!
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}
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type Query {
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  users(limit: Int = 10, offset: Int = 0): UsersResponse!
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}

With this change, a client's query can specify how many results per page, what page to start on, and understand the total number of results that can be paginated:

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query FetchUsers {
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  users(limit: 20, offset: 0) {
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    totalResults
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    data {
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      id
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      firstName
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      lastName
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    }
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  }
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}