It is one of the root types. mutations are operations used to modify data on the server. While queries are used to fetch data, mutations allow you to create, update, or delete data. Essentially, mutations are how GraphQL handles changes to the underlying data.

1
Mutations are used for actions like creating new records, updating existing ones, or deleting data.
2
Similar to queries, mutations return a response based on the operation performed. For instance, after creating a user, the mutation can return the user's ID or details.
3
Unlike traditional APIs, mutations allow clients to define the exact structure of the response they need, reducing over-fetching or under-fetching.

The syntax for mutations is very similar to queries, with the distinction that you use the keyword. Here’s an example of creating a user. The mutation accepts arguments (name, email, age ) to create the user. The response specifies that id, name, and email of the newly created user should be returned.

Difficulty: 6/10

Topics: Schema Design, State Management, Error Handling

Scenario Questions

0-2 years experience

1
Imagine you're building a 'Like' button for a social media post. How would you structure the GraphQL mutation so that the frontend can immediately display the updated like count without having to trigger a separate query?
2
We have a mutation to update a user's profile. If the email update fails validation on the backend but the username update succeeds, how should the mutation response look, and what fields should it return?

2-5 years experience

1
We're using Apollo Client, and after running a 'deleteTodo' mutation, the item disappears from the database but stays on the screen until the user manually refreshes. What is going on here, and how would you fix it?
2
You're designing a mutation to update a shopping cart. You could either pass individual arguments like 'itemId' and 'quantity', or use a single 'UpdateCartInput' object. Which approach would you choose and why?

5-8 years experience

1
In our checkout system, a single mutation needs to charge a card, update inventory, and create an order. If the inventory update fails after the card is charged, how do you handle this partial failure? How do you design the schema's error types to communicate this clearly to the client?
2
We have a mobile app on flaky network connections sending a 'createComment' mutation. Sometimes users double-tap or retry on timeout, causing duplicate comments. How would you design the mutation and backend to guarantee idempotency?

8+ years experience

1
We are migrating a legacy REST API with hundreds of microservices to a unified GraphQL federated gateway. How do you establish governance around mutations to prevent different teams from creating conflicting or redundant write operations, and how do you handle distributed transactions across federated subgraphs?
2
For a high-throughput collaborative document editor, would you use standard GraphQL mutations for every keystroke/change? If not, how would you architect the write path, and where do mutations fit into your CQRS or event-sourcing strategy?

Follow-up Questions

❓Why does GraphQL execute mutations serially while queries run in parallel?
❓How do you handle file uploads within a GraphQL mutation?
❓What is your strategy for implementing optimistic UI updates on slow mutation connections?