Error Handling in Mutations in GraphQL

GraphQL is a powerful open-source query language for APIs. It is known for its flexibility and efficiency in fetching data from server endpoints. GraphQL mutations allow us to update the data in the GraphQL server. Oftentimes, the data update might fail because of several reasons, network failure, retry failure, etc. and so we need some error handling incase of the operation failure to handle it gracefully. In this article, we will explore how we can do error handling in mutations in GraphQL.

What are GraphQL mutations?

Mutations in GraphQL are a set of instructions that allow us to update the data in the API server, and with the help of it, we can create, update, or remove the data in the backend database. They are defined with the "mutations" keyword, and are present in the Schema itself. They are given some input arguments, with the data to modify or insert, and the query uses that information to perform the specified operation.

Error Handling in Mutations

A GraphQL query might face some errors during its execution, such as network failures, or server errors, or rate limit errors. To handle these gracefully, we need to ensure that the end user knows that the operation has failed, and we can either retry the operation in the backend, or show the error message in the UI itself so the user itself can retry on the need basis.

Mutation Example -

In the below example query, we have a createUser mutation that is used to create a new user on the server when the query is hit. It accepts the input data, "name" and "email" and uses the data passed in the arguments to create a new entry for the user the server database.

mutation {
  createUser(input: { username: "john_doe", email: "[email protected]"}) {
    id
    username
    email
  }
}

Error Handling in Mutation Resolver

We will create a resolver function for the above mutation specified, and implement a custom error handling logic that will get triggered whenever the query faces an error while running.

In the below code, we will look for the required fields in the mutations, and if either username or email is missing, we will throw the error of the missing required fields, so the UI would know that the query failed with the valid reason, and can show the message to the end user.

const resolvers = {
  Mutation: {
    createUser: (_, { input }) => {
      // Extract input data
      const { username, email } = input;

      // Perform validation
      if (!username || !email) {
        throw new Error("Missing required fields: username, email");
      }

      // Create user in the database
      try {
        console.log('newUser created!'!);
      } catch (error) {
        // Handle database errors
        console.error("Error creating user:", error);
      }
    },
  },
};

Challenges in Error Handling

Error handling in GraphQL mutations can be challenging because of some factors -

• Proper validation for the data: The new data that is going to be inserted into the DB sometimes need proper and complex validations, unlike the standard ones provided natively by GraphQL
• Atomicity: DB transactions are expected to be atomic at various levels, and ensuring that it happens inside the mutations is essential to maintain data consistency
• Feedback on error messages: The server needs to provide valuable error messages to the client incase of any failure in the mutation

Best Practices for Error Handling in Mutations

• Custom error code: We can predefine a set of custom error codes to send back the client incase of any failure in the mutation
• Combine operations into a single transaction: We can wrap multiple operations in a single GraphQL query into a single transaction to maintain the data consistency
• Using Objects as input parameters: Using objects in the inputs parameters of the mutation enhances readability and the overall validation logic required to run on the data.

Conclusion

In this article, we learnt about the GraphQL mutations, and how can do better error handling to provide meaningful feedback to the end users. The error handling logic often needs to be implemented in the resolver itself, and show return informative messages to the client incase of an error, or perform some retry logic to make the system fault tolerant.