Understanding GraphQL Error Policies: Handling Errors in GraphQL

GraphQL has gained significant popularity as a query language for APIs due to its flexibility and efficiency. One crucial aspect of GraphQL is its error handling mechanism, known as error policies. By understanding and implementing the appropriate error policy, developers can effectively handle and manage errors in GraphQL applications. In this article, we will explore the different error policies in GraphQL and discuss their implications and use cases.

GraphQL operations can result in GraphQL errors or network errors.

GraphQL errors

These are errors related to the server-side execution of a GraphQL operation. They include:

• Syntax errors (e.g., a query was malformed)
• Validation errors (e.g., a query included a schema field that doesn't exist)
• Resolver errors (e.g., an error occurred while attempting to populate a query field)

Syntax error or validation error occurs cause your server not to execute the operation at all because it's invalid. If resolver errors occur, your server can return partial data.

GraphQL errors are included by your server in the errors array of its response to Apollo.

Any GraphQL error that prevents Apollo Server from executing your operation at all, results in a response with a 4xx status code. If executed, the Apollo Server responds with a 200 status code if resolver errors occurred but the response still includes partial data.

Error Policies in GraphQL

GraphQL provides four error policies that dictate how errors are handled in the response: "none," "ignore," and "all."

1. "none" Error Policy

When the "none" error policy is set, GraphQL will return GraphQ errors and the data field will set to undefined ignoring any partial data that may have been returned by the server. This can be used when a GraphQL error will affect the data returned by introducing some sort of discrepancy or integrity issue. This is the default error policy.

2. "ignore" Error Policy

The "ignore" error policy doesn't include the error information in the response. If errors occur, the data response may be undefined or empty as a result of the error. This policy is suitable when error details are not crucial for the client and can be safely ignored whether they cause data to be undefined or not.

3. "all" Error Policy

The "all" error policy is the most comprehensive among the error policies. It includes both the data and error information in the response. Even if errors occur, the response will contain both a "data" field and an "errors" field. This policy is beneficial during development and debugging stages as it provides complete transparency and enables better error handling on the client-side.

Choosing the Right Error Policy

Selecting the appropriate error policy depends on various factors, including the nature of the application, the target audience, and the development stage. Here are some considerations:

1. Production Applications

In production environments, it is often recommended to use the "all" error policy. This ensures that both data and error information are available, allowing for better error handling and troubleshooting.

2. Public-Facing APIs

Public APIs that serve external clients may benefit from the "none" or "ignore" error policies. These policies provide a level of abstraction and security by hiding error details from potential attackers or unauthorized users.

3. Development and Debugging

During development and debugging, using the "all" error policy can be highly valuable. It provides comprehensive error information, helping developers identify and fix issues quickly.

4. Custom Error Handling

In some cases, developers may opt for custom error handling by extending the default error handling mechanism. This approach allows for more fine-grained control over error responses and can be implemented using any of the available error policies.

Setting an error policy

Specify an error policy in the options object you provide your operation hook (such as watchQuery or useQuery), like so:

// Ambient declarations for Apollo GraphQL snippet
var useQuery: any;
var client: any;
var GET_MY_QUERY: any;

// Specify policy for useQuery
const { loading, error, data } = useQuery(GET_MY_QUERY, {
  errorPolicy: 'all'
});

// Specify policy for watchQuery
const observable = client.watchQuery({
  query: GET_MY_QUERY,
  errorPolicy: 'all'
});

The object passed to either watchQuery or useQuery satisfies the following options type:

interface QueryOptions {
  errorPolicy?: 'none' | 'ignore' | 'all';
  // ... other options
}

Conclusion

GraphQL error policies play a crucial role in handling errors effectively in GraphQL applications. By understanding the different error policies and their implications, developers can make informed decisions when choosing the right policy for their specific use cases. Whether it's prioritizing transparency during development or protecting sensitive information in public-facing APIs, the error policy selection significantly impacts the overall user experience and security of GraphQL applications.