Error Handling in GraphQL | CodeSignal Learn

Error Handling in GraphQL | CodeSignal Learn

Introduction

Welcome to the lesson on best practices for error handling in GraphQL . In this lesson, we will explore how to handle errors effectively in your GraphQL API using graphql-ruby and Sinatra. Proper error handling is crucial for building reliable and user-friendly applications.

How GraphQL Handles Errors and Common Error Types

GraphQL treats errors as part of the response format. If any field in a query fails, it includes an errors array in the response. Common error types include: User input errors. Authentication errors. Validation errors. System errors.

Implementing Basic Error Handling in Resolvers

graphql-ruby provides built-in error handling through GraphQL::ExecutionError to help you handle common errors. Let's start with handling missing data and validating user inputs. Here's how you can raise a GraphQL::ExecutionError if a book is not found: Ruby require 'graphql' BOOKS = [{ id: '1', title: 'The Hobbit', author: 'J.R.R. Tolkien' }, { id: '2', title: 'Harry Potter', author: 'J.K. Rowling' }] class BookType < GraphQL::Schema::Object field :id, ID, null: false field :title, String, null: false field :author, String, null: false end class QueryType < GraphQL::Schema::Object field :book, BookType, null: true do argument :id, ID, required: true end def book(id:) book = BOOKS.find { |b| b[:id] == id } raise GraphQL::ExecutionError, 'Book not found' unless book book end end In this code, we first define BookType with fields id, title, and author . Then, in QueryType, when a book with the specified id is not found, a GraphQL::ExecutionError is raised with the message "Book not found."

Example: Validating User Inputs

You can also raise an error if the required inputs are missing or invalid: Rubyclass MutationType < GraphQL::Schema::Object field :add_book, BookType, null: false do argument :title, String, required: true argument :author, String, required: true end def add_book(title:, author:) if title.nil? || title.empty? || author.nil? || author.empty? raise GraphQL::ExecutionError, 'Title and Author are required' end new_book = { id: (BOOKS.length + 1).to_s, title: title, author: author } BOOKS << new_book new_book end end In this code, if either title or author is missing or empty, a GraphQL::ExecutionError is raised with a relevant message.

Advanced Error Handling Techniques

For more complex cases, you might want to create custom error classes that inherit from GraphQL::ExecutionError : Ruby class MyCustomError < GraphQL::ExecutionError def initialize(message) super(message) end end You can then use this custom error class in your resolvers: Ruby def book(id:) book = BOOKS.find { |b| b[:id] == id } raise MyCustomError, 'Book not found' unless book book end

Handling Multiple Errors

You can handle multiple errors by creating a list of errors and raising them as needed: Rubydef add_book(title:, author:) errors = [] errors << 'Title is required' if title.nil? || title.empty? errors << 'Author is required' if author.nil? || author.empty? raise GraphQL::ExecutionError, errors.join(', ') unless errors.empty? new_book = { id: (BOOKS.length + 1).to_s, title: title, author: author } BOOKS << new_book new_book end

Example: Error Handling in a Complete Application

Now, let's put it all together in a complete example. Here is our server code: Rubyrequire 'sinatra' require 'graphql' require 'json' BOOKS = [ { id: '1', title: 'The Hobbit', author: 'J.R.R. Tolkien' }, { id: '2', title: 'Harry Potter', author: 'J.K. Rowling' } ] class BookType < GraphQL::Schema::Object field :id, ID, null: false field :title, String, null: false field :author, String, null: false end class QueryType < GraphQL::Schema::Object field :book, BookType, null: true do argument :id, ID, required: true end def book(id:) book = BOOKS.find { |b| b[:id] == id } raise GraphQL::ExecutionError, 'Book not found' unless book book end end class MutationType < GraphQL::Schema::Object field :add_book, BookType, null: false do argument :title, String, required: true argument :author, String, required: true end def add_book(title:, author:) if title.nil? || title.empty? || author.nil? || author.empty? raise GraphQL::ExecutionError, 'Title and Author are required' end new_book = { id: (BOOKS.length + 1).to_s, title: title, author: author } BOOKS << new_book new_book end end class AppSchema < GraphQL::Schema query QueryType mutation MutationType end post '/graphql' do request_payload = JSON.parse(request.body.read) query = request_payload['query'] variables = request_payload['variables'] || {} result = AppSchema.execute(query, variables: variables) content_type :json result.to_json end set :port, 4000 And here is how we make queries for the server: Rubyrequire 'net/http' require 'json' require 'uri' url = URI.parse('http://localhost:4000/graphql') def fetch_book(url, id) query = <<~GRAPHQL query { book(id: "#{id}") { title author } } GRAPHQL http = Net::HTTP.new(url.host, url.port) request = Net::HTTP::Post.new(url.path, { 'Content-Type' => 'application/json' }) request.body = { query: query }.to_json response = http.request(request) data = JSON.parse(response.body) if data['errors'] puts 'Errors:', data['errors'] else puts 'Data:', data end rescue StandardError => e puts 'Network Error:', e.message end def add_new_book(url, title, author) mutation = <<~GRAPHQL mutation { addBook(title: "#{title}", author: "#{author}") { id title author } } GRAPHQL http = Net::HTTP.new(url.host, url.port) request = Net::HTTP::Post.new(url.path, { 'Content-Type' => 'application/json' }) request.body = { query: mutation }.to_json response = http.request(request) data = JSON.parse(response.body) if data['errors'] puts 'Errors:', data['errors'] else puts 'Data:', data end rescue StandardError => e puts 'Network Error:', e.message end # Test the functions fetch_book(url, '1') add_new_book(url, '1984', 'George Orwell') In this code, errors from the server response are logged to the console using puts. Additionally, any network errors during the HTTP request are caught using Ruby's rescue clause and logged as "Network Error." When you run this code, you will set up a server that handles both queries and mutations with proper error handling.

Lesson Summary

To summarize, in this lesson, you learned the importance of error handling in GraphQL, how to implement basic and advanced error handling techniques using graphql-ruby, and saw how to apply them in a complete application with Sinatra. As you move on to the practice exercises, apply these techniques to solidify your understanding. Keep practicing and explore more advanced topics to enhance your expertise in building secure and resilient GraphQL APIs.

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