What are the differences between GraphQL Query, Mutation, and Subscription

Differences Between GraphQL Query, Mutation, and Subscription

GraphQL provides three main operation types, each with specific purposes and semantics.

1. Query

Definition: Used to fetch data, similar to GET requests in REST.

Characteristics:

• Read-only operation, does not modify data on the server
• Multiple queries can be executed in parallel
• Can nest queries to fetch related data
• Can use parameters to filter or customize results

Example:

graphql
query GetUser($userId: ID!) {
  user(id: $userId) {
    id
    name
    email
    posts {
      id
      title
      createdAt
    }
  }
}

Use Cases:

• Fetching user information
• List queries
• Data detail display
• Report generation

2. Mutation

Definition: Used to modify data on the server, similar to POST, PUT, DELETE requests in REST.

Characteristics:

• Modifies data on the server
• Executes sequentially (serially) to ensure data consistency
• Typically returns modified data or operation results
• Can include input objects and parameters

Example:

graphql
mutation CreateUser($input: CreateUserInput!) {
  createUser(input: $input) {
    id
    name
    email
    createdAt
  }
}

mutation UpdateUser($userId: ID!, $input: UpdateUserInput!) {
  updateUser(id: $userId, input: $input) {
    id
    name
    email
    updatedAt
  }
}

mutation DeleteUser($userId: ID!) {
  deleteUser(id: $userId) {
    success
    message
  }
}

Use Cases:

• Creating new resources
• Updating existing resources
• Deleting resources
• Batch operations

3. Subscription

Definition: Used for real-time data push, clients automatically receive updates when server-side data changes.

Characteristics:

• Establishes persistent connection (typically using WebSocket)
• Server actively pushes data to clients
• Suitable for scenarios with high real-time requirements
• Can include filtering conditions

Example:

graphql
subscription OnUserCreated {
  userCreated {
    id
    name
    email
    createdAt
  }
}

subscription OnPostUpdated($postId: ID!) {
  postUpdated(postId: $postId) {
    id
    title
    content
    updatedAt
  }
}

Use Cases:

• Chat applications
• Real-time notifications
• Real-time data monitoring
• Collaborative editing

Core Differences Comparison

Feature	Query	Mutation	Subscription
Data Modification	Read-only	Modifies data	Read-only
Execution	Parallel	Serial	Persistent connection
Real-time	On-demand request	On-demand request	Real-time push
Network Protocol	HTTP	HTTP	WebSocket
Caching	Cacheable	Not cacheable	Not cacheable
Idempotency	Idempotent	Non-idempotent	Non-idempotent

Best Practices

Query Best Practices

• Avoid over-fetching, request only needed fields
• Use parameters for data filtering
• Implement pagination reasonably
• Leverage GraphQL's type system for data validation

Mutation Best Practices

• Use input objects to encapsulate parameters
• Return complete modified data
• Provide clear success/failure responses
• Implement appropriate error handling and validation

Subscription Best Practices

• Provide filtering conditions to reduce unnecessary data pushes
• Implement connection management and reconnection mechanisms
• Consider using load balancing for handling large numbers of subscriptions
• Set reasonable timeouts and heartbeat mechanisms

Performance Considerations

• Query: Use DataLoader to solve N+1 query problems
• Mutation: Use transactions to ensure data consistency
• Subscription: Use message queues to handle high-concurrency subscriptions