GraphQL & gRPC Skill

# GraphQL & gRPC Skill

Design schemas, resolvers, and service definitions for GraphQL and gRPC APIs.

---

## GraphQL Schema Design

```graphql
# Scalars
scalar DateTime
scalar UUID
scalar JSON

# Enums
enum OrderStatus { PENDING PAID SHIPPED DELIVERED CANCELLED }
enum UserRole { ADMIN EDITOR VIEWER }

# Types
type User {
  id: UUID!
  name: String!
  email: String!
  role: UserRole!
  orders(first: Int, after: String): OrderConnection!
  createdAt: DateTime!
}

type Order {
  id: UUID!
  status: OrderStatus!
  total: Float!
  items: [OrderItem!]!
  user: User!
  createdAt: DateTime!
}

type OrderItem {
  id: UUID!
  product: Product!
  quantity: Int!
  price: Float!
}

# Pagination (Relay cursor spec)
type OrderConnection {
  edges: [OrderEdge!]!
  pageInfo: PageInfo!
  totalCount: Int!
}
type OrderEdge { node: Order!; cursor: String! }
type PageInfo {
  hasNextPage: Boolean!
  hasPreviousPage: Boolean!
  startCursor: String
  endCursor: String
}

# Queries
type Query {
  me: User
  user(id: UUID!): User
  users(first: Int, after: String, role: UserRole): UserConnection!
  order(id: UUID!): Order
  orders(status: OrderStatus, first: Int, after: String): OrderConnection!
}

# Mutations
type Mutation {
  createUser(input: CreateUserInput!): CreateUserPayload!
  updateUser(id: UUID!, input: UpdateUserInput!): UpdateUserPayload!
  deleteUser(id: UUID!): DeletePayload!
  createOrder(input: CreateOrderInput!): CreateOrderPayload!
  cancelOrder(id: UUID!): CancelOrderPayload!
}

# Subscriptions
type Subscription {
  orderStatusChanged(orderId: UUID!): Order!
  newOrder: Order!
}

# Inputs & Payloads
input CreateUserInput { name: String!; email: String!; role: UserRole }
type CreateUserPayload { user: User; errors: [UserError!] }
type UserError { field: String; message: String! }
```

---

## Resolver Pattern (DataLoader — solves N+1)

```javascript
// Without DataLoader: N+1 queries
// With DataLoader: batch all user IDs into one SQL IN(...)

const userLoader = new DataLoader(async (userIds) => {
  const users = await db.query(`SELECT * FROM users WHERE id = ANY($1)`, [userIds]);
  // Return in same order as input IDs
  return userIds.map(id => users.find(u => u.id === id) || null);
});

const resolvers = {
  Order: {
    user: (order, _, { loaders }) => loaders.user.load(order.userId),
  },
  Query: {
    orders: async (_, { status, first = 20, after }) => {
      return paginatedQuery('orders', { status, first, after });
    }
  }
};
```

---

## Error Handling in GraphQL

```json
{
  "data": { "createUser": null },
  "errors": [
    {
      "message": "Email already in use",
      "locations": [{ "line": 2, "column": 3 }],
      "path": ["createUser"],
      "extensions": {
        "code": "USER_EMAIL_TAKEN",
        "field": "email"
      }
    }
  ]
}
```

---

## gRPC Proto Definition

```protobuf
syntax = "proto3";
package users.v1;
option go_package = "github.com/example/api/users/v1";

import "google/protobuf/timestamp.proto";
import "google/protobuf/empty.proto";

service UsersService {
  // Unary RPCs
  rpc GetUser(GetUserRequest) returns (User);
  rpc CreateUser(CreateUserRequest) returns (User);
  rpc UpdateUser(UpdateUserRequest) returns (User);
  rpc DeleteUser(DeleteUserRequest) returns (google.protobuf.Empty);
  rpc ListUsers(ListUsersRequest) returns (ListUsersResponse);

  // Server streaming
  rpc WatchUser(GetUserRequest) returns (stream User);

  // Bidirectional streaming
  rpc SyncUsers(stream SyncRequest) returns (stream SyncResponse);
}

message User {
  string id = 1;
  string name = 2;
  string email = 3;
  string role = 4;
  google.protobuf.Timestamp created_at = 5;
}

message GetUserRequest { string id = 1; }
message CreateUserRequest { string name = 1; string email = 2; string role = 3; }
message UpdateUserRequest { string id = 1; string name = 2; string email = 3; }
message DeleteUserRequest { string id = 1; }
message ListUsersRequest { int32 page = 1; int32 limit = 2; string role = 3; }
message ListUsersResponse { repeated User users = 1; int32 total = 2; }
```

---

## REST vs GraphQL vs gRPC Decision Matrix

| Factor | REST | GraphQL | gRPC |
|--------|------|---------|------|
| Public API | ✓ Best | ✓ Good | ✗ |
| Mobile clients (bandwidth) | ✗ Over-fetch | ✓ Best | ✓ |
| Microservices (internal) | ✓ | ✗ | ✓ Best |
| Streaming / real-time | ✗ | ✓ Subscriptions | ✓ Best |
| Complex queries | ✗ N endpoints | ✓ Best | ✗ |
| Caching | ✓ HTTP cache | ✗ Complex | ✗ |
| Browser native | ✓ | ✓ | ✗ (needs proxy) |

---

## After Completing the API Design

Once the graphql/grpc design output is delivered, ask the user:

"Would you like me to generate API documentation for this design? (yes/no)"

If the user says **yes**:
- Check if the API Documentation skill is available in the installed skills list
- If the skill **is available**:
  - Read and follow the instructions in the API Documentation skill
  - Use the API design output above as the input
- If the skill **is NOT available**:
  - Inform the user: "It looks like the API Documentation skill isn't installed. 
    You can install it and re-run.

If the user says **no**:
- End the task here

---