Implement Command Side Validation and Execution - The Reactor

Implementation

The command side is the write path in CQRS. Before any state change is applied, the command must pass two layers of validation: schema validation (correct fields and types) and business rule validation (domain constraints). Only then is the command executed and a domain event emitted.

Implement a node that processes commands through both validation layers:

// Schema error: missing required field
{ "type": "CreateUser", "msg_id": 1,
  "payload": {"name": "John"} }      // email missing
-> { "type": "validation_failed", "in_reply_to": 1,
    "valid": false, "errors": ["Email is required"] }

// Business rule error: age constraint
{ "type": "CreateUser", "msg_id": 2,
  "payload": {"name": "John", "email": "j@example.com", "age": 16} }
-> { "type": "validation_failed", "in_reply_to": 2,
    "valid": false, "errors": ["User must be 18 or older"] }

// All validation passes: execute and emit domain event
{ "type": "CreateUser", "msg_id": 3,
  "payload": {"name": "John", "email": "j@example.com", "age": 25} }
-> { "type": "command_executed", "in_reply_to": 3,
    "success": true,
    "events": [{"type": "UserCreated", "payload": {"id": "<uuid>"}}] }

When multiple validations fail, collect all errors and return them together. Never execute the command if any validation fails.

Sample Test Cases

Validate command formatTimeout: 5000ms

Input

{
  "src": "client",
  "dest": "commandside",
  "body": {
    "type": "CreateUser",
    "msg_id": 1,
    "payload": {
      "name": "John"
    }
  }
}

Expected Output

{"type": "validation_failed", "in_reply_to": 1, "valid": false, "errors": ["Email is required"]}

Validate business rulesTimeout: 5000ms

Input

{
  "src": "client",
  "dest": "commandside",
  "body": {
    "type": "CreateUser",
    "msg_id": 1,
    "payload": {
      "name": "John",
      "email": "john@example.com",
      "age": 16
    }
  }
}

Expected Output

{"type": "validation_failed", "in_reply_to": 1, "valid": false, "errors": ["User must be 18 or older"]}

Hints

Hint 1▾

Schema validation: check required fields and types before business rules

Hint 2▾

Business rule validation: enforce domain constraints (age >= 18, unique email, etc.)

Hint 3▾

Return all validation errors together, not just the first one

Hint 4▾

Only call the command handler after all validation passes

Hint 5▾

Emit a domain event (UserCreated) that describes what happened, not what was requested

Implementation

Sample Test Cases

Hints

Resources

Theoretical Hub

Key Concepts