Command Pattern

The Problem

Order operations such as placing, cancelling, and refunding orders require different logic. Without the Command pattern, there is no way to maintain a history, undo a recent action, or queue operations for later execution.

The Solution

The Command pattern wraps each operation in a self-contained command object that implements execute() and undo(). An invoker service manages execution, maintains a stack for undo support, and records a full history log.

Structure

Command (interface) — Declares execute() and undo() plus a name identifier.
Concrete Commands — PlaceOrderCommand, CancelOrderCommand, RefundOrderCommand.
Invoker — OrderCommandInvokerService manages execution and undo stack.
Client — CommandController maps HTTP actions to commands.

Implementation

Each command tracks execution state to prevent double-execution. The invoker maintains a stack for undo operations and a full history log:

export interface CommandResult {
  success: boolean;
  message: string;
  data?: Record<string, any>;
}

export interface Command {
  readonly name: string;
  execute(): CommandResult;
  undo(): CommandResult;
}

import { Command, CommandResult } from './command.interface';

export class PlaceOrderCommand implements Command {
  readonly name = 'PlaceOrder';
  private executed = false;

  constructor(
    private readonly orderId: string,
    private readonly amount: number,
  ) {}

  execute(): CommandResult {
    if (this.executed) {
      return {
        success: false,
        message: `Order ${this.orderId} has already been placed`,
      };
    }

    this.executed = true;

    return {
      success: true,
      message: `Order ${this.orderId} placed successfully`,
      data: {
        orderId: this.orderId,
        amount: this.amount,
        status: 'placed',
        timestamp: new Date().toISOString(),
      },
    };
  }

  undo(): CommandResult {
    if (!this.executed) {
      return {
        success: false,
        message: `Order ${this.orderId} was never placed — cannot undo`,
      };
    }

    this.executed = false;

    return {
      success: true,
      message: `Order ${this.orderId} placement reversed`,
      data: {
        orderId: this.orderId,
        status: 'reversed',
        timestamp: new Date().toISOString(),
      },
    };
  }
}

import { Command, CommandResult } from './command.interface';

export class CancelOrderCommand implements Command {
  readonly name = 'CancelOrder';
  private executed = false;

  constructor(
    private readonly orderId: string,
    private readonly reason?: string,
  ) {}

  execute(): CommandResult {
    if (this.executed) {
      return {
        success: false,
        message: `Order ${this.orderId} has already been cancelled`,
      };
    }

    this.executed = true;

    return {
      success: true,
      message: `Order ${this.orderId} cancelled successfully`,
      data: {
        orderId: this.orderId,
        reason: this.reason ?? 'No reason provided',
        status: 'cancelled',
        timestamp: new Date().toISOString(),
      },
    };
  }

  undo(): CommandResult {
    if (!this.executed) {
      return {
        success: false,
        message: `Order ${this.orderId} was never cancelled — cannot undo`,
      };
    }

    this.executed = false;

    return {
      success: true,
      message: `Order ${this.orderId} cancellation reversed — order restored`,
      data: {
        orderId: this.orderId,
        status: 'restored',
        timestamp: new Date().toISOString(),
      },
    };
  }
}

import { Command, CommandResult } from './command.interface';

export class RefundOrderCommand implements Command {
  readonly name = 'RefundOrder';
  private executed = false;

  constructor(
    private readonly orderId: string,
    private readonly amount?: number,
    private readonly reason?: string,
  ) {}

  execute(): CommandResult {
    if (this.executed) {
      return {
        success: false,
        message: `Refund for order ${this.orderId} has already been processed`,
      };
    }

    this.executed = true;

    return {
      success: true,
      message: `Refund for order ${this.orderId} processed successfully`,
      data: {
        orderId: this.orderId,
        refundAmount: this.amount ?? 0,
        reason: this.reason ?? 'No reason provided',
        status: 'refunded',
        timestamp: new Date().toISOString(),
      },
    };
  }

  undo(): CommandResult {
    if (!this.executed) {
      return {
        success: false,
        message: `No refund was issued for order ${this.orderId} — cannot undo`,
      };
    }

    this.executed = false;

    return {
      success: true,
      message: `Refund for order ${this.orderId} reversed — funds reclaimed`,
      data: {
        orderId: this.orderId,
        reclaimedAmount: this.amount ?? 0,
        status: 'refund_reversed',
        timestamp: new Date().toISOString(),
      },
    };
  }
}

import { Injectable } from '@nestjs/common';
import { Command, CommandResult } from './command.interface';

export interface HistoryEntry {
  commandName: string;
  action: 'execute' | 'undo';
  result: CommandResult;
  timestamp: string;
}

@Injectable()
export class OrderCommandInvokerService {
  private readonly executedCommands: Command[] = [];
  private readonly history: HistoryEntry[] = [];

  executeCommand(command: Command): CommandResult {
    const result = command.execute();

    if (result.success) {
      this.executedCommands.push(command);
    }

    this.history.push({
      commandName: command.name,
      action: 'execute',
      result,
      timestamp: new Date().toISOString(),
    });

    return result;
  }

  undoLastCommand(): CommandResult {
    const command = this.executedCommands.pop();

    if (!command) {
      return {
        success: false,
        message: 'No commands to undo',
      };
    }

    const result = command.undo();

    this.history.push({
      commandName: command.name,
      action: 'undo',
      result,
      timestamp: new Date().toISOString(),
    });

    return result;
  }

  getHistory(): HistoryEntry[] {
    return [...this.history];
  }
}

NestJS Integration

The OrderCommandInvokerService is an @Injectable() singleton that persists the command history across requests. Individual command objects are created per request in the controller, keeping them stateful and isolated. The invoker is the only NestJS-managed component; commands are plain classes.

When to Use

You need to parameterize objects with operations, queue operations, or schedule their execution.
You want to support undo/redo functionality.
You need to log all operations for auditing.
You want to decouple the object that invokes the operation from the one that performs it.

When NOT to Use

The operations are simple and don’t need history, undo, or queuing.
Adding command objects for trivial operations introduces unnecessary abstraction.

Memento

Capture and externalize an object's internal state so that the object can be restored to this state later.

Strategy

Define a family of algorithms, encapsulate each one, and make them interchangeable.

Chain of Responsibility

Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request.