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System design patterns

Sharding, load balancing, CAP tradeoffs, and scaling patterns — interview and production ready.

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Domain Driven Design Aggregate Root Example in Python

Model an Order as an aggregate root with invariants enforced through methods, demonstrating DDD principles in Python.

from __future__ import annotations
from dataclasses import dataclass
from typing import List, Optional
from uuid import uuid4


class Money:
    def __init__(self, amount: float, currency: str = "USD"):
        self.amount = amount
        self.currency = currency

    def __add__(self, other: Money) -> Money:
       …

12 0 Open

System design patterns medium

How to Build an Anti-Corruption Layer in Python

Wrap a legacy system with a translation layer that converts awkward legacy data into a clean, modern DTO (Data Transfer Object) for use by new code.

anti-corruption-layer ddd dto

Python

class LegacyOrderSystem:
    """Legacy system with awkward, unstructured data."""
    def get_order(self):
        return {
            "order_id": "ORD-123",
            "cust": "Acme Corp",
            "items": [{"sku": "A1", "qty": 2, "price_each": 10.0}],
            "ship_to": "123 Main St, Springfield"
        }…

15 0 Open

System design patterns medium

How to Implement CQRS with Separate Read and Write Models in Python

Implements Command Query Responsibility Segregation (CQRS) by splitting data into separate write and read models with dedicated repositories, using dataclasses for structure.

cqrs dataclasses repositories

Python

from dataclasses import dataclass, field
from typing import List, Dict, Optional


@dataclass
class OrderWriteModel:
    order_id: int
    customer: str
    items: List[str] = field(default_factory=list)

    def add_item(self, item: str) -> None:
        self.items.append(item)


@dataclass
class OrderReadModel:
    …

14 0 Open

System design patterns medium

How to Implement a Simple MVVM Binding Mock in Python

A minimal Python implementation of the MVVM pattern, mocking data binding so views auto-update when the view model changes.

mvvm binding observer pattern

Python

class BindingMock:
    def __init__(self, view_model):
        self.view_model = view_model
        self.subscribers = []

    def bind(self, property_name, callback):
        self.subscribers.append((property_name, callback))

    def set(self, property_name, value):
        setattr(self.view_model, property_name, va…

18 0 Open

System design patterns medium

How to Migrate a Legacy Facade with the Strangler Fig Pattern in Python

Use a facade to wrap a legacy API and incrementally migrate callers to a modern interface, following the strangler fig pattern.

facade legacy migration

Python

class LegacyAPI:
    """Simulates the legacy system's raw interface."""
    def get_user(self, user_id):
        return {"id": user_id, "name": "Alice", "legacy": True}


class UserService:
    """Facade that wraps the legacy system with a modern interface."""
    def __init__(self, legacy_api=None):
        self.lega…

12 0 Open

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