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System design patterns

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

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How to Build an Adapter to Translate External API Responses in Python

Build an adapter class that translates a mock external API's response shape into your internal representation, keeping callers decoupled from the external contract.

adapter-pattern api architecture

Python

import json
from typing import Dict, Any


class ExternalAPI:
    """Mock external service returning a different data shape."""
    def get_user(self, user_id: int) -> Dict[str, Any]:
        return {
            "id": user_id,
            "full_name": "Jane Doe",
            "email_address": "jane@example.com",
     …

14 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

System design patterns easy

How to Mock Hexagonal Architecture Ports and Adapters in Python

Mock an email adapter in a hexagonal architecture with unittest.mock to test business logic in isolation.

hexagonal-architecture unittest-mock dependency-injection

Python

from unittest.mock import Mock

class EmailService:
    def send(self, recipient, message):
        raise NotImplementedError

class OrderProcessor:
    def __init__(self, email_service):
        self.email_service = email_service
    
    def process_order(self, order_id, customer_email):
        # Business logic
   …

15 0 Open

System design patterns easy

How to mock the domain center in an onion architecture in Python

Define a repository interface and an in-memory mock to test domain services without touching infrastructure.

onion-architecture repository-pattern dependency-injection

Python

from abc import ABC, abstractmethod
from dataclasses import dataclass
from typing import Dict, List, Optional


@dataclass
class Order:
    id: int
    customer: str
    items: List[str]
    total: float


class OrderRepository(ABC):
    @abstractmethod
    def find_by_id(self, order_id: int) -> Optional[Order]:
     …

11 0 Open

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