System design patterns
Sharding, load balancing, CAP tradeoffs, and scaling patterns — interview and production ready.
Create a Data Helper Class in Python
A reusable DataHelper class that saves and loads JSON and CSV files from a configurable base directory, with automatic header detection for CSV.
import json
import csv
from pathlib import Path
class DataHelper:
def __init__(self, base_path="."):
self.base_path = Path(base_path)
self.base_path.mkdir(exist_ok=True)
def save_json(self, data, filename):
path = self.base_path / filename
with open(path, "w") as f:
…How to Build a Health Check System with Instance Up and Down Status in Python
Track instance health by marking them up or down and simulating health checks with a mock class in Python.
from datetime import datetime
import random
class HealthChecker:
def __init__(self):
self.status = {}
def mark_up(self, instance_id):
self.status[instance_id] = {
"state": "up",
"last_check": datetime.now().isoformat(),
"healthy": True
}
…How to Build a Simple Service Discovery Registry in Python
A lightweight in-memory service registry class using a dict — register, deregister, and discover services with host, port, and version.
class ServiceRegistry:
def __init__(self):
self._services = {}
def register(self, name, host, port, version="1.0"):
self._services[name] = {
"host": host,
"port": port,
"version": version
}
def deregister(self, name):
return self._servic…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.
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",
…How to Build an Append-Only Event Store in Python
Implement a simple append-only event store class that stores events in a list and supports retrieval by index range.
class EventStore:
def __init__(self):
self._events = []
def append(self, event):
"""Append an event to the store."""
self._events.append(event)
def get_events(self, start=0, end=None):
"""Return events from start index to end (exclusive)."""
return self._events[sta…How to Build an Immutable Money Value Object in Python
Implement an immutable Money class with rounded decimal amounts, currency, safe equality, and hashing for use as a value object.
class Money:
def __init__(self, amount: float, currency: str):
object.__setattr__(self, "_amount", round(amount, 2))
object.__setattr__(self, "_currency", currency)
def __setattr__(self, name, value):
raise AttributeError(f"Money is immutable: cannot set '{name}'")
def __delattr__…How to Build an MVP Presenter View Mock in Python
A minimal MVP (Model-View-Presenter) mock showing a Presenter controlling a SlideDeck model with slide navigation and typed state via dataclasses.
from dataclasses import dataclass, field
from typing import List
@dataclass
class SlideDeck:
title: str
slides: List[str] = field(default_factory=list)
current_index: int = 0
def next_slide(self) -> str:
if self.current_index < len(self.slides) - 1:
self.current_index += 1
…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.
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:
…How to Implement a Data Helper Class in Python
Build a beginner-friendly DataHelper class using dataclasses and key system design patterns like Command, Strategy, and Map.
from __future__ import annotations
from dataclasses import dataclass, field
from typing import Any, Dict, List, Optional
@dataclass
class DataHelper:
"""A beginner-friendly data utility with common system design patterns."""
data: List[Dict[str, Any]] = field(default_factory=list)
def add_record(self, r…How to Implement a Factory Method by Type String in Python
A factory method maps a type string to a class, creating and returning the appropriate object instance while handling unknown types gracefully.
class Animal:
def speak(self):
raise NotImplementedError
class Dog(Animal):
def speak(self):
return "Woof!"
class Cat(Animal):
def speak(self):
return "Meow!"
class AnimalFactory:
@staticmethod
def create(animal_type: str) -> Animal:
animal_types = {
…How to Implement a Simple Event Bus in Python
Create a publish-subscribe event bus using dataclasses and defaultdict to decouple event producers from consumers.
from collections import defaultdict
from dataclasses import dataclass, field
from typing import Callable, Dict, List, Set
@dataclass
class EventBus:
_subscribers: Dict[str, List[Callable]] = field(
default_factory=lambda: defaultdict(list)
)
def subscribe(self, event_type: str, handler: Callable…How to Implement the Abstract Factory Pattern in Python
Implements the Abstract Factory pattern to create families of related GUI objects (buttons, checkboxes) without specifying their concrete classes.
from abc import ABC, abstractmethod
class Button(ABC):
@abstractmethod
def render(self):
pass
class Checkbox(ABC):
@abstractmethod
def render(self):
pass
class WindowsButton(Button):
def render(self):
return "Rendering Windows-style button"
class WindowsCheckbox(Chec…How to Implement the Prototype Pattern with Deep Copy in Python
Implements the Prototype design pattern using copy.deepcopy to clone complex objects without sharing mutable state.
import copy
from dataclasses import dataclass, field
from typing import List
@dataclass
class Engine:
horsepower: int
@dataclass
class Car:
brand: str
engine: Engine
accessories: List[str] = field(default_factory=list)
def clone_prototype(car: Car) -> Car:
return copy.deepcopy(car)
if __name__ …How to Take Periodic Snapshots of Aggregate State in Python
Build a Python class that accumulates values and periodically captures immutable snapshots of total, count, and average for later analysis.
import time
import random
from collections import defaultdict
class SnapshotAggregator:
def __init__(self):
self.total = 0
self.count = 0
self.history = []
def add(self, value):
self.total += value
self.count += 1
def snapshot(self):
avg = self.total / se…Inbox pattern consumer dedupe mock in Python
Implements a mock inbox consumer that deduplicates incoming messages by ID, with automatic eviction of old seen IDs to prevent unbounded memory growth.
import json
from collections import deque
from dataclasses import dataclass, field
from hashlib import sha256
from typing import Any
@dataclass
class InboxConsumer:
max_seen: int = 1000
seen_ids: set = field(default_factory=set)
seen_history: deque = field(default_factory=deque)
def _mark_seen(self,…Microkernel Plug-in Core Mock in Python
Implements a minimal microkernel plug-in core that registers, unregisters, and executes synchronous or asynchronous plugins via a pluggable manager class.
import json
import abc
import inspect
class MicrokernelCore(abc.ABC):
def __init__(self):
self._plugins = {}
def register(self, name, plugin):
self._plugins[name] = plugin
def unregister(self, name):
return self._plugins.pop(name, None)
def execute(self, name, *args, **kwa…Template Method Workflow Steps Base Class in Python
Define a reusable workflow skeleton in a base class and let subclasses fill in each step with the Template Method design pattern.
from abc import ABC, abstractmethod
class DataPipeline(ABC):
"""Template Method pattern: defines a workflow skeleton."""
def run(self):
"""Template method - defines the algorithm's structure."""
result = {"extracted": False, "transformed": False, "loaded": False}
raw_data = self._ext…Browse by section
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System design patterns — Python code examples
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