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System design patterns

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

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Circuit Breaker Pattern in Python: Closed, Open, and Half-Open States

Implement a circuit breaker with closed, open, and half-open states to prevent repeated calls to failing services and allow recovery after a timeout.

circuit-breaker resilience fault-tolerance

Python

class CircuitBreaker:
    def __init__(self, failure_threshold=3, timeout_seconds=5):
        self.failure_threshold = failure_threshold
        self.timeout_seconds = timeout_seconds
        self.state = "closed"
        self.failure_count = 0
        self.last_failure_time = None

    def record_success(self):
     …

14 0 Open

System design patterns medium

How to Build a Sidecar Logging Proxy in Python

Wrap any object with a proxy that transparently logs every method call, arguments, return value, and execution time to a file — mimicking a sidecar pattern.

proxy logging sidecar

Python

import logging
import time
from datetime import datetime


class LoggingProxy:
    """Sidecar-style proxy that logs all calls to a wrapped object."""

    def __init__(self, target, log_file="proxy.log"):
        self._target = target
        logging.basicConfig(
            filename=log_file,
            level=loggin…

15 0 Open

System design patterns medium

How to Implement the Strategy Pattern in Python

This Python code demonstrates the Strategy design pattern using interchangeable sorting algorithms (bubble sort and quick sort) that can be swapped at runtime.

design-pattern strategy oop

Python

class SortingStrategy:
    def sort(self, data):
        raise NotImplementedError

class BubbleSort(SortingStrategy):
    def sort(self, data):
        result = data.copy()
        n = len(result)
        for i in range(n):
            for j in range(0, n - i - 1):
                if result[j] > result[j + 1]:
      …

13 0 Open

System design patterns easy

How to Mock a Metrics Decorator in Python with unittest.mock

This code demonstrates a timing decorator that wraps a function to measure execution time and prints the duration, with a unit test using unittest.mock to patch the print function and assert it was called.

decorators unittest.mock metrics

Python

import time
from functools import wraps
from unittest.mock import patch

def add_metrics(func):
    @wraps(func)
    def wrapper(*args, **kwargs):
        start = time.perf_counter()
        result = func(*args, **kwargs)
        elapsed = time.perf_counter() - start
        print(f"{func.__name__} took {elapsed:.6f}s…

15 0 Open

System design patterns medium

How to Mock a Timeout per Dependency Call in Python

This code demonstrates how to simulate and test per-call timeouts for external dependencies using Python's unittest.mock and a simple timing wrapper.

mock timeout unittest

Python

```python
import time
from unittest.mock import Mock, patch

def call_dependency(dependency, timeout):
    start = time.time()
    result = dependency.call()
    elapsed = time.time() - start
    if elapsed > timeout:
        raise TimeoutError(f"Dependency call took {elapsed:.2f}s, exceeding timeout {timeout}s")
    …

14 0 Open

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