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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 a Weighted Random Load Balancer in Python

A Python load balancer mock that distributes requests across servers based on configurable weights using a cumulative weighted random selection algorithm.

python how build

Python

import random
from collections import Counter

SERVERS = {
    "server-a": 50,
    "server-b": 30,
    "server-c": 20,
}


def weighted_random_server(servers: dict[str, int]) -> str:
    """Select a server based on its weight (higher weight = more likely)."""
    total_weight = sum(servers.values())
    rand = random.…

13 0 Open

System design patterns easy

Observer Pattern with Mock Metrics in Python

Implement the Observer pattern with a mock metrics collector to track state changes and verify notifications.

observer mock design pattern

Python

import unittest
from unittest.mock import Mock


class Subject:
    def __init__(self):
        self._state = 0
        self._observers = []

    def attach(self, observer):
        self._observers.append(observer)

    def set_state(self, value):
        if value != self._state:
            self._state = value
      …

12 0 Open

System design patterns easy

Round Robin Load Balancer in Python

This code simulates round robin load balancing by distributing a list of requests evenly across a list of servers.

load-balancing round-robin system-design

Python

def round_robin_servers(requests: list[str], servers: list[str]) -> dict[str, list[str]]:
    assignments = {server: [] for server in servers}
    for idx, request in enumerate(requests):
        server = servers[idx % len(servers)]
        assignments[server].append(request)
    return assignments


if __name__ == "_…

13 0 Open

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