How to design a load balancer from scratch?

Designing a load balancer from scratch involves understanding the core concepts of load balancing and implementing the necessary components to distribute traffic efficiently across multiple servers. Here’s a step-by-step guide to help you design a basic load balancer:

Key Concepts

1. Load Balancing Algorithms: Determines how incoming requests are distributed.

   • Round Robin: Distributes requests sequentially.
   • Least Connections: Sends requests to the server with the fewest active connections.
   • IP Hash: Uses the client’s IP address to determine which server receives the request.
   • Weighted Round Robin: Each server is assigned a weight, and requests are distributed based on these weights.

2. Health Checks: Regularly check the status of each server to ensure it's capable of handling requests.

3. Session Persistence (Sticky Sessions): Ensures requests from the same client are always sent to the same server.

4. Failover: Ensures requests are rerouted to healthy servers if a server fails.

5. Scalability: Ability to handle increasing traffic by adding more servers.

Step-by-Step Implementation

1. Define the Server Pool

First, define a pool of servers that the load balancer will distribute traffic to.

class Server:
    def __init__(self, ip, weight=1):
        self.ip = ip
        self.weight = weight
        self.active_connections = 0

servers = [
    Server("192.168.1.1", weight=1),
    Server("192.168.1.2", weight=1),
    Server("192.168.1.3", weight=2)
]

2. Implement Load Balancing Algorithms

Implement a simple round-robin algorithm to distribute requests.

class LoadBalancer:
    def __init__(self, servers):
        self.servers = servers
        self.current_index = 0

    def get_next_server(self):
        server = self.servers[self.current_index]
        self.current_index = (self.current_index + 1) % len(self.servers)
        return server

load_balancer = LoadBalancer(servers)

# Example request handling
for _ in range(6):
    server = load_balancer.get_next_server()
    print(f"Forwarding request to {server.ip}")

Implement the least connections algorithm.

class LeastConnectionsLoadBalancer:
    def __init__(self, servers):
        self.servers = servers

    def get_next_server(self):
        # Get the server with the least active connections
        server = min(self.servers, key=lambda s: s.active_connections)
        server.active_connections += 1
        return server

load_balancer = LeastConnectionsLoadBalancer(servers)

# Example request handling
for _ in range(6):
    server = load_balancer.get_next_server()
    print(f"Forwarding request to {server.ip}")
    server.active_connections -= 1  # Simulate the request being processed

3. Implement Health Checks

Regularly check if servers are healthy and can handle requests.

import requests

def is_server_healthy(server):
    try:
        response = requests.get(f"http://{server.ip}/health")
        return response.status_code == 200
    except requests.RequestException:
        return False

class HealthCheckLoadBalancer:
    def __init__(self, servers):
        self.servers = servers

    def get_next_server(self):
        healthy_servers = [s for s in self.servers if is_server_healthy(s)]
        if not healthy_servers:
            raise Exception("No healthy servers available")
        return min(healthy_servers, key=lambda s: s.active_connections)

load_balancer = HealthCheckLoadBalancer(servers)

# Example request handling
for _ in range(6):
    try:
        server = load_balancer.get_next_server()
        print(f"Forwarding request to {server.ip}")
    except Exception as e:
        print(e)

4. Implement Session Persistence (Sticky Sessions)

Ensure requests from the same client are always sent to the same server.

class StickySessionLoadBalancer:
    def __init__(self, servers):
        self.servers = servers
        self.session_map = {}

    def get_next_server(self, client_id):
        if client_id in self.session_map:
            return self.session_map[client_id]
        server = min(self.servers, key=lambda s: s.active_connections)
        self.session_map[client_id] = server
        return server

load_balancer = StickySessionLoadBalancer(servers)

# Example request handling
client_id = "client1"
for _ in range(6):
    server = load_balancer.get_next_server(client_id)
    print(f"Forwarding request from {client_id} to {server.ip}")

5. Implement Failover

Ensure requests are rerouted to healthy servers if a server fails.

class FailoverLoadBalancer:
    def __init__(self, servers):
        self.servers = servers

    def get_next_server(self):
        for server in self.servers:
            if is_server_healthy(server):
                return server
        raise Exception("No healthy servers available")

load_balancer = FailoverLoadBalancer(servers)

# Example request handling
for _ in range(6):
    try:
        server = load_balancer.get_next_server()
        print(f"Forwarding request to {server.ip}")
    except Exception as e:
        print(e)

Conclusion

Designing a load balancer from scratch involves implementing key functionalities like load balancing algorithms, health checks, session persistence, and failover mechanisms. The provided examples illustrate basic implementations of these concepts. In a production environment, you would need to add more sophisticated features, optimize performance, and ensure security. This foundational understanding will help you build and scale load balancers effectively.