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.

Easy Python 3.9+ Aug 9, 2026 System design patterns 13 views 0 copies

load-balancing round-robin system-design algorithms

Python code

14 lines

Python 3.9+

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__ == "__main__":
    servers = ["server-a", "server-b", "server-c"]
    requests = ["req1", "req2", "req3", "req4", "req5", "req6", "req7"]
    result = round_robin_servers(requests, servers)
    for server, assigned in result.items():
        print(f"{server}: {assigned}")

Output

stdout

server-a: ['req1', 'req4', 'req7']
server-b: ['req2', 'req5']
server-c: ['req3', 'req6']

How it works

The round_robin_servers function uses the modulo operator (%) to cycle through the server list as it iterates over requests. For each request index, idx % len(servers) gives the next server in rotation, ensuring an even distribution. The function builds a dictionary where each server maps to a list of its assigned requests. This pattern is a simple yet effective way to understand core load balancing logic without external dependencies.

Common mistakes

Forgetting to handle an empty servers list, which would cause a ZeroDivisionError
Assuming the function mutates the input lists instead of returning a new dictionary
Not preserving the order of requests when grouping by server
Hard-coding the server count instead of computing it with len(servers)

Variations

Use `itertools.cycle` and `zip` to pair requests with servers in a more functional style
Weighted round robin where each server gets a different number of requests based on capacity

Real-world use cases

Distributing inbound HTTP requests to a pool of backend instances for even load
Assigning tasks to worker processes in a queue-consumer system
Balancing database read replicas based on query volume

Round Robin Load Balancer in Python

Python code

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How it works

Common mistakes

Variations

Real-world use cases

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