Overview
The RoundRobin class implements a round-robin load balancing strategy, which distributes incoming requests sequentially across available backend servers. This ensures even distribution of traffic and prevents any single backend from being overwhelmed.
How it works
The round-robin algorithm maintains an internal index that tracks which backend server should receive the next request. Each time a backend is selected:
- The current index is used to select a backend from the list (using modulo to wrap around)
- The index is incremented for the next selection
- When the index reaches the end of the list, it wraps back to the beginning
This creates a circular rotation through all available backends, ensuring equal distribution over time.
Constructor
const roundRobin = new RoundRobin();
0. No parameters are required.
Methods
pick
Selects the next backend server using the round-robin algorithm.
pick(backends: Backend[]): Backend
Array of backend servers to select from. Each backend must conform to the Backend interface.
Backend - The selected backend server
Throws: Error - If the backends array is empty (“No backend is available”)
Implementation logic
pick(backends: Backend[]): Backend {
if (backends.length === 0){
throw new Error(" No backend is available")
}
const backend = backends[this.index % backends.length]!;
this.index++;
return backend;
}
%) to ensure the index wraps around when it exceeds the array length, creating the circular behavior.
Usage examples
Basic usage
import { RoundRobin } from './balancer/roundRobin';
import type { Backend } from './types/types';
const balancer = new RoundRobin();
const backends: Backend[] = [
{ url: 'http://localhost:3001', health: true },
{ url: 'http://localhost:3002', health: true },
{ url: 'http://localhost:3003', health: true }
];
// First request goes to localhost:3001
const backend1 = balancer.pick(backends);
// Second request goes to localhost:3002
const backend2 = balancer.pick(backends);
// Third request goes to localhost:3003
const backend3 = balancer.pick(backends);
// Fourth request wraps back to localhost:3001
const backend4 = balancer.pick(backends);
Handling empty backend lists
import { RoundRobin } from './balancer/roundRobin';
const balancer = new RoundRobin();
const emptyBackends = [];
try {
const backend = balancer.pick(emptyBackends);
} catch (error) {
console.error(error.message); // " No backend is available"
}
Integration with health checks
import { RoundRobin } from './balancer/roundRobin';
import type { Backend } from './types/types';
const balancer = new RoundRobin();
const allBackends: Backend[] = [
{ url: 'http://localhost:3001', health: true },
{ url: 'http://localhost:3002', health: false }, // Unhealthy
{ url: 'http://localhost:3003', health: true }
];
// Filter to only healthy backends before picking
const healthyBackends = allBackends.filter(b => b.health);
const selectedBackend = balancer.pick(healthyBackends);