Load balancers, proxies and gateways

const BACKENDS = ["api-1", "api-2", "api-3"];

// Three clients, each holding one long-lived HTTP/2 or gRPC connection that carries many requests.
const CONNECTIONS: [string, number][] = [["mobile-gateway", 60], ["web-tier", 30], ["batch-job", 10]];

// An L4 balancer picks a backend per connection, then forwards bytes; it never sees the requests inside.
function l4RoundRobin(): Map<string, number> {
  const load = new Map(BACKENDS.map((b) => [b, 0]));
  CONNECTIONS.forEach(([, requests], n) => {
    const b = BACKENDS[n % BACKENDS.length];
    load.set(b, load.get(b)! + requests);
  });
  return load;
}

// An L7 proxy ends the connection, reads each request and picks a backend per request.
function l7RoundRobin(): Map<string, number> {
  const load = new Map(BACKENDS.map((b) => [b, 0]));
  let n = 0;
  for (const [, requests] of CONNECTIONS) {
    for (let i = 0; i < requests; i++) {
      const b = BACKENDS[n % BACKENDS.length];
      load.set(b, load.get(b)! + 1);
      n++;
    }
  }
  return load;
}

const ROUTES: [string, string][] = [["/api/orders", "orders"], ["/api/", "api"], ["/static/", "cdn"]];

// L7 routing: the longest matching path prefix wins, as in NGINX locations, Envoy routes or a gateway.
function route(path: string): string {
  let best: [string, string] | null = null;
  for (const r of ROUTES) if (path.startsWith(r[0]) && (!best || r[0].length > best[0].length)) best = r;
  return best ? best[1] : "404";
}

function show(name: string, load: Map<string, number>): void {
  console.log(`${name}: ` + BACKENDS.map((b) => `${b}=${load.get(b)}`).join(", "));
}

show("L4, per connection", l4RoundRobin());
show("L7, per request", l7RoundRobin());
for (const path of ["/api/orders/81", "/api/users/7", "/static/logo.svg", "/admin"]) {
  console.log(`route ${path} -> ${route(path)}`);
}

import java.util.*;

public class Main {
    static final List<String> BACKENDS = List.of("api-1", "api-2", "api-3");

    // Three clients, each holding one long-lived HTTP/2 or gRPC connection that carries many requests.
    static final int[] REQUESTS_PER_CONNECTION = {60, 30, 10}; // mobile-gateway, web-tier, batch-job

    // An L4 balancer picks a backend per connection, then forwards bytes; it never sees the requests inside.
    static Map<String, Integer> l4RoundRobin() {
        Map<String, Integer> load = new LinkedHashMap<>();
        BACKENDS.forEach(b -> load.put(b, 0));
        for (int n = 0; n < REQUESTS_PER_CONNECTION.length; n++) {
            load.merge(BACKENDS.get(n % BACKENDS.size()), REQUESTS_PER_CONNECTION[n], Integer::sum);
        }
        return load;
    }

    // An L7 proxy ends the connection, reads each request and picks a backend per request.
    static Map<String, Integer> l7RoundRobin() {
        Map<String, Integer> load = new LinkedHashMap<>();
        BACKENDS.forEach(b -> load.put(b, 0));
        int n = 0;
        for (int requests : REQUESTS_PER_CONNECTION) {
            for (int i = 0; i < requests; i++) load.merge(BACKENDS.get(n++ % BACKENDS.size()), 1, Integer::sum);
        }
        return load;
    }

    static final String[][] ROUTES = {{"/api/orders", "orders"}, {"/api/", "api"}, {"/static/", "cdn"}};

    // L7 routing: the longest matching path prefix wins, as in NGINX locations, Envoy routes or a gateway.
    static String route(String path) {
        String[] best = null;
        for (String[] r : ROUTES) if (path.startsWith(r[0]) && (best == null || r[0].length() > best[0].length())) best = r;
        return best == null ? "404" : best[1];
    }

    static void show(String name, Map<String, Integer> load) {
        StringJoiner j = new StringJoiner(", ");
        load.forEach((b, v) -> j.add(b + "=" + v));
        System.out.println(name + ": " + j);
    }

    public static void main(String[] args) {
        show("L4, per connection", l4RoundRobin());
        show("L7, per request", l7RoundRobin());
        for (String path : List.of("/api/orders/81", "/api/users/7", "/static/logo.svg", "/admin")) {
            System.out.println("route " + path + " -> " + route(path));
        }
    }
}