create the FrontendApplication

src/main/java/com/distributed/search/FrontendApplication.java

+package com.distributed.search;
+
+import com.distributed.search.cluster.LeaderHttpRegistry;
+import com.sun.net.httpserver.HttpExchange;
+import com.sun.net.httpserver.HttpServer;
+import org.apache.zookeeper.WatchedEvent;
+import org.apache.zookeeper.Watcher;
+import org.apache.zookeeper.ZooKeeper;
+
+import java.io.IOException;
+import java.io.OutputStream;
+import java.net.InetSocketAddress;
+import java.net.URI;
+import java.net.URISyntaxException;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.concurrent.Executors;
+
+public class FrontendApplication implements Watcher {
+    private static final String ZOOKEEPER_ADDRESS = "192.168.39.250:2181"; // Use your ZK IP
+    private static final int PORT = 8080; // Main Web Port
+    private final LeaderHttpRegistry leaderHttpRegistry;
+    private final HttpClient httpClient;
+
+    public FrontendApplication(ZooKeeper zooKeeper) {
+        this.leaderHttpRegistry = new LeaderHttpRegistry(zooKeeper);
+        this.httpClient = HttpClient.newBuilder().build();
+    }
+
+    public static void main(String[] args) throws Exception {
+        ZooKeeper zooKeeper = new ZooKeeper(ZOOKEEPER_ADDRESS, 3000, event -> {});
+        FrontendApplication app = new FrontendApplication(zooKeeper);
+        app.startServer();
+    }
+
+    public void startServer() throws IOException {
+        HttpServer server = HttpServer.create(new InetSocketAddress(PORT), 0);
+
+        // Context 1: Serve the HTML UI
+        server.createContext("/", this::handleHomeRequest);
+
+        // Context 2: API Proxy to Leader
+        server.createContext("/api/search", this::handleSearchApiRequest);
+
+        server.setExecutor(Executors.newFixedThreadPool(8));
+        server.start();
+        System.out.println("Frontend Server is running at http://localhost:" + PORT);
+    }
+
+    // Serves the index.html file to the browser
+    private void handleHomeRequest(HttpExchange exchange) throws IOException {
+        byte[] response = Files.readAllBytes(Paths.get("src/main/resources/index.html"));
+        exchange.getResponseHeaders().set("Content-Type", "text/html");
+        sendResponse(response, exchange);
+    }
+
+    // Receives query from browser -> Finds Leader -> Fetches results from Leader -> Returns to browser
+    private void handleSearchApiRequest(HttpExchange exchange) throws IOException {
+        // 1. Only support GET requests
+        if (!"GET".equalsIgnoreCase(exchange.getRequestMethod())) {
+            exchange.sendResponseHeaders(405, -1);
+            return;
+        }
+
+        // 2. Get the RAW query from browser (e.g., query=blue%20car)
+        String query = exchange.getRequestURI().getRawQuery();
+        String leaderUrl = leaderHttpRegistry.getLeaderAddress();
+
+        if (leaderUrl == null) {
+            sendPlainTextResponse("Leader not found in registry", exchange, 502);
+            return;
+        }
+
+        try {
+            // 3. Robust URL Construction
+            // Ensure leaderUrl has protocol and the correct endpoint
+            if (!leaderUrl.startsWith("http")) leaderUrl = "http://" + leaderUrl;
+            if (!leaderUrl.contains("/search")) leaderUrl = leaderUrl.replaceAll("/$", "") + "/search";
+
+            // Create the final URI carefully
+            // concatenation of (base) + (?) + (already encoded query)
+            URI uri = URI.create(leaderUrl + "?" + query);
+            System.out.println("Frontend Proxying to Leader: " + uri);
+
+            // 4. Prepare the HTTP Request to the Leader
+            HttpRequest request = HttpRequest.newBuilder()
+                    .uri(uri)
+                    .GET()
+                    .timeout(java.time.Duration.ofSeconds(30))
+                    .build();
+
+            // 5. Send request and get response as InputStream (Efficiency)
+            HttpResponse<java.io.InputStream> resp = httpClient.send(request, HttpResponse.BodyHandlers.ofInputStream());
+
+            int statusCode = resp.statusCode();
+            exchange.getResponseHeaders().set("Content-Type", "application/json; charset=utf-8");
+
+            if (statusCode != 200) {
+                byte[] errorBytes = resp.body().readAllBytes();
+                sendPlainTextResponse(new String(errorBytes), exchange, statusCode);
+                return;
+            }
+
+            // 6. Stream the results directly from Leader to Browser
+            try (java.io.InputStream is = resp.body();
+                 OutputStream os = exchange.getResponseBody()) {
+
+                // Send headers first (200 OK, 0 means chunked/unknown length)
+                exchange.sendResponseHeaders(200, 0);
+
+                is.transferTo(os); // Copy data directly
+                os.flush();
+            }
+
+        } catch (IllegalArgumentException e) {
+            System.err.println("Invalid Query format: " + e.getMessage());
+            sendPlainTextResponse("Invalid search format", exchange, 400);
+        } catch (Exception e) {
+            e.printStackTrace();
+            sendPlainTextResponse("Error communicating with Leader: " + e.getMessage(), exchange, 502);
+        }
+    }
+
+    private void sendPlainTextResponse(String text, HttpExchange exchange, int status) throws IOException {
+        byte[] bytes = text.getBytes(java.nio.charset.StandardCharsets.UTF_8);
+        exchange.getResponseHeaders().set("Content-Type", "text/plain; charset=utf-8");
+        exchange.sendResponseHeaders(status, bytes.length);
+        try (OutputStream os = exchange.getResponseBody()) {
+            os.write(bytes);
+        }
+    }
+
+    private void sendResponse(byte[] bytes, HttpExchange exchange) throws IOException {
+        exchange.sendResponseHeaders(200, bytes.length);
+        try (OutputStream os = exchange.getResponseBody()) {
+            os.write(bytes);
+        }
+    }
+
+    @Override
+    public void process(WatchedEvent event) {}
+}
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