Use http.createServer((req, res) => {}) to handle incoming HTTP connections, set response headers with res.setHeader(), and end the response using res.end().

HTTP Keep-Alive reuses established TCP sockets for multiple HTTP request/response cycles, eliminating repeated TCP three-way handshake and TLS negotiation latencies. In Node.js, enable socket pooling by passing keepAlive: true and configuring maxSockets and maxFreeSockets on http.Agent or https.Agent instances.

In HTTP/1.1 with persistent connections (keep-alive), requests over a single TCP connection must be served sequentially (head-of-line blocking). HTTP/2 uses binary framing to split messages into independent frames transmitted over a single TCP socket concurrently (multiplexing). The Node.js http2 core module supports stream prioritization, server push, and header compression (HPACK), allowing hundreds of concurrent requests without opening multiple parallel TCP connections.

dns.lookup uses the underlying operating system getaddrinfo(3) function synchronously on Libuv thread pool workers, whereas dns.resolve makes network calls asynchronously without thread pool workers.

Node.js core http.request() resolves DNS hostnames on every single request using dns.lookup(), which executes system calls on the Libuv thread pool without caching results. Under heavy HTTP traffic, this can stall worker threads and cause socket bottlenecks. Developers mitigate this by using custom HTTPS Agents with lookup caching packages (e.g., dnscache) or keep-alive sockets.

WebSockets provide persistent, full-duplex TCP connections for real-time bidirectional communication. Use them for real-time features like chat applications or live data feeds.

Server-Sent Events (SSE) provide unidirectional real-time text streaming from server to client over standard HTTP using the text/event-stream content type. Unlike WebSockets, which are full-duplex TCP connections, SSE runs over standard HTTP, includes native automatic client reconnection, and works seamlessly with standard HTTP proxies and load balancers.

WebSockets require initial HTTP upgrades that transition into persistent TCP sockets. In a clustered Node.js environment, if handshake requests land on worker A but subsequent frames land on worker B, connection state breaks. Sticky sessions ensure that all requests from a single client IP or session cookie are routed consistently to the exact same worker process.