What is the key difference between an L4 and an L7 load balancer?

L4 routes by IP and port only — it never reads HTTP content. L7 inspects headers, URL paths, and cookies, enabling path-based routing and cookie-based sticky sessions.

Why does round-robin fail for long-lived connections like file uploads?

Round-robin ignores how busy each server is. A server handling several slow uploads gets the same new connections as an idle server. Least-connections routes new requests to whichever server has the fewest open connections.

Why do WebSocket apps need sticky sessions?

A WebSocket connection is stateful on one server node. If a reconnect hits a different node, that node has no record of the client's state and the session breaks.

What is IP-hash balancing and when does it break?

IP hash always maps the same client IP to the same server. It breaks when you add or remove servers — the hash table shifts and all existing clients get re-routed, losing their session.

What does a health check do in a load balancer?

It sends a periodic probe (HTTP GET /health or TCP connect) to each backend. Failed probes remove the node from the pool; passing probes add it back — transparent to clients.

When would you choose an L4 balancer over an L7?

When you need raw TCP throughput with no parsing overhead, pass-through TLS (backend handles certificates), or are routing non-HTTP protocols such as database connections.

Load Balancing (for Frontends)

Summary

A load balancer distributes incoming requests across a pool of servers to prevent any single node from becoming a bottleneck. L4 balancers route by TCP/UDP without inspecting content; L7 balancers read HTTP headers, paths, and cookies to make smarter routing decisions. Algorithm choice and health checks determine how evenly load spreads.

Jump to the interview angle

A load balancer sits in front of a server pool and forwards each incoming connection or request to one of the available backends. It tracks server health, removes unhealthy nodes, and re-adds them when they recover. Without it, a single server is both a performance ceiling and a single point of failure.

L4 vs L7 load balancing

	Aspect	L4 (transport layer)
Routing basis	IP address + TCP/UDP port only	HTTP headers, URL path, cookies, host
Content inspection	None — opaque byte stream	Full HTTP body and headers visible
TLS termination	Pass-through (TLS reaches backend)	Terminates TLS; backends see plain HTTP
Sticky sessions	IP-hash only	Cookie-based or header-based affinity
Performance cost	Very low — no parsing	Higher — parses each request
Typical tools	AWS NLB, HAProxy TCP mode	AWS ALB, nginx, Caddy, Envoy

Routing algorithms

**Round-robin** — each request goes to the next server in a fixed cycle; simple, works when servers are equally capable.
**Weighted round-robin** — servers get a share of traffic proportional to their weight; use when instance sizes differ.
**Least-connections** — routes to the server with fewest open connections; beats round-robin for long-lived requests like uploads.
**IP hash** — hashes client IP to pin the same backend; breaks when the server pool changes.
**Power of two choices (P2C)** — picks two servers at random, routes to the less loaded; near-optimal with low overhead.

Health checks run on a timer; failed nodes leave the pool. A sticky cookie pins the client to Server 1 regardless of algorithm.

WebSockets and stateful apps need sticky sessions

A WebSocket connection is long-lived on one server node. If a reconnect lands on a different node, the server has no record of the client — the connection starts from scratch or fails. Use cookie-based affinity on an L7 balancer. The same applies to server-sent SSE streams and in-memory session stores.

Interview angle

Interviewers check whether you can pick L4 vs L7 by the routing signal you need, and whether you know why WebSockets break without sticky sessions. Name a concrete algorithm trade-off.

Soundbite: "L7 for path-based routing and cookie affinity; L4 when you need raw TCP throughput with no parsing cost."

Key terms

L4 load balancer: Routes by IP/port without inspecting HTTP content; low overhead, no TLS termination.
L7 load balancer: Inspects HTTP headers, paths, and cookies to route requests; terminates TLS.
Sticky session: Affinity rule that pins a client to the same backend node across requests.
Health check: Periodic probe (HTTP GET or TCP connect) that removes unhealthy backends from the pool.
Least-connections: Algorithm routing each new request to the backend with the fewest open connections.

Summary

L4 vs L7 load balancing

Routing algorithms

Interview angle

Key terms

Related topics

Further reading

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