Load Balancer Types: L4 vs L7

L4 vs L7 is the foundational LB choice. Pick wrong and you pay forever in operational complexity.

What each does

L4 (TCP/UDP): forwards based on IP/port; cheap; fast.

L7 (HTTP): inspects headers, paths, cookies; routes intelligently.

When L4 wins

• 1. Sub-ms latency required.
• 2. Non-HTTP protocols.
• 3. Pure throughput.
• 4. SSL passthrough preferred.

When L7 wins

Most modern web traffic. Header-based routing. Path-based routing. SSL termination. Rate limiting.

HTTP-aware features only available at L7.

Hybrid posture

Many orgs: L4 at the edge; L7 inside (ingress controllers, service mesh).

Each layer does what it does best.

Antipatterns

• L7 for non-HTTP. Wrong layer; loses features.
• L4 with HTTP-aware needs. Re-implement at app layer.
• Multiple LB layers without thinking. Latency stacks.

What to do this week

Three moves. (1) Apply this pattern to your highest-risk network path. (2) Measure the failure mode rate before/after. (3) Document the change so the next incident-responder inherits the knowledge.