Art of Codeartofcode
Lesson 3 of 17

7. Load Balancers, Reverse Proxies and API Gateways

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The picture to keep in your head
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7.1 Why do we need a load balancer?
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7.2 What's the difference between a layer 4 and a layer 7 load balancer?
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7.3 How does the load balancer choose a server?
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7.4 How does the load balancer know a server is dead?
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7.5 What are sticky sessions, and should you use them?
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7.6 What is SSL termination?
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7.7 If everything goes through the load balancer, what if it fails?
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7.8 What's the difference between a forward proxy and a reverse proxy?
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7.9 What is an API gateway?
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7.10 How do you balance traffic across regions?
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Memory card and where the analogy breaks
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Practice: five levels
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Interview questions
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Solutions
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Sources
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Code lab
7. Load Balancers, Reverse Proxies and API Gateways

7.2 What's the difference between a layer 4 and a layer 7 load balancer?

3 min read+10 XP

At the airport: One kind of queue manager only looks at your boarding pass number and sends you to the next free counter. It's very fast. Another kind reads your ticket: "business class? counter 1. Extra baggage? counter 9." It's slower, but much smarter.

In the system: The "layers" come from the networking model: layer 4 is the transport (TCP, UDP), layer 7 is the application (HTTP).

layer 4 sees addresses and ports; layer 7 sees the HTTP request.

Figure: layer 4 reads the envelope; layer 7 opens it and reads the letter.

  • A layer 4 (L4) load balancer sees only IP addresses and ports. It forwards TCP or UDP connections without looking inside. Very fast and works for any protocol (databases, game servers, MQTT), but it can't route on the URL, headers or cookies.
  • A layer 7 (L7) load balancer understands HTTP. It can see the method, path, host, headers and cookies, so it can:
    • send /api/* to API servers and /images/* to image servers;
    • send shop.example.com and blog.example.com to different pools;
    • terminate TLS (7.6), add headers, retry a failed request on another server, and make sessions sticky with a cookie (7.5).

The cost of L7 is more work per request, because it must decrypt and parse HTTP. Many large systems use both: a fast L4 layer at the edge, then L7 proxies behind it.

Memory code: "L4 reads the envelope, L7 reads the letter."