Handle sessions and client identity
Understand sticky sessions, stateless services, and forwarded client addresses.
- Explain the availability trade-offs of affinity and how proxies preserve client metadata.
Sticky sessions route a client back to the same backend, often using a cookie or source address. This can support in-memory session state but makes traffic less even and creates trouble when that backend fails. A more flexible approach stores session state in a shared durable service or uses signed client tokens. Proxies also need a trusted way to communicate the original client address, such as a configured forwarded header or the PROXY protocol.
clients = {"alice": "app-a", "bob": "app-b"}
for client, backend in clients.items():
print(f"{client} -> {backend}")alice -> app-a bob -> app-b
Forwarded headers are only trustworthy when inserted or sanitized by a trusted proxy. Applications should not blindly accept client-supplied values for access control, rate limits, or audit identity.
Key takeaways
Affinity can help legacy stateful apps but reduces flexibility.
Shared or token-based state makes backends easier to replace.
Trust forwarded client metadata only through a controlled proxy chain.
Lesson quiz
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