Week 1 — Client–Server, DNS, HTTP, TCP/UDP

Client–Server Model

“Client” and “server” are roles, not fixed machines. An Odoo instance simultaneously serves browsers (server role) while acting as a client to PostgreSQL.

Browser → Nginx (L7 LB) → Odoo (app server) → PostgreSQL
   client     server/client      server/client      server

DNS

How app.example.com resolves to an IP:

Browser → Recursive resolver → Root nameserver
       → TLD server (.com) → Authoritative nameserver
       → IP returned + cached for TTL duration

Key record types:

Record	Purpose
A	Domain → IPv4
AAAA	Domain → IPv6
CNAME	Alias → another domain
MX	Mail server
TXT	Verification, SPF

TTL controls how long resolvers cache the answer. Low TTL = fast propagation on change, more DNS queries.

HTTP/HTTPS

Request structure:

GET /api/v1/orders HTTP/1.1
Host: app.example.com
Authorization: Bearer <token>

Status codes that matter:

Code	Meaning
200	OK
201	Created
301/302	Redirect (permanent/temporary)
400	Bad request (client error)
401	Unauthenticated
403	Forbidden
404	Not found
429	Rate limited
500	Server error
503	Service unavailable

HTTPS = HTTP + TLS. The TLS handshake adds ~1 RTT latency but encrypts everything in transit.

TCP vs UDP

	TCP	UDP
Connection	3-way handshake	None
Delivery	Guaranteed, ordered	Best effort
Overhead	Higher	Lower
Use case	HTTP, databases, file transfer	DNS, video streaming, gaming

When to use UDP: When losing a packet is better than waiting for retransmission (e.g. live video — a dropped frame is better than a 2-second freeze).

Practice

Draw the full request journey for: browser opens app.yourcompany.com/web → sees the Odoo login page.

Label every hop: DNS resolution, TCP handshake, TLS, HTTP request, app server processing, DB query, response.

Self-check

• What happens if the DNS TTL is set to 0?
• Why can’t you just scale the app server horizontally without thinking about session state?
• What does a 502 Bad Gateway mean vs a 503 Service Unavailable?