Purpose: Define application rules and message formats.

Application-layer protocols

These protocols operate at the Application Layer (Layer 7) and define how applications exchange data.

* These are application-layer protocols, not transport protocols. * Most application protocols rely on TCP because they require reliable and ordered delivery. * DNS primarily uses UDP for low latency, but switches to TCP for large responses and zone transfers. * HTTPS is essentially:

HTTPS = HTTP + TLS + TCP

* WebSocket starts with an HTTP handshake, then upgrades to a persistent TCP connection.

Client
  ↓
Application Protocol (HTTP, MQTT, AMQP, RESP)
  ↓
Transport Protocol (TCP or UDP)
  ↓
IP
  ↓
Ethernet / Wi-Fi

HTTP/1.1 → TCP → IP → Ethernet

WebSocket → TCP → IP → Ethernet

DNS → UDP → IP → Ethernet

MQTT → TCP → IP → Ethernet

Kafka → TCP → IP → Ethernet

HTTPS → TLS → TCP → IP → Ethernet 

Examples of messages:

GET /orders HTTP/1.1

XREAD BLOCK 2000 STREAMS orders $

Fetch Request

Purpose: Format, serialize, compress, and encrypt data.

Examples:

* JSON * XML * Protobuf * Avro * UTF-8 * Gzip * TLS/SSL

Example:

HTTP + TLS = HTTPS

Purpose: Establish, maintain, and terminate sessions.

Examples:

* HTTP Keep-Alive * WebSocket Session * Session IDs * Authentication Sessions

Purpose: Deliver data between processes.

Purpose: Route packets between networks.

Protocols:

* IPv4 * IPv6 * ICMP

Example IP:

192.168.1.10

Devices:

* Router

Purpose: Transfer frames within a local network.

Protocols:

* Ethernet * Wi-Fi * ARP

Example MAC address:

AA:BB:CC:DD:EE:FF

Devices:

* Switch

Purpose: Transmit raw bits.

Examples:

* Network cables * Fiber optics * Radio waves * Electrical signals

010101010101

Protocol   = language
Connection = communication channel

Examples:

HTTP      + TCP
WebSocket + TCP
RESP      + TCP
AMQP      + TCP
Kafka     + TCP
HTTP/3    + QUIC + UDP

Worker → Database

Worker repeatedly asks:

SELECT * FROM jobs
WHERE STATUS = 'pending';

Worker → Redis

Worker sends:

XREAD BLOCK 2000

Redis waits up to 2 seconds, then replies.

Worker → Redis (SUBSCRIBE)
Redis → Worker (messages)

Redis pushes messages to subscribers.

Worker → RabbitMQ (basic.consume)
RabbitMQ → Worker (messages)

RabbitMQ pushes messages.

Worker → Kafka (Fetch Request)
Kafka → Worker (response)

Consumer repeatedly sends fetch requests.

Example:

XREAD BLOCK 2000 STREAMS orders $

Meaning:

* Read from stream orders * Wait up to 2000 ms if there is no data * Return immediately when a message arrives * Reuse the same TCP connection

Flow:

TCP Connection #1
├── XREAD BLOCK 2000
├── XREAD BLOCK 2000
├── XREAD BLOCK 2000
└── ...

Consumer sends:

Fetch messages from:
- topic: orders
- partition: 0
- offset: 123

Kafka:

* Waits for new data * Returns immediately if data exists * Returns empty response after timeout * Keeps using the same TCP connection

Flow:

TCP Connection #1
├── Fetch Request
├── Fetch Request
├── Fetch Request
└── ...

HTTP/1.1 → TCP
HTTP/2   → TCP
HTTP/3   → QUIC → UDP

* TCP provides reliable communication. * Application protocols define message meaning. * Redis Streams and Kafka use pull + long polling. * Redis Pub/Sub and RabbitMQ use push. * WebSocket provides bidirectional communication. * HTTP/2 is not WebSocket. * HTTP/3 uses QUIC over UDP.