Glossary

Every term here is explained in a few plain sentences. In articles, the first mention of a term shows this definition when you point at it or tap it.

API (Application Programming Interface)

A contract that lets one program use another program. An API says which requests a program accepts and what it returns. In system design, it usually means the network endpoints that clients and other services call.

Read more: Networking: Adding servers

HTTP (Hypertext Transfer Protocol)

The request-response protocol of the web. A client sends a request with a method (GET, POST, …), a path, and headers. The server sends back a status code, headers, and a body. Most public APIs run on HTTP.

HTTP/2

A newer version of HTTP that sends many requests over one connection at the same time. HTTP/2 splits requests into streams on one TCP connection and compresses headers. A lost TCP packet still stops all streams until it is resent.

HTTP/3

The version of HTTP that runs on QUIC instead of TCP. HTTP/3 keeps the many-streams model of HTTP/2, but a lost packet holds up only the streams whose data it carried. It also sets up connections faster.

HTTPS (HTTP Secure)

HTTP sent over an encrypted TLS connection. HTTPS is the same HTTP, but TLS encrypts it and proves the server is who it claims to be. Today almost all web traffic uses HTTPS.

Idempotency key

A unique ID the client attaches to a request, so the server can recognize a retry of it. If the same key comes again, the server returns the saved result instead of doing the work twice. Used for payments and orders.

IP address (Internet Protocol address)

The number that identifies a device on a network. IP, the Internet Protocol, delivers packets to these addresses with no guarantees; TCP and UDP run on top of it. DNS maps names to IP addresses.

Jitter

A random extra wait added before a retry. Without jitter, many clients retry at the same moment and overload the service again.

Keep-alive

Keeping a connection open after a response, so the next request can reuse it. Reusing a connection skips the TCP and TLS handshakes. Clients and servers close idle connections after a timeout.

L4 / L7

Network layers: L4 is the transport layer (TCP, UDP), L7 is the application layer (HTTP). An L4 load balancer routes connections. An L7 load balancer reads each HTTP request and can route by path or header.

Read more: Networking: Adding servers

Least connections

Sending each new connection to the server with the fewest open connections. Good for long-lived connections, where the number of open connections shows the load better than the number of requests.

Load balancer

A component that spreads incoming traffic across several servers. It also stops sending traffic to servers that fail health checks, so clients see one stable address.

Read more: Networking: Adding servers

Long polling

The server holds a request open until it has new data or a timeout, then the client asks again. Long polling gives near real-time updates over plain HTTP, without new infrastructure.

mTLS (Mutual TLS)

TLS in which both sides show a certificate. Each side proves who it is to the other, so only known services can connect. It is common for traffic between internal services.

OSI model (Open Systems Interconnection model)

A model that divides network communication into seven layers. Each layer describes a different part of communication. The layer numbers help distinguish addressing (L3), transport (L4), and application messages (L7) when discussing protocols and load balancers.

Read more: Networking: Adding servers

Polling

The client asks the server for new data again and again on a timer. Polling is simple and works everywhere. The cost is delay (up to one interval) and many empty requests.

Port

A number that identifies a program on a machine, so one IP address can serve many programs. A connection goes to an IP address and a port, for example port 443 for HTTPS. L4 load balancers route traffic by IP address and port.

QUIC

A transport protocol on top of UDP that gives reliable streams with built-in encryption. QUIC does what TCP and TLS do together, but each stream recovers from loss on its own. HTTP/3 runs on QUIC.