nginx Event Loop — Complete Lifecycle Reference
nginx Event Loop — Complete Lifecycle Reference A precise, bottom-up reference covering every buffer, syscall, interrupt, and data movement from the moment a TCP packet hits the NIC to the moment a response is sent back. Two concurrent users are used throughout as a concrete example. Table of Contents Foundations — fd and Socket Hardware Layer — NIC, DMA, Interrupts Kernel Structures and All Buffers epoll — How the Worker Waits Efficiently nginx Startup Sequence Complete Request Lifecycle — Two Concurrent Users What Happens While Worker is Busy All Buffers — Master Reference All Syscalls — Master Reference Failure Modes 1. Foundations 1.1 Everything is a File Linux's core philosophy: every I/O resource — files on disk, network connections, pipes, terminals, devices — is represented as a file. This means one unified API ( read , write , close ) works on all of them. The kernel manages the actual resource. Your process holds a token. 1.2 File Descriptor (fd) A file descriptor is just an integer . It is a per-process token that refers to a kernel-managed resource. The kernel maintains a table per process called the fd table — a simple array where the index is the fd and the value is a pointer into the kernel. Process fd table: ┌─────┬───────────────────────────────┐ │ fd │ points to │ ├─────┼───────────────────────────────┤ │ 0 │ stdin │ │ 1 │ stdout │ │ 2 │ stderr │ │ 3 │ listen socket (nginx) │ │ 5 │ User A client connection │ │ 6 │ User B client connection │ │ 12 │ backend connection for User A │ │ 13 │ backend connection for User B │ └─────┴───────────────────────────────┘ 0, 1, 2 are always pre-assigned. Application fds start from 3 upward. The fd is meaningless on its own. It only means something when passed to a syscall — the kernel uses it to look up the real resource. 1.3 Socket A socket is the kernel's internal data structure representing one end of a network connection. Created when your process calls socket() . Lives entirely in kernel RAM. Your process nev