Which simoom features do I need, and how big is the build?

Q: Does full include compression and WebSocket over HTTP/2?

No. full is every protocol plus TLS; compression, websocket-h2, websocket-h3, ws-deflate, h2-keepalive and the helpers are added by name.

Q: Can I have HTTP/2 without HTTP/1.1?

Yes: default-features = false, features = ["http2"] serves h2c only, or add tls for h2 over TLS. Most clients expect HTTP/1.1 as well, so this is for internal traffic.

Every protocol is a cargo feature: defaults give HTTP/1.1, HTTP/2 and WebSocket, full adds TLS and HTTP/3; HTTP/1.1 alone adds about 215 KB.

simoom only compiles what you ask for. Each build has a size budget, and the project's size script (scripts/size.sh) fails when a build goes over it.

Common choices

toml

# the default: HTTP/1.1, HTTP/2 (h2c), WebSocket, io_uring, watchdog, guards
simoom = "0.1"

# everything: adds TLS 1.3, HTTP/3 over QUIC and the TLS client
simoom = { version = "0.1", features = ["full"] }

# the smallest: HTTP/1.1 only, for a device or a serverless function
simoom = { version = "0.1", default-features = false, features = ["http1"] }

# HTTPS without HTTP/3
simoom = { version = "0.1", features = ["tls"] }

Every feature

feature	adds
`http1` (default)	HTTP/1.1: keep-alive, pipelining, chunked bodies, `100-continue`
`http2` (default)	HTTP/2: h2c with prior knowledge, `Upgrade: h2c`, h2 over TLS
`websocket` (default), alias `ws`	WebSocket on HTTP/1.1
`io-uring` (default)	the Linux io_uring back end; without it `Backend::IoUring` falls back to epoll
`watchdog` (default)	moves handlers that block the loop thread to the offload pool
`guard` (default)	overload and slow-client guards: backpressure admission, slow heads, per-IP reconnect rate
`idle-trim` (default)	an idle loop frees cached buffers and returns memory to the OS after bursts
`tls`	TLS 1.3 listeners: X25519, P-256, RSA, AES-GCM
`tls-extra`	P-384 and Ed25519 certificates, ChaCha20-Poly1305
`http3`	HTTP/3 over QUIC (turns on `tls`)
`tls-client`	the TLS and QUIC client side (used by tests; a server doesn't need it)
`full`	`http1`, `http2`, `http3`, `tls`, `tls-extra`, `tls-client`, `websocket` and the default runtime features
`compression`, `brotli`, `zstd`	response compression and request body decoding
`websocket-h2`, `websocket-h3`	WebSocket over HTTP/2 and HTTP/3
`ws-deflate`	WebSocket permessage-deflate
`h2-keepalive`	HTTP/2 PING keepalive
`multipart`, `cookies`, `conditional`, `cors`, `auth`	helpers for forms, cookies, caching and ranges, CORS, and Basic, Digest and Bearer auth

full doesn't include the last six rows, which keeps it within its size budget. Add them by name: features = ["full", "compression", "websocket-h2"].

What each build adds

Bytes added to the bundled hello example over an empty fn main, stripped, on x86-64 Linux with the dist profile:

build	size	budget
HTTP/1.1 only (`--no-default-features --features http1`)	214,568 B	256,000 B
HTTP/1.1, HTTP/2, TLS, io_uring	583,680 B	614,400 B
everything: HTTP/1.1, HTTP/2, HTTP/3, TLS, WebSocket, io_uring	935,064 B	1,048,576 B

As whole programs in benchmark campaign 4 (2026-11-12), simoom's default build (HTTP/1.1, HTTP/2, WebSocket) was 735 KB stripped and the full server 1,550 KB, against 2,069 KB for hyper and 515 KB for may-minihttp (benchmarks). The measurement script is scripts/size.sh in the repository, and the dates of each figure are in the README.

Building small

For the smallest binary, combine the minimal features with a size-oriented release profile in your own Cargo.toml:

toml

[profile.release]
opt-level = "s"
lto = true
codegen-units = 1
panic = "abort"
strip = true

With panic = "abort", a panic in a handler ends the process instead of becoming a 500, so choose it only where a restart is acceptable.

An edge build without watchdog or guard saves their code, and the related limits are then ignored: a blocking handler stays inline, and slow request heads close at head_timeout.

Cross-compiling

simoom has no C code and no build-time dependencies, so cross-compiling needs only the Rust target (rustup target add aarch64-unknown-linux-gnu) and a linker for it. TLS needs nothing extra. The ARM SIMD and crypto paths compile but have not been benchmarked yet.

Common questions

What is the smallest Rust HTTP server for AWS Lambda or other serverless platforms?

An HTTP/1.1-only simoom build adds about 215 KB. In benchmark campaign 4 (2026-11-12), the default build gave its first answer 0.94 ms after the process started, the fastest of 21 servers, and used 901 KB of memory at rest (benchmarks). Use default-features = false, features = ["http1"].

Does `full` include compression and WebSocket over HTTP/2?

No. full is every protocol plus TLS; compression, websocket-h2, websocket-h3, ws-deflate, h2-keepalive and the helpers are added by name.

Can I have HTTP/2 without HTTP/1.1?

Yes: default-features = false, features = ["http2"] serves h2c only, or add tls for h2 over TLS. Most clients expect HTTP/1.1 as well, so this is for internal traffic.

Does simoom work with `no_std`?

No. simoom is std-only, which is different: it uses the Rust standard library and nothing else, so it needs an operating system (Linux, Windows or macOS).