fastsse

fastsse is a Server-Sent Events codec with a Rust core and thin Node.js / browser bindings.

Goals

• Exact line-oriented SSE parsing, including BOM stripping, CRLF/CR/LF handling, id persistence, and retry control frames.
• Low steady-state allocation pressure in the Rust core.
• Thin target adapters for napi-rs and wasm-bindgen.

Specification

• WHATWG HTML, parsing an event stream: https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream
• WHATWG HTML, interpreting an event stream: https://html.spec.whatwg.org/multipage/server-sent-events.html#interpreting-an-event-stream

Workspace layout

• crates/fastsse: core Rust encoder/decoder.
• crates/fastsse_node: N-API Rust crate for Node.js bindings.
• crates/fastsse_wasm: browser-facing wasm-bindgen wrapper.
• npm/fastsse_node: npm package for the Node.js addon.
• npm/fastsse_wasm: generated browser package output from wasm-pack build.

Development

The repository follows the latest start stack direction: Nix for the dev shell, Node.js 24, and vp as the shared task entrypoint.

nix develop
vp install
vp run fmt
vp run check
vp run test

Release

See RELEASE.md for the full release checklist and versioning contract.

vp run release:patch
vp run release:minor
vp run release:alpha
vp run release:beta

Use -- --dry-run --no-tag to preview a version bump without writing files or creating a tag.

Rust usage

use fastsse::{Decoder, EncodeEvent, encode_event};

let bytes = encode_event(&EncodeEvent {
  event: Some("update"),
  data: "alpha\nbeta",
  id: Some("evt-1"),
  retry: Some(1_500),
})?;

let mut decoder = Decoder::new();
decoder.feed(&bytes, |item| println!("{item:?}"))?;
# Ok::<(), Box<dyn std::error::Error>>(())

Node.js usage

import { Decoder, encodeEvent } from "@matesinc/fastsse-node";

const decoder = new Decoder();
const bytes = encodeEvent({ event: "chat", data: "hello", id: "evt-7" });
console.log(decoder.push(bytes));

Browser usage

import init, { Decoder, encodeEvent } from "./pkg/fastsse_wasm.js";

await init();

const decoder = new Decoder();
const bytes = encodeEvent({ data: "hello from wasm" });
console.log(decoder.push(bytes));