cookbook.md

Cookbook

Examples are organised under examples/ into three tiers — basic, intermediate, and advanced. Each example is a single self-contained file: clone, run, see what changes when you tweak it. Every example is part of pixi run tests and runs on CI, so they stay green with the code. Run any single example with pixi run example-<name> (see pixi.toml for the full task list).

Basic — networking primitives, hello-world HTTP, simple WS

The "first ten minutes with flare" surface. Plain functions, no typed extractors, no middleware composition. If you're skimming to see whether the framework feels right, start here.

File	What it shows
addresses.mojo	IpAddr, SocketAddr, v4 / v6 classification
dns_resolution.mojo	resolve(), resolve_v4(), resolve_v6(), numeric-IP passthrough
error_handling.mojo	typed error hierarchy and the context each error carries
tcp_echo.mojo	TcpListener + TcpStream round-trip, TCP options
udp.mojo	UdpSocket.bind, send_to, recv_from, DatagramTooLarge
encoding.mojo	gzip / deflate compress and decompress
tls.mojo	TlsConfig, TlsStream.connect, raw TLS handshake + GET
http_get.mojo	HttpClient GET / POST / PUT / PATCH / DELETE / HEAD
websocket_echo.mojo	WsClient connect, send, receive
ergonomics.mojo	high-level requests-style API (BufReader, WsMessage, Auth)
http_server.mojo	HttpServer with routing, JSON responses, response helpers
router.mojo	Router with path parameters, method dispatch, 404 / 405
ws_server.mojo	WsServer handshake + frame loop
cookies.mojo	Cookie, CookieJar, parse_cookie_header, parse_set_cookie_header

Intermediate — building a real app

Typed extractors, shared state, middleware composition, sessions, forms, multipart uploads, content-encoding negotiation, CORS, static files, Server-Sent Events. The next 30 minutes after the basics.

File	What it shows
extractors.mojo	Typed extractors (PathInt, QueryStr, HeaderStr, ...), reflective Extracted[H] auto-injection, and State[T] registration-time injection alongside request extractors
typed_extractors.mojo	OptionalPath{Int,Str,Float,Bool} optional path params + JsonAs[T: FromJson] typed-body deserialization
state.mojo	Shared application state via a captured wrapping Handler (a Counters snapshot tagged onto responses) -- the no-injection pattern; for injected state beside extractors see State[T] in extractors.mojo
middleware.mojo	Middleware composition (outside-in): RequestID → Logger → Timing → Recover → RequireAuth → Router
middleware_stack.mojo	Logger + RequestId + Compress + CatchPanic chain
multicore.mojo	HttpServer.serve(..., num_workers=default_worker_count())
static_response.mojo	Pre-encoded StaticResponse + HttpServer.serve_static fast path
cancel.mojo	CancelHandler polling cancel.cancelled() between expensive steps
drain.mojo	HttpServer.drain(timeout_ms) per-worker (caller wires SIGTERM today)
sse.mojo	Streaming response body via ChunkSource (Server-Sent Events shape)
response_from_body.mojo	Opt-in Response[B: Body] ergonomics via response_from_body (buffered InlineBody + chunk-streamed ChunkedBody over a forked server)
request_cookies.mojo	Reading inbound Cookie: headers + the Cookies extractor
forms.mojo	application/x-www-form-urlencoded parsing + the Form extractor
multipart_upload.mojo	multipart/form-data (file uploads) + the Multipart extractor
sessions.mojo	Typed Session[T] over CookieSessionStore (HMAC-SHA256 signed)
cors.mojo	Cors permissive vs allowlist + preflight + credentials
static_files.mojo	FileServer with HEAD + Range + path safety
brotli.mojo	compress_brotli / decompress_brotli + Compress middleware emitting br
ok_json_typed.mojo	Typed JSON request → typed JSON response via ok_json_value
infallible_handler.mojo	HandlerInfallible + WithRaises adapter for provably no-raises paths
trailers.mojo	HTTP/1.1 trailer fields (gRPC-style status trailer): Response.trailers, Trailer: header, smuggling guard
multi_listener.mojo	HttpServer.bind_many over multiple distinct addresses, single accept loop
reliability.mojo	Retry[Inner] + PostHocDeadline[Inner] + RetryPolicy -- RFC 9110 §9.2.2 idempotent-method gate (GET / HEAD / PUT / DELETE / OPTIONS), opt-in exponential backoff with jitter (backoff_base_ms / backoff_max_ms / backoff_jitter_ms), post-hoc 504 wall-clock guard
http_cache.mojo	Cache[Inner, S] middleware over an InMemoryCacheStore -- RFC 9111 freshness check on CacheEntry.is_fresh, Vary-aware secondary key, conditional revalidation (If-None-Match / If-Modified-Since + 304 folding back into the cached entry)
template_inheritance.mojo	Single-level template inheritance: parent layout + child overrides via {% block %} / {% extends %} / Template.render_extending

Advanced — comptime, low-level reactor, HTTP/2, mTLS, work-stealing

The Mojo-unique surfaces (compile-time route tables, direct reactor primitives, io_uring), HTTP/2-specific dispatch, mTLS, ACME-style cert reload, AF_UNIX sidecar IPC, multi-worker handoff. Don't reach for these until the intermediate tier feels natural.

File	What it shows
comptime_router.mojo	ComptimeRouter[routes] with comptime segment parsing and per-route dispatch unroll
reactor.mojo	direct flare.runtime.Reactor usage for custom protocols
work_stealing.mojo	HandoffQueue + WorkerHandoffPool + FLARE_SOAK_WORKERS knob
uds_sidecar.mojo	UnixListener / UnixStream AF_UNIX sidecar IPC
streaming_proxy.mojo	streaming-proxy surface: StreamHandler + attach_upstream(UpstreamChunkSource) + relay_upstream() over serve_streaming, relaying a FrameMux backend with watermark backpressure, no fd, no Span, no per-connection table in front code
cert_reload.mojo	TlsAcceptor.reload() for ACME / Let's Encrypt cert rotation without restart
mtls.mojo	mTLS configuration + construction-time validation
https_server.mojo	In-process HTTPS server: HttpServer.bind_tls / serve_tls (TLS-terminated HTTP/1.1) with a buffered handler and a stream_response chunked body over SSL_write
http2.mojo	Http2Connection driver, ALPN dispatch, h2c upgrade detection
http2_config.mojo	Http2Config SETTINGS knobs + validation
http2_client.mojo	HttpClient(prefer_h2c=True) GET + POST over h2c (cleartext HTTP/2 via prior knowledge)
http2_server_router.mojo	Path-dispatching handler served over HTTP/2 via the unified HttpServer.serve(handler) (auto-dispatches HTTP/1.1 + HTTP/2 on the same port)
h2c_client.mojo	HTTP/2 cleartext client via the Upgrade: h2c + HTTP2-Settings dance (RFC 7540 §3.2): first request flows over h1 then carries forward to h2
client_pool.mojo	HttpClient.with_pool — keyed idle reuse, per-origin caps, stale-conn retry
ws_over_h2.mojo	RFC 8441 WebSockets-over-HTTP/2 (Extended CONNECT + :protocol=websocket)
ws_permessage_deflate.mojo	RFC 7692 permessage-deflate extension: offer / negotiate / compress / decompress
alpn_dispatch_demo.mojo	ALPN -> wire-protocol dispatcher decisions: h1 / h2c / h2 / h3, including RFC 7301 §3.2 server-preference negotiation
http3_server.mojo	Serve the same Handler over HTTP/1.1 + HTTP/2 + HTTP/3 simultaneously: TCP listener for h1 / h2c / h2 alongside a UDP listener for h3
http3_client.mojo	HttpClient(prefer_http3=True) live HTTP/3 GET over QUIC: Alt-Svc discovery, happy-eyeballs race vs h2 / h1, transparent fallback
http3_server_walkthrough.mojo	Http3Connection driver lifecycle walkthrough (pure state-machine surface; pairs with http3_server.mojo)
quic_codec_demo.mojo	QUIC varint / frame codec / transport params / state machine / congestion controller round-trips
h3_codec_demo.mojo	HTTP/3 codec round-trip without a QUIC stream: QPACK field sections + RFC 9114 §7 frame codec + request reader
quic_handler_dispatch.mojo	RFC 9000 §19 transport-frame dispatch via FrameHandler + parse_frame_into (per-type callbacks, no union carrier)
grpc_unary_server.mojo	Reactor-mounted gRPC unary server: a GrpcService served over the unified HttpServer H2 reactor, grpc-status in H2 trailers
grpc_unary_demo.mojo	Sans-I/O gRPC unary adapter (no socket): LPM framing + metadata + status + run_unary_call orchestration
grpc_client_demo.mojo	GrpcClient unary RPC over the HttpClient HTTP/2 path (h2c), against a forked gRPC-speaking server
openai_sse_front.mojo	OpenAI-shaped chat.completion.chunk SSE front relaying an upstream token stream over UpstreamChunkSource / FrameMux with watermark backpressure
unified_middleware.mojo	One Logger(RequestId(Router)) middleware stack shared across both a Handler control plane and a StreamHandler streaming endpoint
production_setup.mojo	Production-shaped server: RequestId + StructuredLogger + CatchPanic + graceful shutdown + healthz

---

"I want to..." quick links

Goal	Start here
Serve hello-world	http_server.mojo
Add a route with a parameter	router.mojo
Make HTTP requests	http_get.mojo
Talk WebSocket	websocket_echo.mojo
Use TLS as a client	tls.mojo
Manage cookies	cookies.mojo
Pass typed inputs to a handler	extractors.mojo
Share state across handlers	state.mojo
Stack middleware	middleware.mojo
Stack Logger / RequestId / Compress / CatchPanic	middleware_stack.mojo
Scale to all cores	multicore.mojo
Skip the parser entirely	static_response.mojo
Detect mid-handler client disconnect	cancel.mojo
Drain on SIGTERM	drain.mojo
Stream a response body / SSE	sse.mojo
Read inbound cookies	request_cookies.mojo
Parse a form POST	forms.mojo
Accept file uploads	multipart_upload.mojo
Use signed-cookie sessions	sessions.mojo
Configure CORS	cors.mojo
Serve static files (with Range)	static_files.mojo
Send Content-Encoding: br	brotli.mojo
Return a typed JSON response	ok_json_typed.mojo
Use a no-raises handler	infallible_handler.mojo
Compile-time route table	comptime_router.mojo
Drive the reactor directly	reactor.mojo
Reload a TLS cert without restart	cert_reload.mojo
Configure mTLS	mtls.mojo
Drive HTTP/2 directly	http2.mojo
Tune HTTP/2 SETTINGS	http2_config.mojo
Make HTTP/2 client requests (h2c via prior knowledge; https:// auto-negotiates h2 vs h1.1 via ALPN)	http2_client.mojo
Serve HTTP/1.1 + HTTP/2 from one port	http2_server_router.mojo
AF_UNIX sidecar IPC	uds_sidecar.mojo
Proxy an external producer's stream with end-to-end backpressure	streaming_proxy.mojo
Even out skewed-keepalive load	work_stealing.mojo
Emit gRPC-style HTTP/1.1 trailers	trailers.mojo
Bind a single worker on multiple addresses	multi_listener.mojo
Speak h2c via the Upgrade dance from a client	h2c_client.mojo
Reuse h1.1 connections via HttpClient.with_pool	client_pool.mojo
Tunnel WebSockets over HTTP/2 (RFC 8441)	ws_over_h2.mojo
Compress WebSocket payloads with permessage-deflate	ws_permessage_deflate.mojo
Stand up a production-shaped server (RequestId + structured logs + graceful shutdown + healthz)	production_setup.mojo
Reuse markup via template inheritance ({% block %} / {% extends %})	template_inheritance.mojo
Retry idempotent requests + bound handler latency	reliability.mojo
Cache GET responses with RFC 9111 freshness + revalidation	http_cache.mojo
Route inbound TLS connections to h1 / h2 / h3 by ALPN	alpn_dispatch_demo.mojo
Serve HTTP/3 alongside HTTP/1.1 + HTTP/2 on the same Handler	http3_server.mojo
Pick the WebSocket carrier wire by negotiated ALPN	flare.ws.WsAutoClient + flare.ws.decide_wire (tests/ws/test_ws_autoclient.mojo)

Reading data from a Request

flare exposes inbound request data through a layered surface — the right shape depends on whether you want a plain field, a parsed primitive, or a typed struct populated by the request as a whole. The table below is the canonical reference; pick the cheapest shape that gets you the value you need.

Shape	What you get	Example	When to reach for it
Plain field access	The raw string / bytes / list / HeaderMap directly off Request	req.method, req.url, req.body, req.headers, req.peer, req.version	You want the wire-level value as the parser saw it. No copying, no extraction.
Path params	Value matched by a Router path segment (:name)	req.param("id") raises -> String, req.has_param, req.params_mut()["id"] = ...	The URL path itself carries the value. param raises if the segment didn't match (use has_param to peek).
Query params	Single value from the URL's query string	req.query_param("k") -> String (returns "" if missing), req.has_query_param	Querystring ?k=v style data; return-value-on-miss makes one-line reads natural.
Cookies	Inbound Cookie header parsed into name/value pairs	req.cookies() -> CookieJar, req.cookie("name") -> String (returns "" if missing), req.has_cookie	Inspecting an inbound Cookie header directly. For typed extraction prefer the Cookies extractor.
Body decoding	Body interpreted as text / JSON / raw bytes	req.text() -> String, req.json() raises -> json.Value, req.body: List[UInt8]	Reading the body opportunistically inside a handler without an extractor.
*.extract(req) extractors	Typed value from the body or a header set	Form.extract(req) -> Form, Multipart.extract(req) -> MultipartForm, Cookies.extract(req) -> Cookies	Hand-written extractor pipeline; reach for this when the auto-injection adapter is overkill but you still want typed parsing.
Comptime-keyed extractors	Single typed primitive keyed at compile time	PathInt["id"], QueryStr["q"], OptionalQueryInt["page"], HeaderStr["Authorization"], Json[T]	Building blocks for the auto-injection shape (next row); each one is a Defaultable struct with a value field.
Extracted[H] auto-injection	A handler struct whose fields are the extractor set; the adapter walks the field list per request and populates each	r.get("/users/:id", Extracted[GetUser]()) where GetUser(HandlerExtractor) declares id: PathInt["id"] etc.	Production handler shape: declarative, typed, monomorphised. The adapter raises 400 with the parser error on extractor failure; serve raises propagate to 500.

Examples that exercise each shape, in order: the router.mojo example covers plain field + path param shapes; request_cookies.mojo walks the cookie surface; forms.mojo and multipart_upload.mojo use the *.extract(req) extractors; extractors.mojo and ok_json_typed.mojo show comptime-keyed extractors and the Extracted[H] auto-injection adapter.