API reference

HTTP/1.1 + HTTP/2 server and client, WebSocket server and client (RFC 6455), TLS 1.2/1.3 (OpenSSL with ALPN), TCP, UDP, DNS. The HTTP server and client are version-aware: HttpServer.serve(handler) peeks the first 24 bytes of every accepted connection and dispatches HTTP/1.1 or HTTP/2 to the same handler. HttpClient.get("https://...") advertises ALPN ["h2", "http/1.1"] and switches wires from what the server picks. The application surface (Router, middleware, typed extractors, Auth, Session) doesn't know which wire is talking to it.

Small FFI footprint: libc syscalls, OpenSSL for TLS, zlib + brotli for content encoding. No HTTP framework dependency.

from flare import HttpServer, Router, Request, Response, ok, SocketAddr

def hello(req: Request) raises -> Response:
    return ok("hello")

def main() raises:
    var r = Router()
    r.get("/", hello)
    var srv = HttpServer.bind(SocketAddr.localhost(8080))
    srv.serve(r^, num_workers=4)

What flare is

One reactor per worker (kqueue on macOS, epoll on Linux, opt-in io_uring on Linux >= 6.0 via FLARE_BUFRING_HANDLER=1), a Handler trait that takes plain def functions or compiled-down structs, an RFC 7230 parser under continuous fuzzing (62 fuzz harnesses across the tree, 9M+ runs combined, zero known crashes), and a Cancel token plumbed to handlers via CancelHandler. num_workers=1 is a single-threaded reactor; num_workers=N with N >= 2 runs N workers (on Mojo's AsyncRT pool) behind per-worker SO_REUSEPORT listeners by default (matches actix_web's listener strategy and gives the highest steady-state throughput). Set FLARE_REUSEPORT_WORKERS=0 to opt into the single shared listener with EPOLLEXCLUSIVE -- trades 7-22 % req/s (handler vs static fast path) for a uniformly tighter p99.99 tail under sustained load. See docs/benchmark.md for the head-to-head numbers. Static endpoints can skip the parser entirely with serve_static(resp, num_workers=N).

The operational core: per-request / handler / body-read deadlines, HttpServer.drain(timeout_ms) for graceful shutdown, sanitised error responses, Request.peer threaded from the accept path, zero-copy reads (RequestView[origin] + ViewHandler), streaming response primitives (Body / ChunkSource / StreamingResponse[B]; stream_response(source) streams the same handler byte-identically over h1 / h2 / h3 / https), and server-side TLS (TlsAcceptor over OpenSSL, plus in-process HTTPS termination via HttpServer.bind_tls + serve).

The application layer: Router with path params, typed extractors (PathInt / QueryInt / HeaderStr / Form / Multipart / Cookies / ...), generic middleware (Logger / RequestId / Compress / CatchPanic), Cors, FileServer with HEAD + Range, gzip + brotli content negotiation, RFC 6265 cookie jars, HMAC-SHA256 signed cookies, and signed Session stores.

Architecture

flare.io       - BufReader
flare.ws       - WebSocket client + server (RFC 6455, permessage-deflate
                 with context-takeover, WS-over-h2 via RFC 8441 and
                 WS-over-h3 via RFC 9220 Extended CONNECT)
flare.http2    - HTTP/2 frame codec + HPACK (with table-driven Huffman
                 fast decoder) + h2c upgrade (RFC 9113 / 7541)
flare.http     - HTTP/1.1 client + reactor server + Router /
                 extractors + middleware (Logger / RequestId / Compress /
                 Cors / Retry / PostHocDeadline / Conditional) + FileServer +
                 forms + cookies + sessions + content-encoding + SSE
                 + template engine with {% block %} / {% extends %}
                 inheritance + serve_streaming streaming-proxy surface
                 (StreamHandler / StreamConn, UpstreamChunkSource,
                 watermark backpressure, admission control) + sans-I/O
                 parser sublayer under flare.http.proto.*
flare.http.cache - RFC 9111 cache primitives (CacheControl directive
                 parser, CacheKey, InMemoryCacheStore)
flare.grpc     - gRPC primitives on flare.http2: LPM message framing,
                 canonical Status codes (with optional
                 ``grpc-status-details-bin`` payload), Metadata
                 carrier, the unary server adapter
                 (``GrpcRequestHeaders`` typed request-headers
                 carrier -> ``GrpcUnaryReply`` typed handler return
                 (``ok`` / ``err`` factories) -> ``run_unary_call``
                 never-raises orchestrator that maps header / LPM /
                 handler failures to typed ``INVALID_ARGUMENT`` /
                 ``INTERNAL`` outcomes), and a ``GrpcClient`` that
                 drives unary RPCs over the ``HttpClient`` HTTP/2 path
                 plus server- / client- / bidi-streaming RPCs
                 (``GrpcServerStream`` / ``GrpcBidiStream``) that
                 surface LPM messages incrementally on a long-lived
                 HTTP/2 stream
flare.openapi  - OpenAPI 3.1 spec model + deterministic JSON emitter
flare.quic     - Sans-I/O QUIC v1 codec primitives (varint, long /
                 short packet headers, all 22 RFC 9000 §19 transport
                 frames dispatched via the ``FrameHandler`` trait +
                 ``parse_frame_into[H]`` (per-type callbacks; no
                 intermediate union carrier), with per-type
                 ``encode_*(payload, mut out: List[UInt8])`` writers,
                 RFC 9000 §18 transport parameters, and RFC 9000 §3 /
                 §13 connection + stream state machines, plus the live
                 ``QuicListener`` UDP reactor, the ``QuicClientConnection``
                 client driver (incl. resumption + 0-RTT EarlyData
                 send flight, NEW_TOKEN, ECN, ACK_FREQUENCY and path-MTU
                 discovery), and the rustls QUIC TLS drive that
                 carry HTTP/3 end-to-end
flare.http3       - Sans-I/O HTTP/3 frame codec, SETTINGS payload,
                 request-stream reader (``Http3RequestReader`` +
                 ``Http3RequestEventHandler`` + ``feed_into[H]`` --
                 typed ``on_headers`` / ``on_data`` / ``on_trailers``
                 callbacks), response-stream writer with QPACK-
                 encoded field sections and the ``mut out:
                 List[UInt8]`` buffer-reuse contract (RFC 9114 §4 +
                 §7), plus the client side: ``Http3ClientConnection``
                 (request writer + response reader over the QUIC
                 client driver) for end-to-end HTTP/3 requests,
                 including ``fetch_0rtt`` -- idempotent-method-only
                 0-RTT: emits the request in the first EarlyData flight
                 on a resumed connection and transparently replays it
                 at 1-RTT if the server rejects early data
flare.qpack    - Sans-I/O static-only QPACK encoder / decoder for
                 HTTP/3 field sections (RFC 9204 Appendix A static
                 table + literal + Huffman, shared with HPACK)
flare.crypto   - HMAC-SHA256, base64url
flare.tls      - TLS 1.2/1.3 (OpenSSL, client + server, ALPN, session resumption,
                 SNI certificates, certificate hot reload)
flare.tcp      - TcpStream + TcpListener (IPv4 + IPv6)
flare.udp      - UdpSocket (IPv4 + IPv6)
flare.uds      - UnixListener + UnixStream (AF_UNIX sidecar IPC);
                 FrameMux / FrameDemux multiplex logical streams over
                 one UnixStream (encode_frame / decode_frame)
flare.dns      - getaddrinfo (dual-stack) + an additive TTL
                 ``DnsCache`` over the sync resolver, plus an
                 off-reactor ``resolve_async`` (getaddrinfo on a pool
                 thread) and ``order_happy_eyeballs`` address ordering
flare.net      - IpAddr, SocketAddr, RawSocket
flare.runtime  - Reactor (kqueue / epoll, opt-in io_uring on Linux),
                 TimerWheel, Scheduler, HandoffQueue +
                 WorkerHandoffPool, BufferPool, vectored I/O
flare.testing  - TestClient[H] (in-process handler exerciser) +
                 fork_server / kill_forked_server for integration tests
flare.utils    - POSIX FFI thunks (fork / waitpid / kill / usleep /
                 exit / getpid) the Mojo stdlib doesn't expose yet

Each layer mostly imports from layers below it. The runtime scheduler is generic over a Frontend trait so that flare.runtime no longer imports from flare.http; the HTTP worker bring-up lives in flare.http.multicore and plugs into the scheduler via that trait. flare.http still re-imports flare.http2 for the unified reactor dispatch (the only remaining intra-http cycle, tracked as cleanup in the source).

HTTP requests

from flare.http import get, post

def main() raises:
    var resp = get("https://httpbin.org/get")
    print(resp.status, resp.ok()) # 200 True

    var r = post("https://httpbin.org/post", '{"hello": "flare"}')
    r.raise_for_status()
    var data = r.json()
    print(data["json"]["hello"].string_value())

post with a String body sets Content-Type: application/json automatically.

The server-side sections below build gradually: each one adds one new concept on top of the previous example.

Routing: Router

One event loop (kqueue on macOS, epoll on Linux), non-blocking sockets, a per-connection state machine, a hashed timing wheel for idle timeouts. Routes carry path parameters (:name) and methods; unknown paths return 404, known paths with the wrong method return 405 with an auto-generated Allow: header.

from flare.http import Router, Request, Response, ok, HttpServer
from flare.net import SocketAddr

def home(req: Request) raises -> Response:
    return ok("home")

def get_user(req: Request) raises -> Response:
    return ok("user " + req.param("id"))

def create_user(req: Request) raises -> Response:
    return ok("created")

def main() raises:
    var r = Router()
    r.get("/", home)
    r.get("/users/:id", get_user)
    r.post("/users", create_user)

    var srv = HttpServer.bind(SocketAddr.localhost(8080))
    srv.serve(r^)

Typed inputs: Extracted[H]

Declare the handler's extractors as the fields of a Handler struct and wrap it in Extracted[H]. The adapter reflects on the struct's field types at compile time to pull each one from the request before calling the inner serve. Parse failures become automatic 400 responses; serve is only reached when every field has a value of the right type.

from flare.http import (
    Router, Handler, Request, Response, ok, HttpServer,
    Extracted, PathInt, OptionalQueryInt, HeaderStr,
)
from flare.net import SocketAddr

@fieldwise_init
struct GetUser(Copyable, Defaultable, Handler):
    var id: PathInt["id"]
    var page: OptionalQueryInt["page"]
    var auth: HeaderStr["Authorization"]

    def __init__(out self):
        self.id = PathInt["id"]()
        self.page = OptionalQueryInt["page"]()
        self.auth = HeaderStr["Authorization"]()

    def serve(self, req: Request) raises -> Response:
        return ok("user=" + String(self.id.value))

def main() raises:
    var r = Router()
    r.get("/users/:id", Extracted[GetUser]())
    var srv = HttpServer.bind(SocketAddr.localhost(8080))
    srv.serve(r^)

The concrete PathInt / PathStr / QueryInt / HeaderStr / etc. extractors expose .value directly as the parsed primitive. Custom types are handled by writing your own Extractor struct. Value-constructor extractors (PathInt["id"].extract(req)) are also available for use inside plain def handlers when the struct shape is overkill.

Static route tables: ComptimeRouter

Same routing surface as Router, but the route list is a compile-time value. Segment parsing happens at build time and the dispatch loop unrolls per route, so the runtime does zero string-compares on unknown paths.

from flare.http import (
    ComptimeRoute, ComptimeRouter, Request, Response, Method, ok, HttpServer,
)
from flare.net import SocketAddr

def home(req: Request) raises -> Response:
    return ok("home")

def get_user(req: Request) raises -> Response:
    return ok("user=" + req.param("id"))

def create_user(req: Request) raises -> Response:
    return ok("created")

comptime ROUTES: List[ComptimeRoute] = [
    ComptimeRoute(Method.GET, "/", home),
    ComptimeRoute(Method.GET, "/users/:id", get_user),
    ComptimeRoute(Method.POST, "/users", create_user),
]

def main() raises:
    var r = ComptimeRouter[ROUTES]()
    var srv = HttpServer.bind(SocketAddr.localhost(8080))
    srv.serve(r^)

Shared state via captured handlers

When a handler needs application-scoped state, build a wrapping Handler struct that captures the state by value. Middleware itself is a Handler that holds another Handler, so you stack layers by nesting constructors, no callback chain to thread through.

from flare.http import Router, Request, Response, Handler, ok, HttpServer
from flare.net import SocketAddr

@fieldwise_init
struct Counters(Copyable):
    var hits: Int

def home(req: Request) raises -> Response:
    return ok("home")

@fieldwise_init
struct WithHits[Inner: Handler](Handler):
    var inner: Self.Inner
    var counters: Counters

    def serve(self, req: Request) raises -> Response:
        var resp = self.inner.serve(req).lower()
        resp.headers.set("X-Hits", String(self.counters.hits))
        return resp^

def main() raises:
    var router = Router()
    router.get("/", home)

    var srv = HttpServer.bind(SocketAddr.localhost(8080))
    srv.serve(WithHits(inner=router^, counters=Counters(hits=37)))

Scale knob: num_workers

Every server example above takes an optional num_workers. At 1 (the default) it's the single-threaded reactor. Set it to default_worker_count() and flare runs N workers, each with its own reactor + timer wheel, with per-core pinning on Linux. The handler type is unchanged.

By default each worker binds its own SO_REUSEPORT listener (the kernel hashes new 4-tuples to one of N listeners; matches actix_web's listener strategy and gives the highest steady-state throughput on dev-box workloads). Export FLARE_REUSEPORT_WORKERS=0 before launch to opt back into the single shared listener with EPOLLEXCLUSIVE -- 7-22 % less req/s (handler vs static fast path) for a uniformly tighter p99.99 σ under sustained load (the kernel offers each accept event to whichever worker is currently parked in epoll_wait, so idle workers absorb spikes). See docs/benchmark.md for the head-to-head numbers and the rationale.

from flare.http import HttpServer, Router, Request, Response, ok
from flare.net import SocketAddr
from flare.runtime import default_worker_count

def hello(req: Request) raises -> Response:
    return ok("hello")

def main() raises:
    var r = Router()
    r.get("/", hello)
    var srv = HttpServer.bind(SocketAddr.localhost(8080))
    srv.serve(r^, num_workers=default_worker_count())

pin_cores=True (default) pins worker N to core N % num_cpus() on Linux and is a no-op on macOS. Upper bound on num_workers is 256.

Fixed-body endpoints: serve_static

For endpoints that always return the same bytes (health checks, TFB plaintext, single-URL microservices), precompute_response builds the full HTTP wire form at startup and HttpServer.serve_static runs a specialised reactor that skips the parser's Request-construction and handler-dispatch step entirely.

from flare.http import HttpServer, precompute_response
from flare.net import SocketAddr

def main() raises:
    var resp = precompute_response(
        status=200,
        content_type="text/plain; charset=utf-8",
        body="Hello, World!",
    )
    var srv = HttpServer.bind(SocketAddr.localhost(8080))
    srv.serve_static(resp)

Keep-alive and Connection: close wire forms are both pre-encoded; the reactor picks the right one per request from the parsed Connection header. serve_static(resp, num_workers=N) fans the same fast path out across N worker threads, each with its own listener.

Comptime config checks

ServerConfig.check[config]() validates a comptime ServerConfig through Mojo comptime assert so misconfigured servers fail the build rather than the first request:

from flare.http import HttpServer, Request, Response, ok
from flare.http.handler import FnHandler
from flare.http.server import ServerConfig
from flare.net import SocketAddr

def hello(req: Request) raises -> Response:
    return ok("hello")

comptime CONFIG: ServerConfig = ServerConfig(
    max_header_size=4096,
    max_body_size=64 * 1024, # must be >= max_header_size (compile time)
    max_keepalive_requests=1000,
    idle_timeout_ms=30_000,
)

def main() raises:
    ServerConfig.check[CONFIG]()
    var srv = HttpServer.bind(SocketAddr.localhost(8080), materialize[CONFIG]())
    srv.serve(FnHandler(hello))

Break any invariant (e.g. max_body_size < max_header_size) and Mojo rejects the build with a pointed error. The impossible state doesn't compile, so no runtime guard is needed.

HTTP client with auth

from flare.http import HttpClient, BasicAuth, BearerAuth

def main() raises:
    var client = HttpClient("https://api.example.com", BearerAuth("tok_abc"))
    var items = client.get("/items").json()
    client.post("/items", '{"name": "new"}').raise_for_status()

HTTP/2: same client, same server, version-aware

There is no separate Http2Client / Http2Server to learn: the same :class:flare.http.HttpClient and :class:flare.http.HttpServer are HTTP-version-aware internally.

from flare.http import HttpClient

def main() raises:
    # https:// auto-negotiates via TLS+ALPN. If the server picks
    # h2 the request is driven through HTTP/2 internally; if it
    # picks http/1.1 (or doesn't speak ALPN at all) the
    # existing HTTP/1.1 wire is used. Either way you get a
    # flare.http.Response back.
    with HttpClient() as c:
        var r = c.get("https://nghttp2.org/")
        print(r.status, r.text())

    # http:// is HTTP/1.1 by default; opt into HTTP/2 cleartext
    # via prior knowledge with prefer_h2c=True:
    with HttpClient(prefer_h2c=True, base_url="http://localhost:8080") as c:
        var r = c.get("/api/users")
        r.raise_for_status()

    # https:// can prefer HTTP/3 over QUIC with prefer_http3=True. The
    # same call site applies; an idempotent GET races h3 against h2/h1
    # (happy-eyeballs) and any QUIC failure falls back transparently:
    with HttpClient(prefer_http3=True, base_url="https://cloudflare.com") as c:
        var r = c.get("/")
        print(r.status, r.text())

The client surface is additive and composable: with_redirect_policy, with_retry, with_cookies, and with_pool chain off the constructor, and auto_decompress (default on) transparently decodes gzip / deflate / br responses. The H3 / QUIC client drivers stay internal -- you reach HTTP/3 through HttpClient, never a separate type.

The server side is symmetric -- one accept loop dispatches both wires per connection (preface peek for cleartext, ALPN h2 for TLS):

from flare.http import HttpServer, Request, Response, ok
from flare.net import SocketAddr

def hello(req: Request) raises -> Response:
    return ok("hi")

def main() raises:
    var srv = HttpServer.bind(SocketAddr.localhost(8080))
    # The same handler is dispatched whether the client speaks
    # HTTP/1.1 or HTTP/2 over the same port.
    srv.serve(hello, num_workers=4)

WebSocket

from flare.ws import WsClient

def main() raises:
    with WsClient.connect("ws://echo.websocket.events") as ws:
        ws.send_text("hello")
        var msg = ws.recv_message()
        if msg.is_text:
            print(msg.as_text())

Cookies

from flare.http import Cookie, CookieJar, parse_set_cookie_header

def main() raises:
    var jar = CookieJar()
    jar.set(Cookie("session", "abc123", secure=True, http_only=True))
    print(jar.to_request_header()) # session=abc123

    var c = parse_set_cookie_header("id=42; Path=/; Max-Age=3600")
    print(c.name, c.value, c.max_age) # id 42 3600

Low-level API

IP addresses and DNS

from flare.net import IpAddr, SocketAddr
from flare.dns import resolve

def main() raises:
    var ip = IpAddr.parse("192.168.1.100")
    print(ip.is_private()) # True

    var addr = SocketAddr.parse("[::1]:8080")
    print(addr.ip.is_v6(), addr.port) # True 8080

    var addrs = resolve("example.com") # returns both IPv4 and IPv6
    print(addrs[0])

TCP

from flare.tcp import TcpStream

def main() raises:
    var conn = TcpStream.connect("localhost", 8080)
    _ = conn.write("Hello\n".as_bytes())

    var buf = List[UInt8](capacity=4096)
    buf.resize(4096, 0)
    var n = conn.read(buf.unsafe_ptr(), len(buf))
    conn.close()

TLS

from flare.tls import TlsStream, TlsConfig

def main() raises:
    var tls = TlsStream.connect("example.com", 443, TlsConfig())
    _ = tls.write("GET / HTTP/1.0\r\nHost: example.com\r\n\r\n".as_bytes())
    tls.close()

WebSocket frames

from flare.ws import WsClient, WsFrame

def main() raises:
    var ws = WsClient.connect("ws://echo.websocket.events")
    ws.send_text("ping")
    var frame = ws.recv()
    print(frame.text_payload())
    ws.close()

Reactor (advanced)

from flare.runtime import Reactor, Event, INTEREST_READ

def main() raises:
    var r = Reactor()
    # Register a non-blocking fd; see ``Reactor`` docs for a full example.