Crate argan

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A web framework for Rust.


Argan applications are built by composing Resources into a tree. Each resource in the tree corresponds to one of the path segments of the URI. When a request comes, it passes through the resources that match its URI path segments. If a resource that matches the last path segment exists, that resource handles the request. Otherwise, a resource that matches the farthest segment (not the last segment) responds with its custom or with the default mistargeted request handler that returns a "404 Not Found" response.

use argan::{

//  "/resource_0_0" --- "/resource_1_0" --- "/resource_2_0"
//                 |                   |
//                 |                    --- "/resource_2_1" --- "/resource_3_0"
//                 |
//                  --- "/resource_1_1"

// A resource can be created with the pattern of its corresponding segment.
let mut resource_0_0 = Resource::new("/resource_0_0");

let mut resource_1_1 = Resource::new("/resource_1_1");
resource_1_1.set_handler_for(|| async { "resource_1_1" }));

// It can also be created with an absolute path pattern.
let mut resource_2_0 = Resource::new("/resource_0_0/resource_1_0/resource_2_0");
resource_2_0.set_handler_for(|| async { "resource_2_0" }));

resource_0_0.add_subresource([resource_1_1, resource_2_0]);

// We can also create a new subresource or get an existing one with a relative
// path pattern from a parent.
let mut resource_2_1 = resource_0_0.subresource_mut("/resource_1_0/resource_2_1");
resource_2_1.set_handler_for(|| async { "resource_2_1" }));
    .set_handler_for(|| async { "resource_3_0" }));

let resource_service = resource_0_0.into_arc_service();

A Resource itself is mainly made of three components: request receiver, request passer, and request handler. All three of them are Handlers and can be wrapped with middleware.

The request receiver, as its name suggests, is responsible for receiving a request and handing it over to the request passer if the resource is not the request’s target resource, or to the request handler if it is.

The request passer finds the next resource that matches the next path segment and passes the request to that resource. If there is no matching resource, the request passer calls the resource’s custom or the default mistargeted request handler to generate a "404 Not Found" response unless there is a subtree handler among the parent resources. In that case, it returns the request back to the subtree handler parent that can handle the request.

The request handler contains all the HTTP method handlers of the resource. It is responsible for calling the corresponding method handler that matches the request’s method.


A Host is another node in Argan that can be converted into a service. But unlike a Router and Resource nodes, a Host doesn’t have any Handler components. Instead, it contains a root resource and guards the resource tree against the request’s "Host".

use argan::{

async fn hello_world() -> &'static str {
    "Hello, World!"

let mut root = Resource::new("/");

let host_service = Host::new("", root).into_arc_service();


When there is a need for multiple resource trees with different host guards, a Router node can be used.

use argan::{

let mut router = Router::new();

let mut example_com_root = Resource::new("/");
example_com_root.set_handler_for(|| async { "" }));

let mut abc_example_com_root = Resource::new("/");
abc_example_com_root.set_handler_for(|| async { "" }));

    Host::new("", example_com_root),
    Host::new("", abc_example_com_root),

// If we create a resource with a host pattern, it's automatically placed under that
// host when we add it to a router.

let mut bca_example_com_root = Resource::new("");
bca_example_com_root.set_handler_for(|| async { "" }));

// We can also add a hostless resource tree for requests with a "Host" that doesn't
// match any of our hosts.

let mut hostless_resource = Resource::new("/resource");
hostless_resource.set_handler_for(|| async { "resource" }));

router.add_resource([bca_example_com_root, hostless_resource]);

let router_service = router.into_leaked_service();


There are three kinds of patterns in Argan: static, regex, and wildcard. Path segment patterns can be of any kind, while host patterns can only be static or regex. Hosts and resources with static patterns have the highest priority. That is, when the request passer is trying to find the next resource, first it matches the next path segment against resources with static patterns. Then come resources with regex patterns, and finally a resource with a wildcard pattern.

The static pattern matches the request’s path segment exactly.

use argan::Resource;

// Resources with static patterns:

let news = Resource::new("/news");
let items = Resource::new("/items");

Regex patterns are available only when the "regex" feature flag is enabled. They can have static and regex subpatterns. Regex patterns match if both subpatterns match the request’s path segment in the exact order. Regex subpatterns are written in curly braces with their name and regex parts separated by a colon: "{name:regex}". If the regex subpattern is the last suppattern or the following subpattern is a static subpattern that starts with a dot '.', then the regex part can be omitted to match anything.

use argan::{Host, Resource};

// Resources with regex patterns:

let sub_domain = Host::new("http://{sub}", Resource::new("/"));

// Here, `number_of_days` is a path parameter name that can have a value of
// `5` or `10`, and '-days-forecast' is a static subpattern.
let n_days_forecast = Resource::new("/{number_of_days:5|10}-days-forecast");

// Here, 'id:' is a static subpattern, and the `prefix` and `number` are the
// path parameter names.
let id = Resource::new(r"/id:{prefix:A|B|C}{number:\d{5}}");

// Here, we have `name` and `ext` path parameter names. Both match anything
// that's separated with a dot '.'.
let file = Resource::new("/{name}.{ext}");

The wildcard pattern matches anything in the request’s path segment. The wildcard pattern has only a name in the curly braces.

use argan::Resource;

// Resources with wildcard patterns:

let echo = Resource::new("/{echo}");
let black_box = Resource::new("/{black_box}");

A Resource may contain any number of child resources with static and regex patterns. But it can contain only one resource with a wildcard pattern.

Patterns can be joined together to form a path pattern or a URI pattern. Note that a resource with a prefix URI pattern should be used with a Router. Otherwise, its host pattern, if it exists, and prefix segment patterns will be ignored when it is converted into a service. Likewise, a resource with a prefix path pattern should be used as a subresource instead of being converted into a service.

use argan::Resource;

// A resource with only its own segment pattern.
let resource = Resource::new("/products");

// A resource with a prefix path pattern. A path pattern is always considered
// to be an absolute path.
let resource = Resource::new("/groups/{group_id}/users/{user_id}");

// A resource with a prfix URI pattern.
let resource = Resource::new("http://{lang:en|fr}");

A resource pattern may end with a trailing slash /, meaning the request targeting the resource must also have a trailing slash in its path. If not, the resource redirects the request to a path with a trailing slash. When the resource pattern doesn’t have a trailing slash but the request’s path does, then the resource redirects the request to a path without a trailing slash. This is the default behavior.

A resource pattern can also have configuration symbols, !, ?, and *, attached after a single space. They can be used to configure the resource to drop or to handle the request in the absence or presence of a trailing slash / in the request’s path, and also to handle requests that target non-existent subresources, aka as a subtree handler. An ! configures the resource to drop the request when the absence or presence of a trailing slash in its path doesn’t match the resource’s pattern. A ? configures the resource to be lenient to a trailing slash in the request’s path, which means the resource in any case handles the request. ! and ?configuration symbols are mutually exclusive. An * configures the resource to be a subtree handler. Configuration symbols can only be specified on the resource’s own pattern. Prefix segment patterns given when the resource is being created or retrieved cannot have configuration symbols.

symbol(s) on a patternresource action
"/some_pattern"redirects the requests with a trailing slash
"/some_pattern/"redirects the requests without a trailing slash
r"/some_pattern *"redirects the requests with a trailing slash; subtree handler
r"/some_pattern/ *"redirects the requests without a trailing slash; subtree handler
"/some_pattern !"sends a "404" when there is a trailing slash
"/some_pattern/ !"sends a "404" when there is no trailing slash
r"/some_pattern !*"sends a "404" when there is a tralling slash; subtree handler
r"/some_pattern/ !*"sends a "404" when there is no tralling slash; subtree handler
"/some_pattern ?"handles the requests with or without a trailing slash
"/some_pattern/ ?"handles the requests with or without a trailing slash
r"/some_pattern ?*"handles the requests with or without a trailing slash; subtree handler
r"/some_pattern/ ?*"handles the requests with or without a trailing slash; subtree handler

Note that patterns must be specified without a percent-encoding. An exception to this is the slash /. If a path segment should contain a slash /, then it should be replaced with %2f or %2F.

use argan::Resource;

let resource = Resource::new("/resource%2F1%2F/resource%2F2%2F");


A handler is a type that implements the Handler trait. At the high level, Argan applications use async functions. These functions take a specific set of parameters in a specific order and must return a value of a type that implements either the IntoResponse or ErrorResponse traits. Following are the parameters in the required order: RequestHead, FromRequest implementor, Args.

use argan::{

use serde::Deserialize;

// Note that `()` implements the `IntoResponse` trait and can be used
// as a "success" response with a "200 OK" status code and an empty body.

// --------------------------------------------------------------------------------
// `async` functions that can be used as request handlers.

// No params.
async fn handler_1() {
    // ...

// With a `RequestHead`.
async fn handler_2(head: RequestHead) {
    // ...

// With an extractor that implements the `FromRequest` trait.
async fn handler_3(Text(text): Text) {
    // ...

// Here, `()` in the `Args<'static, ()>` can be a type provided as a handler
// extension via the `IntoHandler::with_extension()` method.
// With handler `Args`.
async fn handler_4(args: Args<'static, ()>) {
    // ...

// With a `RequestHead` and an extractor.
async fn handler_5(head: RequestHead, Text(text): Text) {
    // ...

// With a `RequestHead` and handler `Args`.
async fn handler_6(head: RequestHead, args: Args<'static, ()>) {
    // ...

// With an extractor and handler `Args`.
async fn handler_7(Text(text): Text, args: Args<'static, ()>) {
    // ...

// With all three parameters.
async fn handler_8(head: RequestHead, Text(text): Text, args: Args<'static, ()>) {
    // ...

In addition to the combinations of these three main parameters, handler functions can take up to 12 parameters that implement the ExtractorGuard trait. Those parameters must come before the combinations of the three main parameters.

§Error handling

Errors in Argan can be handled in the handler functions or at some points in the node tree using a middleware. If the handler decides to return an ErrorResponse, that error passes through the nodes in the node tree up to the first node that was converted into service. The first node may be a Resource, a Host, or a Router. If the error reaches the service without being handled, the service will convert it into a Response before passing it to the hyper.

An error handler middleware can be layered on the components of the Resource and Router via the ErrorHandlerLayer. The constructor of the ErrorHandlerLayer takes a parameter that implements the ErrorHandler trait. The ErrorHandler trait is blanketly implemented for functions with a signature async fn(BoxedErrorResponse) -> Result<Response, BoxedErrorResponse>.

use argan::{
		form::{Form, FormError},
		json::{Json, JsonError},
	handler::{IntoHandler, _get, _post},
	middleware::{ErrorHandlerLayer, _request_receiver},
	request::{PathParamsError, RequestHead},
	response::{BoxedErrorResponse, IntoResponse, IntoResponseResult, Response},
use serde::{Deserialize, Serialize};

// --------------------------------------------------
// Error handlers

async fn path_error_handler(error: BoxedErrorResponse) -> Result<Response, BoxedErrorResponse> {
	let path_error = error.downcast::<PathParamsError>()?;

	// ...


async fn general_errors_handler(error: BoxedErrorResponse) -> Result<Response, BoxedErrorResponse> {
	if let Some(form_error) = error.downcast_ref::<FormError>() {
		// ...

	if let Some(json_error) = error.downcast_ref::<JsonError>() {
		// ...

	// ...


// --------------------------------------------------
// Method handlers

// Deserialization of the form data may fail. In such a case, the `login` handler won't
// be called, and the error response will be generated. If we want to deal with the error
// inside the handler, we can get the result of the extraction by using the expression
// `result: Result<Form<Credentials>, FormError>` as a function parameter.
async fn login(Form(credentials): Form<Credentials>) -> Json<Token> {
	// ...

	let token = Token {
		jwt: "JWT".to_owned(),


struct Credentials {
	user_name: String,
	password: String,

struct Token {
	jwt: String,

// -------------------------

// Here, if the handler returns different kinds of error responses, a `BoxedErrorResponse`
// can be used instead of the `PathParamsError`.
async fn item_data(head: RequestHead) -> Result<Json<ItemData>, PathParamsError> {
	let (category, item) = head.path_params_as::<(&str, &str)>()?;

	// ...

	let item_datan = ItemData {
		// ...

	// Serialization may also fail, and an error response will be generated.

struct ItemData {
	// ...

// --------------------------------------------------

let mut root = Resource::new("/");
// It's best for general errors to be handled up in the hierarchy.


    // The most specific or custom errors can be handled by layering the method handlers.

// Unhandled errors will automatically be converted into a `Response` by the service.
let service = root.into_arc_service();


Argan has a flexible middleware system. Middlewares can be applied to handlers and resource components using the Layer trait implementors. In addition to its own trait, Argan is also compatible with the Tower layers.

use std::{future::ready, time::Duration};

use argan::{
	handler::{Args, BoxableHandler, Handler, IntoHandler, _get},
	middleware::{IntoLayer, _request_handler},
	response::{BoxedErrorResponse, IntoResponse, Response},
use tower_http::{
	compression::CompressionLayer, decompression::DecompressionLayer, timeout::TimeoutLayer,

// --------------------------------------------------
// Middleware

struct Middleware<H> {
	name: &'static str,
	handler: H,

impl<H> Handler for Middleware<H>
	H: BoxableHandler,
	type Response = Response;
	type Error = BoxedErrorResponse;
	type Future = BoxedFuture<Result<Self::Response, Self::Error>>;

	fn handle(&self, request_context: RequestContext, args: Args<'_, ()>) -> Self::Future {
		println!("middleware: {}",;

		self.handler.handle(request_context, args)

// --------------------------------------------------

// The `IntoLayer` trait is blankedly implemented for functions that take a handler
// and return a handler.

fn layer_a<H>(handler: H) -> Middleware<H> {
	Middleware { name: "A", handler }

fn layer_b<H>(handler: H) -> Middleware<H> {
	Middleware { name: "B", handler }

fn layer_c<H>(handler: H) -> Middleware<H> {
	Middleware { name: "C", handler }

// --------------------------------------------------

let mut resource = Resource::new("/resource");

// Layers are applied from right to left and from bottom to top.
// When the resource's request handler is called, middlewares print `ABC`.
    _request_handler((layer_b.into_layer(), layer_c.into_layer())),

// The GET method handler will be layered in the following order:
// timeout layer {
//   compression layer {
//     decompression layer {
//       GET method handler
//     }
//   }
// }
resource.set_handler_for(|| async {}).wrapped_in((

See also Router::add_layer_to(), Resource::add_layer_to(), and IntoHandler::wrapped_in() for more information.

§Feature flags

feature flagenables
“regex”regex patterns
“private-cookies”cookies, private cookies, and a cookie Key
“signed-cookies”cookies, signed cookies, and a cookie Key
“query-params”query params
“json”the JSON extractor and response type Json
“form”the form extractor and response type Form
“multipart-form”the multipart form extractor MultipartForm
“sse”server-sent events
“file-stream”static file streaming
“websockets”the WebSockets
“full”all the features


  • Types and traits for request and response bodies.
  • Common types and functions.
  • Extractors and response types for some data formats.
  • Traits and types for request handling.
  • Host service types.
  • Middleware system types and traits.
  • HTTP request types.
  • Resource service types.
  • HTTP response types.
  • Router service types.


  • Representation of the host subcomponent of the URI.
  • Representation of the web resource that corresponds to the path segment component of the URI.
  • A type that can contain hosts and a root resource.
  • StaticFilesfile-stream
    A resource that serves static files from a directory.