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//! Using an event based interaction pattern for callback communication is often praised as ideal, but, //! for many cases we've found this results in complexity, bugs, and unnecessary work. `Fish` //! provides imperative non-blocking runtime interaction for callback and webhook based APIs. //! //! Let's contrive an example to see how this works: imagine you are the developer of some plugin for a delivery app. //! //! The app will notify you (... via a webhook) when a new delivery region opens. //! There is an endpoint that allows you to register a webhook for new orders //! in that region. //! //! Whenever an order is received, you can send a request to accept it. The delivery app //! will then send you a callback if your request is granted. Oh, I forgot to mention, //! you are a broker, so, whenever the order is received, you need to pass it along to //! your fulfillment client, who will likewise send a callback if they desire to fulfill the order. //! //! Along each step, you have domain-specific logic and state that is often used throughout the //! entire lifecycle. //! //! In `Fish`, this interaction looks something like: //! //! ```rust,ignore //! // Step 1: Register webhook for new delivery regions //! let orders = server.spawn(); //! //! app.send("region", RegisterRegion { //! webhook_url: orders.url(), //! region_id //! }).await; //! //! // orders is a Stream. You could concatenate multiple regions' orders together, //! // or maybe handle them separately //! while let Ok(order) = orders.next().await { //! // Step 2: Let's see if our fulfillment partner is interested in the order //! let fulfillment = server.spawn(); //! //! partner.send("order", NotifyOrder { //! callback: fulfillment.url(), //! ..order //! }).await; //! //! // The partner will send us back a POST request if they want to fulfill the order, //! // and do-nothing otherwise. We have a time limit to adhere to, so we'll give them //! // 5 seconds to respond //! if let Ok(_) = timeout(Duration::from_secs(5), fulfillment.next().await) { //! // Step 3: OK, we're set, lets let the app know we are interested! //! let granted = server.spawn(); //! app.send("order", AcceptOrder { //! callback_url: url, //! .. //! }).await; //! //! // granted.next() and so on! //! } //! } //! ``` //! //! Ok, we could've drawn this out a lot further. And sorely missing is the domain-specific logic, //! state updates, caching, and so on that happens during this orchestration. We've found doing //! this in an event-handler-based manner takes an enormous effort. //! //! What about you? Have you found this to be challenging? Any suggestions? //! //! //! Feedback, contributions, and bug reports welcome. use futures::Stream; use std::collections::HashMap; use std::net::SocketAddr; use std::pin::Pin; use std::sync::{Arc, Mutex}; use std::task::{Context, Poll, Waker}; use url::Url; use uuid::Uuid; use async_channel::{unbounded, Receiver, Sender, TryRecvError}; use hyper::service::{make_service_fn, service_fn}; use hyper::{Body, Request, Response}; use thiserror::Error; const QUERY_PARAM_NAME: &'static str = "_async_webhook_id"; #[derive(Debug, Error)] enum Error { #[error("Webhook receiver expected a unique UID but none was found")] WebhookMissingUid, } /// Responsible for negotiating outgoing requests and incoming callbacks; resolves /// the Webhook future with received values struct Registry { /// This channel will be notified when a Webhook is dropped /// to ensure it is removed drop: Sender<Uuid>, /// Outstanding requests pending a callback requests: Arc<Mutex<HashMap<Uuid, (Arc<Mutex<Option<Waker>>>, Sender<Request<Body>>)>>>, } impl Clone for Registry { fn clone(&self) -> Self { Registry { drop: self.drop.clone(), requests: Arc::clone(&self.requests), } } } impl Registry { /// Create a new Registry fn new() -> Self { let (drop, mut handle_drop) = unbounded::<Uuid>(); let requests = Arc::new(Mutex::new(HashMap::new())); { let requests = requests.clone(); use tokio::stream::StreamExt; tokio::task::spawn(async move { while let Some(id) = handle_drop.next().await { requests.lock().unwrap().remove(&id); } }); } Self { drop, requests } } fn register(&self, uid: Uuid) -> (Arc<Mutex<Option<Waker>>>, Receiver<Request<Body>>) { let (sender, receiver) = unbounded(); let mut g = self.requests.lock().unwrap(); let waker = Arc::new(Mutex::new(None)); g.insert(uid, (waker.clone(), sender)); (waker, receiver) } fn notify(&self, uid: Uuid, t: Request<Body>) { let mut g = self.requests.lock().unwrap(); if let Some((waker, sender)) = g.remove(&uid) { // Dispatch the request to the async Webhook // Can we be certain that the opposite side is not closed? sender.try_send(t).expect( "Webhook couldn't have been droped, because, otherwise, requests would have been locked and the uid removed!", ); // We put the request on the channel, time to wake up the pending Webhook: Future waker.lock().unwrap().as_ref().map(|waker| { waker.wake_by_ref(); }); } } } struct WebhookInner { url: Url, uid: Uuid, value: Receiver<Request<Body>>, dropper: Sender<Uuid>, waker: Arc<Mutex<Option<Waker>>>, } impl Drop for WebhookInner { fn drop(&mut self) { self.dropper .try_send(self.uid.clone()) // can we handle this more appropriately .expect("Registry should have an open channel to receive dropped requests"); } } pub struct Webhook { inner: WebhookInner, } impl Webhook { /// Return pub fn url(&self) -> &Url { &self.inner.url } } impl Stream for Webhook { type Item = Request<Body>; fn poll_next(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Option<Self::Item>> { let this = Pin::into_inner(self); // Set the waker. This will wake whenever a matching ID // is sent to the associated Server *this.inner.waker.lock().unwrap() = Some(_cx.waker().clone()); match this.inner.value.try_recv() { // .. Ok(l) => Poll::Ready(Some(l)), // Nothing yet! Err(TryRecvError::Empty) => Poll::Pending, // If the server drops Err(TryRecvError::Closed) => Poll::Ready(None), } } } /// A fully kitted-callback handling server that can handle async communication /// over HTTP-Webhooks #[derive(Clone)] pub struct Server { inner: Arc<Inner>, } // This is so that cloned servers don't trigger the drop condition. Is it a common pattern? struct Inner { endpoint: url::Url, registry: Registry, stop: Sender<()>, } impl Drop for Inner { fn drop(&mut self) { self.stop.try_send(()).ok(); } } impl Server { /// Create a new callback server on the provided address /// /// # Example /// /// ```ignore /// let fish = Server::start("127.0.0.1:3306"); /// /// let hook = fish.spawn(); /// /// let _ = async { /// reqwest::get(format!("http://some.api/?callback={}", hook.url())).await /// }; /// /// // don't forget to spin up the server! /// server.await pub fn start<A: Into<SocketAddr>>(addr: A) -> Self { let addr = addr.into(); let endpoint = Url::parse(format!("http://{}", addr).as_str()).unwrap(); Self::with_endpoint(addr, endpoint) } /// Create a callback server but with a proxy URL that will forward traffic /// there. E.g. you could use `ngrok` to tunnel a public Url to your /// development server's `fish::Server` pub fn start_with_proxy<A: Into<SocketAddr>>(addr: A, proxy: Url) -> Self { Self::with_endpoint(addr.into(), proxy) } fn with_endpoint(addr: SocketAddr, endpoint: Url) -> Self { let registry = Registry::new(); type BoxError = Box<dyn std::error::Error + Send + Sync>; async fn handle_request( registry: Registry, request: Request<Body>, ) -> Result<Response<Body>, BoxError> { let registry = registry.clone(); // println!("Received request at {:?}", request.uri()); // this is a relative URL. Quick hack to make this parse to extract the query // parameter -- let url = format!("http://localhost{}", request.uri()); let url = Url::parse(url.as_str()).expect("hyper::Uri is url::Url parsable"); let uid = url .query_pairs() // the webhook should contain our outgoing query parameter // with a UID .find(|(k, _)| k.as_ref() == QUERY_PARAM_NAME) .map(|(_, uid)| { // that UID should be a UUID Uuid::parse_str(uid.as_ref()).map_err(|_| Error::WebhookMissingUid) }) .ok_or_else(|| Error::WebhookMissingUid)??; registry.notify(uid, request); Ok(Response::new(Body::from("Hello World"))) } let keep_registry = registry.clone(); let make_service = make_service_fn(move |_conn| { let registry = registry.clone(); async move { Ok::<_, Error>(service_fn(move |request| { let registry = registry.clone(); handle_request(registry, request) })) } }); let (stop, on_stop) = async_channel::bounded::<()>(16); // Then bind and serve... let server = hyper::Server::bind(&addr) .serve(make_service) .with_graceful_shutdown(async move { on_stop.recv().await.ok(); }); // Lost to sea tokio::spawn(async { server.await }); Server { inner: Arc::new(Inner { registry: keep_registry, stop, endpoint, }), } } /// Spawn a webhook /// /// **Example** /// /// ```rust,ignore /// let server = Server::new(([127, 0, 0, 1], 3031)); /// let webhook = server.spawn(); /// // make your call /// // e.g. http_client.body(webhook.url()).send().await /// webhook.await /// ``` pub fn spawn(&self) -> Webhook { let uid = Uuid::new_v4(); let url = { let mut url = self.inner.endpoint.clone(); url.query_pairs_mut() .append_pair(QUERY_PARAM_NAME, uid.to_string().as_str()); url }; let (waker, value) = self.inner.registry.register(uid.clone()); let dropper = self.inner.registry.drop.clone(); Webhook { inner: WebhookInner { url, uid, value, dropper, waker, }, } } } #[cfg(test)] mod tests { use crate::{Server, Webhook}; use futures::{StreamExt, TryStreamExt}; use serde::{Deserialize, Serialize}; #[tokio::test] async fn test_callback_pattern() { // Launches a dummy API and callback server // and tests that the callback is received and equals what was // expected use serde::{Deserialize, Serialize}; #[derive(Serialize, Deserialize)] struct Request { message: String, callback: String, } #[derive(Serialize, Deserialize)] struct Response { message: String, } eprintln!("Spawning API Server"); // The API Server gets hit and then sends a HTTP request to the URL // provided in the POST body `Request.callback` let api_server = { use warp::Filter; let api_route = warp::any() .and(warp::body::json::<Request>()) .and_then(|req: Request| async { let client = reqwest::Client::new(); eprintln!( "API received message, sending callback to: {}", req.callback.as_str() ); match client .post(req.callback.as_str()) .json(&Response { message: req.message, }) .send() .await { Ok(r) => { eprintln!("Webhook server responded with status: {}", r.status()); Ok(warp::reply()) } Err(_) => Err(warp::reject::reject()), } }); tokio::spawn(warp::serve(api_route).run(([127, 0, 0, 1], 3032))) }; eprintln!("Starting callback server"); // Launch fish server on 3031 let server: Server = Server::start(([127, 0, 0, 1], 3031)); // Run the tests! woot eprintln!("Sending API request"); let mut webhook: Webhook = server.spawn(); let client = reqwest::Client::new(); client .post("http://localhost:3032") .json(&Request { callback: webhook.url().to_string(), message: "hey".to_string(), }) .send() .await; let res: Option<Response> = if let Some(response) = webhook.next().await { let bytes = response .into_body() .try_fold(Vec::new(), |mut data, chunk| async move { data.extend_from_slice(&chunk); Ok(data) }) .await .unwrap(); serde_json::from_slice(bytes.as_slice()).unwrap() } else { None }; assert_eq!(res.unwrap().message.as_str(), "hey"); } }