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//! Easily round-robin between servers providing the same service, automatically reconnecting to the //! next server should an error happen. //! //! # Example //! //! ```rust //! use async_trait::async_trait; //! use std::{io::Error, net::IpAddr}; //! use tokio::{io::AsyncReadExt, net::TcpStream, sync::Mutex}; //! use tourniquet::{Connector, RoundRobin}; //! //! struct Conn(u16); //! //! #[async_trait] //! impl Connector<IpAddr, Mutex<TcpStream>, Error> for Conn { //! async fn connect(&self, src: &IpAddr) -> Result<Mutex<TcpStream>, Error> { //! let Conn(ref port) = self; //! TcpStream::connect((*src, *port)).await.map(Mutex::new) //! } //! } //! //! #[tokio::main] //! async fn main() { //! let rr = RoundRobin::new( //! vec!["185.30.166.38".parse().unwrap(), "66.110.9.37".parse().unwrap()], //! Conn(6667), //! ); //! //! let hello = rr.run(|sock| async move { //! let mut sock = sock.lock().await; //! let mut buf = [0; 50]; //! sock.read_exact(&mut buf).await.map(|_| String::from_utf8(buf.to_vec()).unwrap()) //! }).await.unwrap(); //! //! assert!(hello.contains("freenode.net")); //! } //! ``` use core::future::Future; use std::{ fmt::{Debug, Display}, marker::PhantomData, sync::atomic::{AtomicUsize, Ordering}, sync::Arc, }; pub use async_trait::async_trait; use tokio::sync::RwLock; #[cfg(feature = "tracing")] use tracing::{ field::{debug, display, Empty}, instrument, Instrument, Span, }; /// Trait indicating wether an error mandates trying the next service. /// /// It is returned by the round-robin handler or the connector, and indicates wether we should /// try the next service, or if it should abort and bubble up the error to the caller. /// /// A next error mandates trying the next service in the line. It is an error that could be solved /// by trying another server. This includes IO errors, server-side errors, etc. On the other hand, /// business errors should not be a next error since another server will most likely yield the same /// error (resource not found, permission denied, ...). /// /// Basically, a server that yields a Next error should be considered unhealthy. /// /// # Example /// /// ```rust /// # use tourniquet::Next; /// # /// enum MyError { /// NotFound, /// PermissionDenied, /// InternalError, /// Timeout, /// } /// /// impl Next for MyError { /// fn is_next(&self) -> bool { /// match self { /// // Business logic error, that are likely to happen on all servers /// Self::NotFound | Self::PermissionDenied => false, /// // Server specific error: server software down, host down, etc. Try the next one /// Self::InternalError | Self::Timeout => true, /// } /// } /// } /// ``` pub trait Next { /** * If true, the error is non-fatal and the next service in the list will be tried. */ fn is_next(&self) -> bool; } impl Next for std::io::Error { fn is_next(&self) -> bool { use std::io::ErrorKind::*; match self.kind() { ConnectionRefused | ConnectionReset | ConnectionAborted | NotConnected | BrokenPipe | TimedOut | Interrupted | UnexpectedEof => true, NotFound | PermissionDenied | AddrInUse | AddrNotAvailable | AlreadyExists | WouldBlock | InvalidInput | InvalidData | Other => false, _ => false, } } } /// Trait to be implemented by connector types. Used to get a connected service from its connection /// information. /// /// Note that the implementor type can hold data for connection information shared across all /// providers, like TLS certificates, port, database name, ... /// /// # Example /// /// ```rust /// # use async_trait::async_trait; /// # use std::sync::Mutex; /// use std::{io::Error, net::IpAddr}; /// use tokio::net::TcpStream; /// use tourniquet::Connector; /// /// struct Conn(u16); /// /// #[async_trait] /// impl Connector<IpAddr, Mutex<TcpStream>, Error> for Conn { /// async fn connect(&self, src: &IpAddr) -> Result<Mutex<TcpStream>, Error> { /// let Conn(ref port) = self; /// TcpStream::connect((*src, *port)).await.map(Mutex::new) /// } /// } /// ``` #[async_trait] pub trait Connector<SvcSrc, Svc, E> { async fn connect(&self, src: &SvcSrc) -> Result<Svc, E>; } /// Round Robin manager. /// /// This holds a list of services, a way to connect to said services, and a way to run stuff against /// this connected service. pub struct RoundRobin<SvcSrc, Svc, E, Conn> where Conn: Connector<SvcSrc, Svc, E>, { /// Sources used to connect to a service. Usually some form of URL to attempt a connection, /// e.g. `amqp://localhost:5672` sources: Vec<SvcSrc>, /// Async connection handler. connector: Conn, /// How many services to try before giving up. Defaults to the service count. max_attempts: usize, /// Already connected service handler. We use Arc here to be able to easily clone the service /// handler and avoid issues with references, as they don't play nicely with futures. service: RwLock<Option<Arc<Svc>>>, /// Current service source being connected current: AtomicUsize, _phantom: PhantomData<E>, } impl<SvcSrc, Svc, E, Conn> RoundRobin<SvcSrc, Svc, E, Conn> where SvcSrc: Debug, E: Next + Display, Conn: Connector<SvcSrc, Svc, E>, { /// Build a new round-robin manager. /// /// The connector is a struct that yields a connected handler to the service, from its service /// "source" (usually an URL). Note that the connector is lazily executed on demand when a /// connection is needed. /// /// # Example /// /// ```rust /// # use async_trait::async_trait; /// # use std::sync::Mutex; /// use std::{io::Error, net::IpAddr}; /// use tokio::net::TcpStream; /// use tourniquet::{Connector, RoundRobin}; /// /// struct Conn(u16); /// /// #[async_trait] /// impl Connector<IpAddr, Mutex<TcpStream>, Error> for Conn { /// async fn connect(&self, src: &IpAddr) -> Result<Mutex<TcpStream>, Error> { /// let Conn(ref port) = self; /// TcpStream::connect((*src, *port)).await.map(Mutex::new) /// } /// } /// /// # #[tokio::main] /// # async fn main() { /// let rr = RoundRobin::new( /// vec!["185.30.166.38".parse().unwrap(), "66.110.9.37".parse().unwrap()], /// Conn(6667), /// ); /// # } /// ``` pub fn new(sources: Vec<SvcSrc>, connector: Conn) -> Self { Self { max_attempts: sources.len(), sources, connector, service: RwLock::new(None), current: AtomicUsize::new(0), _phantom: PhantomData::default(), } } /// Set how many times we will try the next service in case of failure. pub fn set_max_attempts(&mut self, count: usize) { self.max_attempts = count; } #[cfg_attr( feature = "tracing", tracing::instrument(skip(self, run), err, fields(service = Empty, index = Empty)), )] async fn run_inner<Run, RunFut, T>(&self, run: &Run, current: usize) -> Result<T, E> where Run: Fn(Arc<Svc>) -> RunFut, RunFut: Future<Output = Result<T, E>>, { let index = current % self.sources.len(); #[cfg(feature = "tracing")] { let span = Span::current(); span.record("index", &display(index)); span.record("service", &debug(&self.sources[index])); } // Connect if not already connected if self.service.read().await.is_none() { *self.service.write().await = Some(Arc::new(self.connector.connect(&self.sources[index]).await?)); } // Run let fut = run(self.service.read().await.clone().unwrap()); #[cfg(feature = "tracing")] let fut = fut.instrument(tracing::debug_span!("run_fn")); let res = fut.await; if let Err(ref e) = res { // Trash handler only if that's a next error and if we didn't already move to the next // provider (e.g. in another concurrent task). if e.is_next() && current == self.current.load(Ordering::Relaxed) { *self.service.write().await = None; } } res } /// Run the provided async function against an established service connection. /// /// The connection to the service will be established at this point if not already established. #[cfg_attr(feature = "tracing", instrument(skip(self, run), err))] pub async fn run<R, Fut, T>(&self, run: R) -> Result<T, E> where R: Fn(Arc<Svc>) -> Fut, Fut: Future<Output = Result<T, E>>, { let n_svc = self.sources.len(); let mut attempts = 0usize; loop { let current = self.current.load(Ordering::Relaxed); match self.run_inner(&run, current).await { Ok(t) => return Ok(t), Err(e) => { if e.is_next() { #[cfg(feature = "tracing")] tracing::error!("Service {}/{} failed: {}", current % n_svc, n_svc, e); #[cfg(not(feature = "tracing"))] eprintln!("Service {}/{} failed: {}", current % n_svc, n_svc, e); self.current.fetch_add(1, Ordering::Relaxed); attempts += 1; if attempts < self.max_attempts { continue; } } return Err(e); } } } } } #[cfg(test)] mod tests { use super::*; use std::sync::{ atomic::{AtomicUsize, Ordering}, Arc, }; #[derive(Debug, PartialEq)] enum Error { Timeout, NotFound, } impl std::fmt::Display for Error { fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result { write!(f, "{:?}", self) } } impl Next for Error { fn is_next(&self) -> bool { *self == Self::Timeout } } struct Conn { count: Arc<AtomicUsize>, ok_from: i32, } #[async_trait] impl Connector<i32, i32, Error> for Conn { async fn connect(&self, src: &i32) -> Result<i32, Error> { self.count.fetch_add(1, Ordering::Relaxed); if *src < self.ok_from { Err(Error::Timeout) } else { Ok(*src) } } } fn build_rr( svcs: Vec<i32>, ok_from: i32, ) -> (RoundRobin<i32, i32, Error, Conn>, Arc<AtomicUsize>) { let count = Arc::new(AtomicUsize::new(0)); let cnt = count.clone(); (RoundRobin::new(svcs, Conn { count: cnt, ok_from }), count) } #[tokio::test] async fn test_first_called() { let (rr, count_conn) = build_rr(vec![0, 1], 0); let count_run = AtomicUsize::new(0); rr.run(|_| async { Ok(count_run.fetch_add(1, Ordering::Relaxed)) }).await.unwrap(); // Async blocks should be called only once assert_eq!(count_conn.load(Ordering::Relaxed), 1); assert_eq!(count_run.load(Ordering::Relaxed), 1); rr.run(|_| async { Ok(count_run.fetch_add(1, Ordering::Relaxed)) }).await.unwrap(); // Connector should not have been called a second time, though the run block should. assert_eq!(count_conn.load(Ordering::Relaxed), 1); assert_eq!(count_run.load(Ordering::Relaxed), 2); } #[tokio::test] async fn test_exhausted_run() { let (mut rr, count_conn) = build_rr(vec![0, 1], 0); let count = AtomicUsize::new(0); // With a single attemt, options will be exhausted rr.set_max_attempts(1); let res = rr .run(|n| { count.fetch_add(1, Ordering::Relaxed); async move { match *n { 0 => Err(Error::Timeout), // Next error _ => Ok(n), } } }) .await; assert_eq!(count_conn.load(Ordering::Relaxed), 1); match res { Ok(_) => panic!("Run did not error"), Err(Error::Timeout) => (), Err(_) => panic!("Wrong error"), } assert_eq!(count.load(Ordering::Relaxed), 1); } #[tokio::test] async fn test_try_next_run() { let (rr, count_conn) = build_rr(vec![0, 1], 0); let count = AtomicUsize::new(0); rr.run(|n| { count.fetch_add(1, Ordering::Relaxed); async move { match *n { 0 => Err(Error::Timeout), // Next error _ => Ok(n), } } }) .await .unwrap(); assert_eq!(count_conn.load(Ordering::Relaxed), 2); assert_eq!(count.load(Ordering::Relaxed), 2); } #[tokio::test] async fn test_exhausted_connector() { let (mut rr, count_conn) = build_rr(vec![0, 1], 1); let count = AtomicUsize::new(0); // With a single attemt, options will be exhausted rr.set_max_attempts(1); let res = rr.run(|_| async { Ok(count.fetch_add(1, Ordering::Relaxed)) }).await; assert_eq!(count_conn.load(Ordering::Relaxed), 1); match res { Ok(_) => panic!("Connect did not error"), Err(Error::Timeout) => (), Err(_) => panic!("Wrong error"), } assert_eq!(count.load(Ordering::Relaxed), 0); } #[tokio::test] async fn test_try_next_connector() { let (rr, count_conn) = build_rr(vec![0, 1], 1); let count = AtomicUsize::new(0); rr.run(|_| async { Ok(count.fetch_add(1, Ordering::Relaxed)) }).await.unwrap(); assert_eq!(count_conn.load(Ordering::Relaxed), 2); assert_eq!(count.load(Ordering::Relaxed), 1); } #[tokio::test] async fn test_abort() { let (rr, _) = build_rr(vec![0, 1], 1); let res = rr.run(|_| async { Err::<(), _>(Error::NotFound) }).await; match res { Ok(_) => panic!("Run did not error"), Err(Error::NotFound) => (), Err(Error::Timeout) => panic!("Connector error aborted"), } } }