neon/proxy/src/rate_limiter/limiter.rs

use std::{
    sync::{
        atomic::{AtomicUsize, Ordering},
        Arc,
    },
    time::Duration,
};

use dashmap::DashMap;
use parking_lot::Mutex;
use smol_str::SmolStr;
use tokio::sync::{Mutex as AsyncMutex, Semaphore, SemaphorePermit};
use tokio::time::{timeout, Instant};
use tracing::info;

use super::{
    limit_algorithm::{LimitAlgorithm, Sample},
    RateLimiterConfig,
};

// Simple per-endpoint rate limiter.
//
// Check that number of connections to the endpoint is below `max_rps` rps.
// Purposefully ignore user name and database name as clients can reconnect
// with different names, so we'll end up sending some http requests to
// the control plane.
//
// We also may save quite a lot of CPU (I think) by bailing out right after we
// saw SNI, before doing TLS handshake. User-side error messages in that case
// does not look very nice (`SSL SYSCALL error: Undefined error: 0`), so for now
// I went with a more expensive way that yields user-friendlier error messages.
//
// TODO: add a better bucketing here, e.g. not more than 300 requests per second,
//       and not more than 1000 requests per 10 seconds, etc. Short bursts of reconnects
//       are noramal during redeployments, so we should not block them.
pub struct EndpointRateLimiter {
    map: DashMap<SmolStr, Arc<Mutex<(chrono::NaiveTime, u32)>>>,
    max_rps: u32,
    access_count: AtomicUsize,
}

impl EndpointRateLimiter {
    pub fn new(max_rps: u32) -> Self {
        Self {
            map: DashMap::new(),
            max_rps,
            access_count: AtomicUsize::new(1), // start from 1 to avoid GC on the first request
        }
    }

    /// Check that number of connections to the endpoint is below `max_rps` rps.
    pub fn check(&self, endpoint: SmolStr) -> bool {
        // do GC every 100k requests (worst case memory usage is about 10MB)
        if self.access_count.fetch_add(1, Ordering::AcqRel) % 100_000 == 0 {
            self.do_gc();
        }

        let now = chrono::Utc::now().naive_utc().time();
        let entry = self
            .map
            .entry(endpoint)
            .or_insert_with(|| Arc::new(Mutex::new((now, 0))));
        let mut entry = entry.lock();
        let (last_time, count) = *entry;

        if now - last_time < chrono::Duration::seconds(1) {
            if count >= self.max_rps {
                return false;
            }
            *entry = (last_time, count + 1);
        } else {
            *entry = (now, 1);
        }
        true
    }

    /// Clean the map. Simple strategy: remove all entries. At worst, we'll
    /// double the effective max_rps during the cleanup. But that way deletion
    /// does not aquire mutex on each entry access.
    pub fn do_gc(&self) {
        info!(
            "cleaning up endpoint rate limiter, current size = {}",
            self.map.len()
        );
        self.map.clear();
    }
}

/// Limits the number of concurrent jobs.
///
/// Concurrency is limited through the use of [Token]s. Acquire a token to run a job, and release the
/// token once the job is finished.
///
/// The limit will be automatically adjusted based on observed latency (delay) and/or failures
/// caused by overload (loss).
pub struct Limiter {
    limit_algo: AsyncMutex<Box<dyn LimitAlgorithm>>,
    semaphore: std::sync::Arc<Semaphore>,
    config: RateLimiterConfig,

    // ONLY WRITE WHEN LIMIT_ALGO IS LOCKED
    limits: AtomicUsize,

    // ONLY USE ATOMIC ADD/SUB
    in_flight: Arc<AtomicUsize>,

    #[cfg(test)]
    notifier: Option<std::sync::Arc<tokio::sync::Notify>>,
}

/// A concurrency token, required to run a job.
///
/// Release the token back to the [Limiter] after the job is complete.
#[derive(Debug)]
pub struct Token<'t> {
    permit: Option<tokio::sync::SemaphorePermit<'t>>,
    start: Instant,
    in_flight: Arc<AtomicUsize>,
}

/// A snapshot of the state of the [Limiter].
///
/// Not guaranteed to be consistent under high concurrency.
#[derive(Debug, Clone, Copy)]
pub struct LimiterState {
    limit: usize,
    available: usize,
    in_flight: usize,
}

/// Whether a job succeeded or failed as a result of congestion/overload.
///
/// Errors not considered to be caused by overload should be ignored.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Outcome {
    /// The job succeeded, or failed in a way unrelated to overload.
    Success,
    /// The job failed because of overload, e.g. it timed out or an explicit backpressure signal
    /// was observed.
    Overload,
}

impl Outcome {
    fn from_reqwest_error(error: &reqwest_middleware::Error) -> Self {
        match error {
            reqwest_middleware::Error::Middleware(_) => Outcome::Success,
            reqwest_middleware::Error::Reqwest(e) => {
                if let Some(status) = e.status() {
                    if status.is_server_error()
                        || reqwest::StatusCode::TOO_MANY_REQUESTS.as_u16() == status
                    {
                        Outcome::Overload
                    } else {
                        Outcome::Success
                    }
                } else {
                    Outcome::Success
                }
            }
        }
    }
    fn from_reqwest_response(response: &reqwest::Response) -> Self {
        if response.status().is_server_error()
            || response.status() == reqwest::StatusCode::TOO_MANY_REQUESTS
        {
            Outcome::Overload
        } else {
            Outcome::Success
        }
    }
}

impl Limiter {
    /// Create a limiter with a given limit control algorithm.
    pub fn new(config: RateLimiterConfig) -> Self {
        assert!(config.initial_limit > 0);
        Self {
            limit_algo: AsyncMutex::new(config.create_rate_limit_algorithm()),
            semaphore: Arc::new(Semaphore::new(config.initial_limit)),
            config,
            limits: AtomicUsize::new(config.initial_limit),
            in_flight: Arc::new(AtomicUsize::new(0)),
            #[cfg(test)]
            notifier: None,
        }
    }
    // pub fn new(limit_algorithm: T, timeout: Duration, initial_limit: usize) -> Self {
    //     assert!(initial_limit > 0);

    //     Self {
    //         limit_algo: AsyncMutex::new(limit_algorithm),
    //         semaphore: Arc::new(Semaphore::new(initial_limit)),
    //         timeout,
    //         limits: AtomicUsize::new(initial_limit),
    //         in_flight: Arc::new(AtomicUsize::new(0)),
    //         #[cfg(test)]
    //         notifier: None,
    //     }
    // }

    /// In some cases [Token]s are acquired asynchronously when updating the limit.
    #[cfg(test)]
    pub fn with_release_notifier(mut self, n: std::sync::Arc<tokio::sync::Notify>) -> Self {
        self.notifier = Some(n);
        self
    }

    /// Try to immediately acquire a concurrency [Token].
    ///
    /// Returns `None` if there are none available.
    pub fn try_acquire(&self) -> Option<Token> {
        let result = if self.config.disable {
            // If the rate limiter is disabled, we can always acquire a token.
            Some(Token::new(None, self.in_flight.clone()))
        } else {
            self.semaphore
                .try_acquire()
                .map(|permit| Token::new(Some(permit), self.in_flight.clone()))
                .ok()
        };
        if result.is_some() {
            self.in_flight.fetch_add(1, Ordering::AcqRel);
        }
        result
    }

    /// Try to acquire a concurrency [Token], waiting for `duration` if there are none available.
    ///
    /// Returns `None` if there are none available after `duration`.
    pub async fn acquire_timeout(&self, duration: Duration) -> Option<Token<'_>> {
        info!("acquiring token: {:?}", self.semaphore.available_permits());
        let result = if self.config.disable {
            // If the rate limiter is disabled, we can always acquire a token.
            Some(Token::new(None, self.in_flight.clone()))
        } else {
            match timeout(duration, self.semaphore.acquire()).await {
                Ok(maybe_permit) => maybe_permit
                    .map(|permit| Token::new(Some(permit), self.in_flight.clone()))
                    .ok(),
                Err(_) => None,
            }
        };
        if result.is_some() {
            self.in_flight.fetch_add(1, Ordering::AcqRel);
        }
        result
    }

    /// Return the concurrency [Token], along with the outcome of the job.
    ///
    /// The [Outcome] of the job, and the time taken to perform it, may be used
    /// to update the concurrency limit.
    ///
    /// Set the outcome to `None` to ignore the job.
    pub async fn release(&self, mut token: Token<'_>, outcome: Option<Outcome>) {
        tracing::info!("outcome is {:?}", outcome);
        let in_flight = self.in_flight.load(Ordering::Acquire);
        let old_limit = self.limits.load(Ordering::Acquire);
        let available = if self.config.disable {
            0 // This is not used in the algorithm and can be anything. If the config disable it makes sense to set it to 0.
        } else {
            self.semaphore.available_permits()
        };
        let total = in_flight + available;

        let mut algo = self.limit_algo.lock().await;

        let new_limit = if let Some(outcome) = outcome {
            let sample = Sample {
                latency: token.start.elapsed(),
                in_flight,
                outcome,
            };
            algo.update(old_limit, sample).await
        } else {
            old_limit
        };
        tracing::info!("new limit is {}", new_limit);
        let actual_limit = if new_limit < total {
            token.forget();
            total.saturating_sub(1)
        } else {
            if !self.config.disable {
                self.semaphore.add_permits(new_limit.saturating_sub(total));
            }
            new_limit
        };
        crate::proxy::RATE_LIMITER_LIMIT
            .with_label_values(&["expected"])
            .set(new_limit as i64);
        crate::proxy::RATE_LIMITER_LIMIT
            .with_label_values(&["actual"])
            .set(actual_limit as i64);
        self.limits.store(new_limit, Ordering::Release);
        #[cfg(test)]
        if let Some(n) = &self.notifier {
            n.notify_one();
        }
    }

    /// The current state of the limiter.
    pub fn state(&self) -> LimiterState {
        let limit = self.limits.load(Ordering::Relaxed);
        let in_flight = self.in_flight.load(Ordering::Relaxed);
        LimiterState {
            limit,
            available: limit.saturating_sub(in_flight),
            in_flight,
        }
    }
}

impl<'t> Token<'t> {
    fn new(permit: Option<SemaphorePermit<'t>>, in_flight: Arc<AtomicUsize>) -> Self {
        Self {
            permit,
            start: Instant::now(),
            in_flight,
        }
    }

    #[cfg(test)]
    pub fn set_latency(&mut self, latency: Duration) {
        use std::ops::Sub;

        self.start = Instant::now().sub(latency);
    }

    pub fn forget(&mut self) {
        if let Some(permit) = self.permit.take() {
            permit.forget();
        }
    }
}

impl Drop for Token<'_> {
    fn drop(&mut self) {
        self.in_flight.fetch_sub(1, Ordering::AcqRel);
    }
}

impl LimiterState {
    /// The current concurrency limit.
    pub fn limit(&self) -> usize {
        self.limit
    }
    /// The amount of concurrency available to use.
    pub fn available(&self) -> usize {
        self.available
    }
    /// The number of jobs in flight.
    pub fn in_flight(&self) -> usize {
        self.in_flight
    }
}

#[async_trait::async_trait]
impl reqwest_middleware::Middleware for Limiter {
    async fn handle(
        &self,
        req: reqwest::Request,
        extensions: &mut task_local_extensions::Extensions,
        next: reqwest_middleware::Next<'_>,
    ) -> reqwest_middleware::Result<reqwest::Response> {
        let start = Instant::now();
        let token = self
            .acquire_timeout(self.config.timeout)
            .await
            .ok_or_else(|| {
                reqwest_middleware::Error::Middleware(
                    // TODO: Should we map it into user facing errors?
                    crate::console::errors::ApiError::Console {
                        status: crate::http::StatusCode::TOO_MANY_REQUESTS,
                        text: "Too many requests".into(),
                    }
                    .into(),
                )
            })?;
        info!(duration = ?start.elapsed(), "waiting for token to connect to the control plane");
        crate::proxy::RATE_LIMITER_ACQUIRE_LATENCY.observe(start.elapsed().as_secs_f64());
        match next.run(req, extensions).await {
            Ok(response) => {
                self.release(token, Some(Outcome::from_reqwest_response(&response)))
                    .await;
                Ok(response)
            }
            Err(e) => {
                self.release(token, Some(Outcome::from_reqwest_error(&e)))
                    .await;
                Err(e)
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use std::{pin::pin, task::Context, time::Duration};

    use futures::{task::noop_waker_ref, Future};

    use super::{Limiter, Outcome};
    use crate::rate_limiter::RateLimitAlgorithm;

    #[tokio::test]
    async fn it_works() {
        let config = super::RateLimiterConfig {
            algorithm: RateLimitAlgorithm::Fixed,
            timeout: Duration::from_secs(1),
            initial_limit: 10,
            disable: false,
            ..Default::default()
        };
        let limiter = Limiter::new(config);

        let token = limiter.try_acquire().unwrap();

        limiter.release(token, Some(Outcome::Success)).await;

        assert_eq!(limiter.state().limit(), 10);
    }

    #[tokio::test]
    async fn is_fair() {
        let config = super::RateLimiterConfig {
            algorithm: RateLimitAlgorithm::Fixed,
            timeout: Duration::from_secs(1),
            initial_limit: 1,
            disable: false,
            ..Default::default()
        };
        let limiter = Limiter::new(config);

        // === TOKEN 1 ===
        let token1 = limiter.try_acquire().unwrap();

        let mut token2_fut = pin!(limiter.acquire_timeout(Duration::from_secs(1)));
        assert!(
            token2_fut
                .as_mut()
                .poll(&mut Context::from_waker(noop_waker_ref()))
                .is_pending(),
            "token is acquired by token1"
        );

        let mut token3_fut = pin!(limiter.acquire_timeout(Duration::from_secs(1)));
        assert!(
            token3_fut
                .as_mut()
                .poll(&mut Context::from_waker(noop_waker_ref()))
                .is_pending(),
            "token is acquired by token1"
        );

        limiter.release(token1, Some(Outcome::Success)).await;
        // === END TOKEN 1 ===

        // === TOKEN 2 ===
        assert!(
            limiter.try_acquire().is_none(),
            "token is acquired by token2"
        );

        assert!(
            token3_fut
                .as_mut()
                .poll(&mut Context::from_waker(noop_waker_ref()))
                .is_pending(),
            "token is acquired by token2"
        );

        let token2 = token2_fut.await.unwrap();

        limiter.release(token2, Some(Outcome::Success)).await;
        // === END TOKEN 2 ===

        // === TOKEN 3 ===
        assert!(
            limiter.try_acquire().is_none(),
            "token is acquired by token3"
        );

        let token3 = token3_fut.await.unwrap();
        limiter.release(token3, Some(Outcome::Success)).await;
        // === END TOKEN 3 ===

        // === TOKEN 4 ===
        let token4 = limiter.try_acquire().unwrap();
        limiter.release(token4, Some(Outcome::Success)).await;
    }

    #[tokio::test]
    async fn disable() {
        let config = super::RateLimiterConfig {
            algorithm: RateLimitAlgorithm::Fixed,
            timeout: Duration::from_secs(1),
            initial_limit: 1,
            disable: true,
            ..Default::default()
        };
        let limiter = Limiter::new(config);

        // === TOKEN 1 ===
        let token1 = limiter.try_acquire().unwrap();
        let token2 = limiter.try_acquire().unwrap();
        let state = limiter.state();
        assert_eq!(state.limit(), 1);
        assert_eq!(state.in_flight(), 2); // For disabled limiter, it's expected.
        limiter.release(token1, None).await;
        limiter.release(token2, None).await;
    }
}