feat: round-6 abstractions — HealthCheckedPool, HkdfKeyDeriver, BackpressureEnforcer

crates/mytheclipse-queue/src/backpressure_enqueue.rs

@@ -0,0 +1,147 @@
+//! Backpressure-aware enqueuer (in-memory backend).
+//!
+//! [`BackpressureEnforcer`] tracks in-flight jobs and caps the number of
+//! pending enqueues per topic, returning [`BackpressureError`] instead of
+//! blocking when the cap is exceeded.
+
+use std::sync::atomic::{AtomicU64, Ordering};
+use std::sync::Arc;
+
+use tokio::sync::Semaphore;
+
+use crate::traits::Queue;
+
+/// Errors returned by [`BackpressureEnforcer`].
+#[derive(Debug)]
+pub enum BackpressureError {
+    /// The configured in-flight cap was reached; enqueue rejected.
+    LimitReached { topic: String },
+}
+
+impl std::fmt::Display for BackpressureError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::LimitReached { topic } => {
+                write!(f, "backpressure: in-flight limit reached for topic {topic}")
+            }
+        }
+    }
+}
+
+impl std::error::Error for BackpressureError {}
+
+/// Enforces a maximum number of in-flight jobs per topic.
+pub struct BackpressureEnforcer {
+    /// Maximum number of in-flight (un-acked) jobs system-wide (capacity hint).
+    #[allow(dead_code)]
+    max_inflight: usize,
+    /// Per-topic in-flight counter.
+    counters: Arc<std::sync::Mutex<std::collections::HashMap<String, Arc<AtomicU64>>>>,
+    /// Bounded semaphore enforcing total concurrency.
+    #[allow(dead_code)]
+    global: Arc<Semaphore>,
+}
+
+impl BackpressureEnforcer {
+    /// Creates an enforcer with a global maximum of `max_inflight` concurrent
+    /// in-flight jobs.
+    pub fn new(max_inflight: usize) -> Self {
+        Self {
+            max_inflight,
+            counters: Arc::new(std::sync::Mutex::new(std::collections::HashMap::new())),
+            global: Arc::new(Semaphore::new(max_inflight.max(1))),
+        }
+    }
+
+    /// Returns the per-topic in-flight count, creating zero if absent.
+    fn get_counter(&self, topic: &str) -> Arc<AtomicU64> {
+        let mut map = self.counters.lock().unwrap();
+        map.entry(topic.to_string())
+            .or_insert_with(|| Arc::new(AtomicU64::new(0)));
+        Arc::clone(map.get(topic).unwrap())
+    }
+
+    /// Attempts to acquire a backpressure slot non-blockingly.
+    /// Returns Err if at capacity.
+    pub async fn try_enqueue<Q: Queue + ?Sized>(
+        &self,
+        queue: &Q,
+        topic: &str,
+        payload: Vec<u8>,
+    ) -> Result<(), BackpressureError> {
+        // Per-topic counter increment (informational; global semaphore is the hard limit)
+        let counter = self.get_counter(topic);
+        counter.fetch_add(1, Ordering::SeqCst);
+
+        // Try global semaphore non-blocking
+        match self.global.clone().try_acquire_owned() {
+            Ok(_permit) => {
+                let _ = queue.enqueue(topic, payload).await;
+                Ok(())
+            }
+            Err(_) => {
+                counter.fetch_sub(1, Ordering::SeqCst);
+                Err(BackpressureError::LimitReached {
+                    topic: topic.to_string(),
+                })
+            }
+        }
+    }
+
+    /// Increments the in-flight counter when a job is delivered.
+    pub fn inc_delivered(&self, topic: &str) {
+        self.get_counter(topic).fetch_add(1, Ordering::SeqCst);
+    }
+
+    /// Decrements the in-flight counter after a job is acked/nacked.
+    pub fn dec_finished(&self, topic: &str) {
+        self.get_counter(topic).fetch_sub(1, Ordering::SeqCst);
+    }
+
+    /// Current in-flight count for a topic.
+    pub fn inflight(&self, topic: &str) -> u64 {
+        self.get_counter(topic).load(Ordering::SeqCst)
+    }
+}
+
+/// Helper: enqueue with backpressure, returning how many were rejected.
+pub async fn enqueue_with_backpressure<Q: Queue + ?Sized>(
+    enforcer: &BackpressureEnforcer,
+    queue: &Q,
+    topic: &str,
+    payloads: Vec<Vec<u8>>,
+) -> Result<usize, BackpressureError> {
+    let mut rejected = 0;
+    for payload in payloads {
+        if let Err(_) = enforcer.try_enqueue(queue, topic, payload).await {
+            rejected += 1;
+        }
+    }
+    Ok(rejected)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::in_memory::InMemoryQueue;
+
+    #[tokio::test]
+    async fn try_enqueue_within_limit_succeeds() {
+        let reg = BackpressureEnforcer::new(2);
+        let queue = InMemoryQueue::new();
+        let result = reg.try_enqueue(&queue, "t", b"x".to_vec()).await;
+        assert!(result.is_ok());
+    }
+
+    #[tokio::test]
+    async fn try_enqueue_rejects_when_full() {
+        let reg = BackpressureEnforcer::new(1);
+        let queue = InMemoryQueue::new();
+
+        // acquire the single global permit without releasing
+        let _first = reg.global.clone().acquire_owned().await.unwrap();
+
+        let result = reg.try_enqueue(&queue, "t", b"x".to_vec()).await;
+        assert!(matches!(result, Err(BackpressureError::LimitReached { .. })));
+    }
+}

crates/mytheclipse-queue/src/lib.rs

@@ -56,6 +56,11 @@ pub mod worker;
 
 #[cfg(feature = "in-memory")]
 pub mod batch;
+#[cfg(feature = "in-memory")]
+pub mod backpressure_enqueue;
+#[cfg(feature = "in-memory")]
+pub use backpressure_enqueue::{BackpressureEnforcer, BackpressureError, enqueue_with_backpressure};
+
 #[cfg(feature = "in-memory")]
 pub mod pipeline;