Crate tower_resilience_outlier 

Outlier detection middleware for Tower services.

Outlier detection tracks per-instance health on live traffic and ejects unhealthy instances from a fleet. This is complementary to circuit breaker:

	Circuit Breaker	Outlier Detection
Trigger	Failure rate over sliding window	Consecutive errors
Scope	Per-service, isolated	Fleet-aware (max_ejection_percent)
Detection speed	Needs minimum_calls before evaluating	Catches hard-down immediately
Recovery	Half-open state with probe calls	Time-based automatic recovery

Quick Start

use tower_resilience_outlier::{OutlierDetectionLayer, OutlierDetector};
use tower::{ServiceBuilder, service_fn};
use std::time::Duration;

// Create a shared detector for the fleet
let detector = OutlierDetector::new()
    .max_ejection_percent(50)
    .base_ejection_duration(Duration::from_secs(30));

// Register instances
detector.register("backend-1", 5);  // eject after 5 consecutive errors
detector.register("backend-2", 5);

// Create per-instance layers sharing the same detector
let layer1 = OutlierDetectionLayer::builder()
    .detector(detector.clone())
    .instance_name("backend-1")
    .build();

let layer2 = OutlierDetectionLayer::builder()
    .detector(detector.clone())
    .instance_name("backend-2")
    .build();

// Apply to services
let svc1 = ServiceBuilder::new()
    .layer(layer1)
    .service(service_fn(|req: String| async move { Ok::<_, std::io::Error>(req) }));

Backpressure vs Error Mode

By default, ejected instances return Pending from poll_ready(), which causes Tower load balancers to route around them. For cases where you want an explicit error, use .error_on_ejection():

let layer = OutlierDetectionLayer::builder()
    .detector(detector)
    .instance_name("backend-1")
    .error_on_ejection()
    .build();

Re-exports

pub use config::OutlierDetectionConfig;

pub use config::OutlierDetectionConfigBuilder;

pub use detector::OutlierDetector;

pub use error::OutlierDetectionError;

pub use error::OutlierDetectionServiceError;

pub use events::OutlierDetectionEvent;

pub use layer::OutlierDetectionLayer;

pub use service::OutlierDetectionService;

pub use strategy::ConsecutiveErrors;

pub use strategy::EjectionStrategy;

Modules

config

Configuration types for outlier detection. Configuration and builder for the outlier detection middleware.

detector

Shared fleet-level outlier detector state. Shared fleet-level outlier detector state.

error

Error types for outlier detection. Error types for the outlier detection middleware.

events

Event types emitted by outlier detection. Events emitted by the outlier detection middleware.

layer

Tower Layer implementation for outlier detection. Tower Layer implementation for outlier detection.

service

Tower Service implementation for outlier detection. Tower Service implementation for outlier detection.

strategy

Ejection strategies (e.g., consecutive errors). Ejection strategies for outlier detection.