# Hystrix — Latency and Fault Tolerance Library for Distributed Systems

> Hystrix is a Netflix open-source library that isolates remote service calls, prevents cascading failures, and provides real-time monitoring dashboards for JVM-based microservice architectures.

## Install

Save in your project root:

# Hystrix — Latency and Fault Tolerance for Distributed Systems

## Quick Use
```xml
<!-- Maven dependency -->
<dependency>
  <groupId>com.netflix.hystrix</groupId>
  <artifactId>hystrix-core</artifactId>
  <version>1.5.18</version>
</dependency>
```
```java
public class GetOrderCommand extends HystrixCommand<Order> {
    protected Order run() { return orderService.get(id); }
    protected Order getFallback() { return Order.empty(); }
}
```

## Introduction
Hystrix implements the circuit breaker pattern for JVM applications. When a downstream service becomes slow or unavailable, Hystrix short-circuits requests and routes them to a fallback, protecting the calling service from cascading failures.

## What Hystrix Does
- Wraps remote calls in isolated thread pools or semaphores to contain failures
- Opens a circuit breaker when error rates exceed a configurable threshold
- Routes failed or timed-out requests to developer-defined fallback logic
- Publishes real-time metrics via an event stream for the Hystrix Dashboard
- Provides request caching and request collapsing for batching

## Architecture Overview
Each Hystrix command runs inside its own thread pool (bulkhead pattern), preventing a slow dependency from consuming all application threads. A sliding-window health monitor tracks success, failure, timeout, and rejection rates. When failures cross the threshold, the circuit opens and all requests go to the fallback for a configurable sleep window before half-open probing resumes.

## Self-Hosting & Configuration
- Add hystrix-core as a Maven or Gradle dependency in any JVM project
- Configure per-command thread pool sizes, timeouts, and circuit breaker thresholds via Archaius or static properties
- Deploy the Hystrix Dashboard WAR alongside your service for real-time monitoring
- Use Turbine to aggregate metrics streams across a cluster
- Integrates with Spring Cloud Netflix for annotation-driven usage

## Key Features
- Bulkhead isolation keeps failing dependencies from taking down the whole service
- Configurable circuit breaker with automatic recovery probing
- Real-time streaming dashboard for observing command health
- Request collapsing reduces fan-out by batching concurrent calls
- Mature and battle-tested at Netflix scale

## Comparison with Similar Tools
- **Resilience4j** — modern lightweight successor designed for Java 8+; recommended for new projects since Hystrix is in maintenance mode
- **Sentinel (Alibaba)** — broader flow control and system protection; more features but heavier
- **Polly (.NET)** — equivalent circuit breaker library for the .NET ecosystem
- **Istio/Envoy** — implements circuit breaking at the service mesh layer rather than in application code

## FAQ
**Q: Is Hystrix still maintained?**
A: Hystrix entered maintenance mode in 2018. Netflix recommends Resilience4j for new projects, but Hystrix remains stable and widely deployed.

**Q: Can I use Hystrix without Spring Cloud?**
A: Yes. Hystrix is a standalone library; Spring Cloud Netflix is an optional integration layer.

**Q: How does the circuit breaker decide when to open?**
A: It uses a rolling time window to track error percentages. When the error rate exceeds the configured threshold and minimum request volume, the circuit opens.

**Q: Does Hystrix work with reactive frameworks?**
A: Hystrix includes an RxJava-based observable command variant for reactive pipelines.

## Sources
- https://github.com/Netflix/Hystrix
- https://github.com/Netflix/Hystrix/wiki

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