# Finagle — Extensible RPC System for the JVM

> A protocol-agnostic RPC framework from Twitter built on Netty. Finagle provides connection pooling, load balancing, retries, and distributed tracing for building resilient microservices on the JVM.

## Install

Save as a script file and run:

# Finagle — Extensible RPC System for the JVM

## Quick Use
```scala
// build.sbt
libraryDependencies += "com.twitter" %% "finagle-http" % "24.2.0"
```
```scala
import com.twitter.finagle.Http
import com.twitter.finagle.http.{Request, Response, Status}
import com.twitter.util.{Await, Future}

val service = (req: Request) => {
  val rep = Response(req.version, Status.Ok)
  rep.contentString = "Hello from Finagle"
  Future.value(rep)
}
val server = Http.serve(":8080", service)
Await.ready(server)
```

## Introduction
Finagle is a JVM-based RPC framework developed by Twitter (now X) for building high-throughput, fault-tolerant distributed systems. It abstracts transport details behind a uniform client-server API and provides built-in observability, load balancing, and failure management.

## What Finagle Does
- Provides protocol-agnostic client and server building blocks for HTTP, Thrift, gRPC, MySQL, and more
- Implements client-side load balancing with pluggable strategies (round-robin, least-loaded, aperture)
- Handles retries, timeouts, circuit breaking, and rate limiting out of the box
- Integrates with Zipkin for distributed tracing across service boundaries
- Exposes detailed metrics via an admin HTTP interface on each service

## Architecture Overview
Finagle is built on Netty for asynchronous I/O and uses Twitter's ``Future`` and ``Service`` abstractions. A ``Service`` is a function from request to ``Future[Response]``, and a ``Filter`` is a composable middleware that wraps services. Clients maintain connection pools and use a load balancer to distribute requests across endpoints discovered via service registries or DNS.

## Self-Hosting & Configuration
- Add the protocol-specific module (``finagle-http``, ``finagle-thrift``, etc.) to your build tool
- Configure via the builder pattern: ``Http.client.withRetries(...).withLoadBalancer(...)``
- Enable the admin HTTP server for metrics and health checks on a dedicated port
- Deploy as a standard JVM process; no external sidecar required
- Use Flags (Finagle's config system) or standard JVM properties for runtime tuning

## Key Features
- Protocol-agnostic design supports HTTP/1.1, HTTP/2, Thrift, gRPC, MySQL, Redis, and Memcached
- Composable filters let you layer cross-cutting concerns (auth, logging, tracing) cleanly
- Aperture load balancer minimizes connections while distributing load fairly
- Built-in distributed tracing compatible with Zipkin
- Battle-tested at scale handling billions of requests per day at Twitter

## Comparison with Similar Tools
- **gRPC** — focused on Protocol Buffers over HTTP/2; Finagle supports multiple protocols and richer client-side features
- **Spring Cloud** — heavier Java ecosystem with config server and gateway; Finagle is a focused RPC library
- **Akka HTTP** — actor-based model; Finagle uses futures and services for a simpler async model
- **Netty** — raw async I/O framework; Finagle adds RPC semantics, load balancing, and observability on top
- **Envoy** — sidecar proxy approach; Finagle is an in-process library with no extra deployment

## FAQ
**Q: Can I use Finagle with Java?**
A: Yes. While examples often use Scala, Finagle has a Java-friendly API and works well in pure Java projects.

**Q: Is Finagle still actively maintained?**
A: Yes. Twitter continues to maintain Finagle, and it powers critical infrastructure at X (formerly Twitter).

**Q: How does Finagle compare to a service mesh?**
A: Finagle handles load balancing, retries, and tracing in-process. A service mesh like Istio does similar work via sidecar proxies. Finagle avoids the extra hop but requires library integration.

**Q: Does Finagle support HTTP/2?**
A: Yes. The ``finagle-http`` module supports both HTTP/1.1 and HTTP/2 with configuration.

## Sources
- https://github.com/twitter/finagle
- https://twitter.github.io/finagle/

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Source: https://tokrepo.com/en/workflows/asset-0eccef2e
Author: Script Depot