A New Ruby Application Server: NGINX Unit

by Nate Berkopec (@nateberkopec) of (who?), a Rails performance consultancy.
Summary: NGINX Inc. has just released Ruby support for their new multi-language application server, NGINX Unit. What does this mean for Ruby web applications? Should you be paying attention to NGINX Unit? (2057 words/10 minutes)

There’s a new application server on the block for Rubyists - NGINX Unit. As you could probably guess by the name, it’s a project of NGINX Inc., the for-profit open-source company that owns the NGINX web server. In fall of 2017, they announced the NGINX Unit project. It’s essentially an application server designed to replace all of the various application servers used with NGINX. In Ruby’s case, that’s Puma, Unicorn, and Passenger.1(For a far more in-depth comparison of these application servers, read my article about configuring Puma, Passenger and Unicorn)1 For a far more in-depth comparison of these application servers, read my article about configuring Puma, Passenger and Unicorn NGINX Unit also runs Python, Go, PHP and Perl.

The overarching idea seems to be to make microservice administration a lot easier. One NGINX Unit process can run any number of applications running any number of languages - for example, one NGINX Unit server can manage a half-dozen different Ruby applications, each running a different version of the Ruby runtime. Or you can run a Ruby application and a Python application side-by-side. The combinations are only limited by your system resources.

Unfortunately, the “microservice” space is quite prone to buzzword-laden marketing pages.2(I really don’t like when software projects advertise themselves as “modern”. It’s like “subtweeting” all pre-existing software projects in this problem space and saying they’re all old and busted, and this is the New Way To Do Things. Why it’s better than the “old busted ways” is never explicitly stated. This kind of marketing preys on software developer’s fear of becoming obsolete in their skillset, rather than making any substantive point.)2 I really don’t like when software projects advertise themselves as “modern”. It’s like “subtweeting” all pre-existing software projects in this problem space and saying they’re all old and busted, and this is the New Way To Do Things. Why it’s better than the “old busted ways” is never explicitly stated. This kind of marketing preys on software developer’s fear of becoming obsolete in their skillset, rather than making any substantive point. Words like “dynamic”, “modular”, “lightweight” are mixed in with “service mesh”, “seamless” and “graceful”. This article is going to be about cutting through the marketing and getting into what NGINX Unit means for those of us running production Ruby applications.

Before I move on to more about NGINX Unit’s architecture and what makes it unique, let’s make sure we all understand the difference between an application server and a web server. A web server connects to clients over HTTP, and usually serves static files or proxies to other HTTP-enabled servers, and acts as a middleman. An application server is the thing which actually starts and runs the language runtime. In Ruby, these functions are sometimes combined. For example, all of the major Ruby application servers also are web servers. However, many web servers, such as Nginx and Apache, are not also application servers. Nginx UNIT is both a web and application server.

NGINX Unit runs four different types of processes: main, router, controller, and application. Application processes are the self-explanatory ones - this would just be the Ruby runtime running your Rails application. The router and controller processes, and how they interact with each other and the application processes, is what defines how NGINX Unit works.

The main process creates the router and application processes. That’s really all it does. Application processes in NGINX Unit are dynamic, however - the number of processes running can be changed at any time, Ruby versions can be changed, or even entire new applications can be added while the server is running. The thing that tells the main process what application processes to run is the controller process.

The controller process (like the main process, there’s only one) has two jobs: expose a JSON configuration API over HTTP, and configure the router and main processes. This is probably the most novel and interesting part of NGINX Unit for Rubyists. Rather than working with configuration files, you POST JSON objects to the controller process to tell it what to do. For example, with this json file:

    "listeners": {
        "*:3000": {
            "application": "rails"

    "applications": {
        "rails": {
            "type": "ruby",
            "processes": 5,
            "script": "/www/railsapp/config.ru"

… we can PUT it to an NGINX Unit controller process with this (assuming our NGINX Unit server is listening on port 8443):

curl -d "myappconfig.json" -X PUT ''

… and create a new Ruby application.

NGINX Unit’s JSON configuration object is divided into listeners and applications. Applications are the actual apps you want to run. Listeners are where those apps are exposed to the world (i.e. what port they’re on).

Changes in application and listener configuration are supposed to be seamless. For example, a “hot deploy” of a new version of your application would be accomplished by adding a new application to the configuration:

  "rails-new": {
      "type": "ruby",
      "processes": 5,
      "script": "/www/rails-new-app/config.ru"

curl -d "mynewappconfig.json" -X PUT

and then switching the listener to the new application:

curl -X PUT -d '"rails-new"' '*:3000/application`

This transition is (supposedly) seamless, and clients won’t notice. This is similar to a Puma “phased restart”. In a phased restart in Puma, each worker process is restarted one at a time, which means that the other works processes are up and available to take requests. Puma accomplishes this using a control server (managed by the pumactl utility). However, unlike Puma, NGINX Unit “hot restarts” will not have two versions of the application taking requests at the same time.

In a Puma phased restart, say your application has six workers. Halfway through the phased restart, 3 works will be running the old code, and half will be running new code. This can cause some problems with database schema changes, for example. NGINX Unit restarts happen “all at once”, so while two versions of the code will be running at once, only one version will be taking requests at any point in time.

This functionality seems quite useful to those who are running their own Ruby applications on a service such as AWS, where you have to manage your own deployment. However, Heroku users won’t find any of this useful, as you’ve already had this sort of “hot deploy” functionality using Heroku’s preboot system. However, these two features aren’t doing exactly the same thing. Heroku creates an entirely new virtual server and hot-swaps the whole thing, whereas NGINX Unit is just changing processes on a single machine, but they’re completely the same from a client perspective.

The router process is pretty much what it sounds like - the thing which turns HTTP connections from clients into requests to the web application processes. NGINX claims a single Unit router can handle thousands of simultaneous connections. The router works a lot like an NGINX web server, and has a number of worker threads to accept, buffer and parse incoming connections.

To me, this is one of the most exciting parts of NGINX Unit for Rubyists. It is very difficult for Ruby application servers to deal with HTTP connections without some kind of reverse proxy in front of the app server. Unicorn, for example, is recommended for use only behind a reverse proxy because it cannot buffer requests. That is, if a client sends one byte of their request and then stops (due to network conditions, a bad cellphone connection perhaps), then the Unicorn process just stops all work and cannot continue until that request has finished buffering. Using NGINX, for example, in front of Unicorn allows NGINX to buffer that request before it reaches Unicorn. Since NGINX is written in highly optimized C and it’s not restricted by Ruby’s GVL, it can buffer hundreds of connections for Unicorn. Passenger solves this problem by basically just being an addon for NGINX or Apache3(Now you know why it’s called Passenger!)3 Now you know why it’s called Passenger! (mod_ruby!) and offloading all of the connection-related work to the webserver. In this way, NGINX Unit is more similar to Passenger than it is to Unicorn.

The application configuration has a processes key. This key can have a minimum number and maximum number of processes:

  "rails-new": {
      "type": "ruby",
      "processes": {
        "spare": 5
        "max": 10
      "script": "/www/rails-new-app/config.ru"

For some reason, the “minimum” number of processes is called “spare”. The config above will start 5 processes immediately, and will scale to 10 if the load requires it.

No word yet on if any settings like Puma’s preload_app! and similar settings in Passenger and Unicorn are available so you will be able to start up processes before they are needed and take advantage of copy-on-write memory.

This leaves the application processes. The interesting and novel thing here is that the router does not communicate with the application processes via HTTP - it uses Unix sockets and shared memory. This looks like an optimization aimed at microservice architectures, as communicating between services on the same machine will be considerably faster without any HTTP in between. I have yet to see any Ruby code examples of how this could work, however.

It is unclear to me in the long-term if it is intended for you to run NGINX in front of NGINX Unit, or if NGINX Unit can run on it’s own without anything in front of it. As of right now (Q1 2018), you should probably be running NGINX in front of NGINX Unit as a reverse proxy, because NGINX Unit lacks static file serving, HTTPS (TLS), and HTTP/2. Obviously, the integration is pretty seamless.

NGINX Unit is approaching a stable 1.0 release. You can’t really run it in production right now for Ruby applications: As I write this sentence, the Ruby module is literally 5 days old. It’s still under very active development right now - minor versions are released every few weeks. TLS and HTTP-related features seem like the next “big features” to come down the pipe, with static file serving being next. There is some discussion about support for Java, which could probably be turned into support for JRuby and TruffleRuby as well.

There is no Windows support, and I don’t think I would hold my breath for any in the future. NGINX Unit only supports Ruby 2.0 and above.

I will not be benchmarking NGINX Unit in this post. It’s Ruby module is extremely new and probably not ready for any kind of benchmarking. However, the real reason I won’t be benchmarking NGINX Unit against Puma, Unicorn or Passenger is because application server choice in Ruby is not a matter of speed (techincally, latency) but throughput. Application servers tend to differ in how many requests they can serve in parallel, rather than how quickly they do it. Application servers impose very little latency overhead on the applications they serve, probably on the order of a couple of milliseconds.

The most important Ruby application server setting which affects throughput is threading. The reason is that it is the only application server setting which can increase the number of requests served concurrently. A multithreaded Ruby application server can make greater and more efficient use of the available CPU and memory resources and serve more requests-per-minute than a single-threaded Ruby application process.

Currently, the only free application server which runs Ruby web applications in multiple threads is Puma. Passenger Enterprise will do it, but you must pay for a license.

NGINX Unit plans support for multiple threads in Python applications, so it is not inconceivable that it will support Ruby applications in multiple threads sometime in the future.

So, how does NGINX Unit currently “shake out” in comparison to Unicorn, Passenger and Puma? I think that the traditional Rails application setup: one monolithic application, run on a Plaform-as-a-Service provider like Heroku will probably not see any benefit at all from NGINX Unit’s current features and planned roadmap. Puma already serves these users very well.

NGINX Unit may be interesting for Unicorn users who want to stop using a reverse proxy. Once NGINX Unit’s HTTP features are fleshed out, it could replace a Unicorn/NGINX setup with just a single NGINX Unit server.

NGINX Unit is probably most directly comparable to Phusion Passenger, which also recently went into the “microservice” realm by supporting Javascript and Python as well as Ruby applications. NGINX Unit currently supports more languages and will probably support even more in the future, so those that need greater language support will probably switch. However, Phusion is a Ruby-first company, so I expect Passenger to always “support” Ruby in a better, more complete way than NGINX Unit ever will. And, as mentioned above, Phusion Passenger Enterprise supports multithreaded execution today.

So, what is the ideal NGINX Unit app? If you’re running your own cloud (that is, not on a service which manages the routing for you, like Heroku) and you have many Ruby applications running on different Ruby versions or many services in many different languages and those services/apps need to talk to each other, quickly, it looks like NGINX Unit was designed for you. If you don’t fit that profile, though, it’s probably best to stick to the existing top three options (Puma, Passenger, and Unicorn).


Want a faster website?

I'm Nate Berkopec (@nateberkopec). I write online about web performance from a full-stack developer's perspective. I primarily write about frontend performance and Ruby backends. If you liked this article and want to hear about the next one, click below. I don't spam - you'll receive about 1 email per week. It's all low-key, straight from me.

The Complete Guide to Rails Performance

Look what I wrote! The Complete Guide to Rails Performance is a full-stack course that gives you the tools to make Ruby on Rails applications faster, more scalable, and simpler to maintain. It includes a 361 page PDF, private Slack, and over 15 hours of video content.

Learn more

More Posts

3 ActiveRecord Mistakes That Slow Down Rails Apps: Count, Where and Present

Many Rails developers don't understand what causes ActiveRecord to actually execute a SQL query. Let's look at three common cases: misuse of the count method, using where to select subsets, and the present? predicate. You may be causing extra queries and N+1s through the abuse of these three methods.

Read more

The Complete Guide to Rails Performance, Version 2

I've completed the 'second edition' of my course, the CGRP. What's changed since I released the course two years ago? Where do I see Rails going in the future?

Read more

Malloc Can Double Multi-threaded Ruby Program Memory Usage

Memory fragmentation is difficult to measure and diagnose, but it can also sometimes be very easy to fix. Let's look at one source of memory fragmentation in multi-threaded CRuby programs: malloc's per-thread memory arenas.

Read more

Configuring Puma, Unicorn and Passenger for Maximum Efficiency

Application server configuration can make a major impact on the throughput and performance-per-dollar of your Ruby web application. Let's talk about the most important settings.

Read more

Is Ruby Too Slow For Web-Scale?

Choosing a new web framework or programming language for the web and wondering which to pick? Should performance enter your decision, or not?

Read more

Railsconf 2017: The Performance Update

Did you miss Railsconf 2017? Or maybe you went, but wonder if you missed something on the performance front? Let me fill you in!

Read more

Understanding Ruby GC through GC.stat

Have you ever wondered how the heck Ruby's GC works? Let's see what we can learn by reading some of the statistics it provides us in the GC.stat hash.

Read more

Rubyconf 2016: The Performance Update

What happened at RubyConf 2016 this year? A heck of a lot of stuff related to Ruby performance, that's what.

Read more

What HTTP/2 Means for Ruby Developers

Full HTTP/2 support for Ruby web frameworks is a long way off - but that doesn't mean you can't benefit from HTTP/2 today!

Read more

How Changing WebFonts Made Rubygems.org 10x Faster

WebFonts are awesome and here to stay. However, if used improperly, they can also impose a huge performance penalty. In this post, I explain how Rubygems.org painted 10x faster just by making a few changes to its WebFonts.

Read more

Page Weight Doesn't Matter

The total size of a webpage, measured in bytes, has little to do with its load time. Instead, increase network utilization: make your site preloader-friendly, minimize parser blocking, and start downloading resources ASAP with Resource Hints.

Read more

Hacking Your Webpage's Head Tags for Speed and Profit

One of the most important parts of any webpage's performance is the content and organization of the head element. We'll take a deep dive on some easy optimizations that can be applied to any site.

Read more

How to Measure Ruby App Performance with New Relic

New Relic is a great tool for getting the overview of the performance bottlenecks of a Ruby application. But it's pretty extensive - where do you start? What's the most important part to pay attention to?

Read more

Ludicrously Fast Page Loads - A Guide for Full-Stack Devs

Your website is slow, but the backend is fast. How do you diagnose performance issues on the frontend of your site? We'll discuss everything involved in constructing a webpage and how to profile it at sub-millisecond resolution with Chrome Timeline, Google's flamegraph-for-the-browser.

Read more

Action Cable - Friend or Foe?

Action Cable will be one of the main features of Rails 5, to be released sometime this winter. But what can Action Cable do for Rails developers? Are WebSockets really as useful as everyone says?

Read more

rack-mini-profiler - the Secret Weapon of Ruby and Rails Speed

rack-mini-profiler is a powerful Swiss army knife for Rack app performance. Measure SQL queries, memory allocation and CPU time.

Read more

Scaling Ruby Apps to 1000 Requests per Minute - A Beginner's Guide

Most "scaling" resources for Ruby apps are written by companies with hundreds of requests per second. What about scaling for the rest of us?

Read more

Secrets to Speedy Ruby Apps On Heroku

Ruby apps in the memory-restrictive and randomly-routed Heroku environment don't have to be slow. Achieve <100ms server response times with the tips laid out below.

Read more

Speed Up Your Rails App by 66% - The Complete Guide to Rails Caching

Caching in a Rails app is a little bit like that one friend you sometimes have around for dinner, but should really have around more often.

Read more

100ms to Glass with Rails and Turbolinks

Is Rails dead? Can the old Ruby web framework no longer keep up in this age of "native-like" performance? Turbolinks provides one solution.

Read more


Get notified on new posts.

Straight from the author. No spam, no bullshit. Frequent email-only content.