iodine - HTTP / Websocket Server with Pub/Sub support, optimized for Ruby MRI on Linux / BSD


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Notice: iodine's core library, is being re-vamped with many updates and changes. This is a time to ask - what features are important for you? let me know here.

The development branch is the v.0.5-rewrite branch.

Iodine is a fast concurrent web server for real-time Ruby applications, with native support for:

  • Websockets;
  • Pub/Sub (with optional Redis Pub/Sub scaling);
  • Static file service (with automatic gzip support for pre-compressed versions);
  • HTTP/1.1 keep-alive and pipelining;
  • Asynchronous event scheduling and timers;
  • Client connectivity (attach client sockets to make them evented);
  • Custom protocol authoring;
  • and more!

Iodine is an evented framework with a simple API that builds off the low level C code library with support for epoll and kqueue - this means that:

  • Iodine can handle thousands of concurrent connections (tested with more then 20K connections)!

  • Iodine supports only Linux/Unix based systems (i.e. macOS, Ubuntu, FreeBSD etc'), which are ideal for evented IO (while Windows and Solaris are better at IO completion events, which are totally different).

Iodine is a C extension for Ruby, developed and optimized for Ruby MRI 2.2.2 and up... it should support the whole Ruby 2.0 MRI family, but Rack requires Ruby 2.2.2, and so iodine matches this requirement.

Iodine::Rack == fast & powerful HTTP + Websockets server with native Pub/Sub

Iodine includes a light and fast HTTP and Websocket server written in C that was written according to the Rack interface specifications and the Websocket draft extension.

With Iodine.listen2http it's possible to run multiple HTTP applications in addition to (or instead of) the default Iodine::Rack HTTP service.

Iodine also supports native process cluster Pub/Sub and a native RedisEngine to easily scale iodine's Pub/Sub horizontally.

Running the web server

Using the iodine server is easy, simply add iodine as a gem to your Rack application:

gem 'iodine', '~>0.4'

Iodine will calculate, when possible, a good enough default concurrency model for lightweight applications... this might not fit your application if you use heavier database access or other blocking calls.

To get the most out of iodine, consider the amount of CPU cores available and the concurrency level the application requires.

The common model of 16 threads and 4 processes can be easily adopted:

bundler exec iodine -p $PORT -t 16 -w 4

Static file serving support

Iodine supports an internal static file service that bypasses the Ruby layer and serves static files directly from "C-land".

This means that iodine won't lock Ruby's GVL when sending static files. The files will be sent directly, allowing for true native concurrency.

Since the Ruby layer is unaware of these requests, logging can be performed by turning iodine's logger on.

To use native static file service, setup the public folder's address before starting the server.

This can be done when starting the server from the command line:

bundler exec iodine -p $PORT -t 16 -w 4 -www /my/public/folder

Or by adding a single line to the application. i.e. (a example):

require 'iodine'
# static file service
Iodine::Rack.public = '/my/public/folder'
# application
out = [404, {"Content-Length" => "10"}, ["Not Found."]].freeze
app = { out }
run app

To enable logging from the command line, use the -v (verbose) option:

bundler exec iodine -p $PORT -t 16 -w 4 -www /my/public/folder -v


Ruby can leverage static file support (if enabled) by using the X-Sendfile header in the Ruby application response.

This allows Ruby to send very large files using a very small memory footprint, as well as (when possible) leveraging the sendfile system call.

i.e. (example for iodine):

app = proc do |env|
  request =
  if request.path_info == '/source'.freeze
    [200, { 'X-Sendfile' => File.expand_path(__FILE__) }, []]
  elsif request.path_info == '/file'.freeze
    [200, { 'X-Header' => 'This was a Rack::Sendfile response sent as text.' },]
    [200, { 'Content-Type' => 'text/html',
            'Content-Length' => request.path_info.length.to_s },
# # optional:
# use Rack::Sendfile
run app

Go to localhost:3000/source to download the file using the X-Sendfile extension.

Pre-Compressed assets / files

Simply gzip your static files and iodine will automatically recognize and send the gz version if the client (browser) supports the gzip transfer-encoding.

For example, to offer a compressed version of style.css, run (in the terminal):

  $  gzip -k -9 style.css

Now, you will have two files in your folder, style.css and style.css.gz.

When a browser that supports compressed encoding (which is most browsers) requests the file, iodine will recognize that a pre-compressed option exists and will prefer the gzip compressed version.

It's as easy as that. No extra code required.

Special HTTP Upgrade support

Iodine's HTTP server includes special support for the Upgrade directive using Rack's env Hash, allowing the application to focus on services and data while iodine takes care of the network layer.

Upgrading an HTTP connection can be performed either using iodine's Websocket Protocol support with env['upgrade.websocket'] or by implementing your own protocol directly over the TCP/IP layer - be it a websocket flavor or something completely different - using env['upgrade.tcp'].


When an HTTP Upgrade request is received, iodine will set the Rack Hash's upgrade property to true, so that: env[upgrade.websocket?] == true

To "upgrade" the HTTP request to the Websockets protocol, simply provide iodine with a Websocket Callback Object instance or class: env['upgrade.websocket'] = MyWebsocketClass or env['upgrade.websocket'] =

Iodine will adopt the object, providing it with network functionality (methods such as write, each, defer and close will become available) and invoke it's callbacks on network events.

Here is a simple chatroom example we can run in the terminal (irb) or easily paste into a file:

require 'iodine'
class WebsocketChat
  def on_open
    # Pub/Sub directly to the client (or use a block to process the messages)
    subscribe channel: :chat
    # Writing directly to the socket
    write "You're now in the chatroom."
  def on_message data
    # Strings and symbol channel names are equivalent.
    publish channel: "chat", message: data
end do |env|
  if env['upgrade.websocket?'.freeze] && env["HTTP_UPGRADE".freeze] =~ /websocket/i.freeze
    env['upgrade.websocket'.freeze] = WebsocketChat # or:
    [0,{}, []] # It's possible to set cookies for the response.
    [200, {"Content-Length" => "12"}, ["Welcome Home"] ]
# Pus/Sub can be server oriented as well as connection bound
root_pid =
Iodine.subscribe(channel: :chat) {|ch, msg| puts msg if == root_pid }
# By default, Pub/Sub performs in process cluster mode.
Iodine.processes = 4
# static file serving can be set manually as well as using the command line:
Iodine::Rack.public = "www/public"

Native Pub/Sub with optional Redis scaling

Iodine's core, offers a native Pub/Sub implementation. The implementation is totally native to iodine, it covers the whole process cluster and it can be easily scaled by using Redis (which isn't required except for horizontal scaling).

Here's an example that adds horizontal scaling to the chat application in the previous example, so that Pub/Sub messages are published across many machines at once:

require 'uri'
# initialize the Redis engine for each iodine process.
  uri = URI(ENV["REDIS_URL"])
  Iodine.default_pubsub =, uri.port, 0, uri.password)
  puts "* No Redis, it's okay, pub/sub will still run on the whole process cluster."

# ... the rest of the application remain unchanged.

The new Redis client can also be used for asynchronous Redis command execution. i.e.:

if(Iodine.default_pubsub.is_a? Iodine::PubSub::RedisEngine)
  # Ask Redis about all it's client connections and print out the reply.
  Iodine.default_pubsub.send("CLIENT LIST") { |reply| puts reply }

Details and Limitations:

  • Iodine does not use a Hash table for the Pub/Sub channels, it uses a 4 bit trie.

    The cost is higher memory consumption per channel and a limitation of 1024 bytes per channel name (shorter names are better).

    The bonus is high lookup times, zero chance of channel conflicts and an optimized preference for shared prefix channels (i.e. "user:1", "user:2"...).

    Another added bonus is pattern publishing (is addition to pattern subscriptions) which isn't available when using Redis (since Redis doesn't support this feature).

  • Iodine's Redis client does not support multiple databases. This is both because database scoping is ignored by Redis during pub/sub and because Redis Cluster doesn't support multiple databases. This indicated that multiple database support just isn't worth the extra effort.

  • The iodine Redis client will use two Redis connections per process (one for subscriptions and the other for publishing and commands). Both connections will be automatically re-established if timeouts or errors occur.

TCP/IP (raw) sockets

Upgrading to a custom protocol (i.e., in order to implement your own Websocket protocol with special extensions) is performed almost the same way, using env['upgrade.tcp']. In the following (terminal) example, we'll use an echo server without direct socket echo:

require 'iodine'
class MyProtocol
  def on_message data
    # regular socket echo - NOT websockets - notice the upgrade code
    write data
end = do |env|
  if env['upgrade.tcp?'.freeze] && env["HTTP_UPGRADE".freeze] =~ /echo/i.freeze
    env['upgrade.tcp'.freeze] = MyProtocol
    # no HTTP response will be sent when the status code is 0 (or less).
    # to upgrade AFTER a response, set a valid response status code.
    [1000,{}, []]
    [200, {"Content-Length" => "12"}, ["Welcome Home"] ]

A few notes

This design has a number of benefits, some of them related to better IO handling, resource optimization (no need for two IO polling systems), etc. This also allows us to use middleware without interfering with connection upgrades and provides backwards compatibility.

Iodine::Rack imposes a few restrictions for performance and security reasons, such as that the headers (both sending and receiving) must be less than 8Kb in size. These restrictions shouldn't be an issue and are similar to limitations imposed by Apache.

Of course, if you still want to use Rack's hijack API, iodine will support you - but be aware that you will need to implement your own reactor and thread pool for any sockets you hijack, as well as a socket buffer for non-blocking write operations (why do that when you can write a protocol object and have the main reactor manage the socket?).

Performance oriented design - but safety first

Iodine is an evened server, similar in it's architecture to nginx and puma. It's different than the simple "thread-per-client" design that is often taught when we begin to learn about network programming.

By leveraging epoll (on Linux) and kqueue (on BSD), iodine can listen to multiple network events on multiple sockets using a single thread.

All these events go into a task queue, together with the application events and any user generated tasks, such as ones scheduled by

In pseudo-code, this might look like this


def server_cycle
      QUEUE << get_next_32_socket_events # these events schedule the proper user code to run
    QUEUE << server_cycle

def run_server
      while ((event = QUEUE.pop))

In pure Ruby (without using C extensions or Java), it's possible to do the same by using select... and although select has some issues, it works well for lighter loads.

The server events are fairly fast and fragmented (longer code is fragmented across multiple events), so one thread is enough to run the server including it's static file service and everything...

...but single threaded mode should probably be avoided.

The thread pool is there to help slow user code.

It's very common that the application's code will run slower and require external resources (i.e., databases, a custom pub/sub service, etc'). This slow code could "starve" the server, which is patiently waiting to run it's tasks on the same thread.

The slower your application code, the more threads you will need to keep the server running in a responsive manner (note that responsiveness and speed aren't always the same).

How does it compare to other servers?

Personally, after looking around, the only comparable servers are Puma and Passenger, which iodine significantly outperformed on my tests (I didn't test Passenger's enterprise version).

Since the HTTP and Websocket parsers are written in C (with no RegExp), they're fairly fast.

Also, iodine's core and parsers are running outside of Ruby's global lock, meaning that they enjoy true concurrency before entering the Ruby layer (your application) - this offers iodine a big advantage over other Ruby servers.

Another assumption iodine makes is that it is behind a load balancer / proxy (which is the normal way Ruby applications are deployed) - this allows iodine to disregard some header validity checks (we're not checking for invalid characters) and focus it's resources on other security and performance concerns.

I recommend benchmarking the performance for yourself using wrk or ab:

$ wrk -c200 -d4 -t12 http://localhost:3000/
# or
$ ab -n 100000 -c 200 -k

Create a simple file with a hello world app:

App = do |env|
     {   "Content-Type" => "text/html".freeze,
         "Content-Length" => "16".freeze },
     ['Hello from Rack!'.freeze]  ]

run App

Then start comparing servers. Here are the settings I used to compare iodine and Puma (4 processes, 16 threads):

$ RACK_ENV=production iodine -p 3000 -t 16 -w 4
# vs.
$ RACK_ENV=production puma -p 3000 -t 16 -w 4
# Review the `iodine -?` help for more command line options.

When benchmarking with wrk, iodine performed significantly better, (~62K req/sec vs. ~44K req/sec) while keeping a lower memory foot print (~60Mb vs. ~111Mb).

When benchmarking with ab, I got different results, where iodine still performed significantly better, (~72K req/sec vs. ~36K req/sec and ~61Mb vs. ~81.6Mb). I suspect the difference between the two benchmarks has to do with system calls to write and possible packet fragmentation, but I have no real proof.

Remember to compare the memory footprint after running some requests - it's not just speed that C is helping with, it's also memory management and object pooling (i.e., iodine uses a buffer packet pool management).

Can I try before I buy?

Well, it is free and open source, no need to buy.. and of course you can try it out.

It's installable just like any other gem on MRI, run:

$ gem install iodine

If building the native C extension fails, please note that some Ruby installations, such as on Ubuntu, require that you separately install the development headers (ruby.h and friends). I have no idea why they do that, as you will need the development headers for any native gems you want to install - so hurry up and get them.

If you have the development headers but still can't compile the iodine extension, open an issue with any messages you're getting and I'll be happy to look into it.

Mr. Sandman, write me a server

Iodine allows custom TCP/IP server authoring, for those cases where we need raw TCP/IP (UDP isn't supported just yet).

Here's a short and sweet echo server - No HTTP, just use telnet:

require 'iodine'

# an echo protocol with asynchronous notifications.
class EchoProtocol
  # `on_message` is an optional alternative to the `on_data` callback.
  # `on_message` has a 1Kb buffer that recycles itself for memory optimization.
  def on_message buffer
    # writing will never block and will use a buffer written in C when needed.
    write buffer
    # close will be performed only once all the data in the write buffer
    # was sent. use `force_close` to close early.
    close if buffer =~ /^bye[\r\n]/i
    # use buffer.dup to save the data from being recycled once we return.
    data = buffer.dup
    # run asynchronous tasks with ease
    run do
      sleep 1
      puts "Echoed data: #{data}"

# listen on port 3000 for the echo protocol.
Iodine.listen 3000, EchoProtocol
Iodine.threads = 1
Iodine.processes = 1

Why not EventMachine?

You can go ahead and use EventMachine if you like. They're doing amazing work on that one and it's been used a lot in Ruby-land... really, tons of good developers and people on that project, I'm sure...

But me, I prefer to make sure my development software runs the exact same code as my production software. So here we are.

Also, I don't really understand all the minute details of EventMachine's API, it kept crashing my system every time I reached 1K-2K active connections... I'm sure I just don't know how to use EventMachine, but that's just that.

Besides, you're here - why not take iodine out for a spin and see for yourself?

Can I contribute?

Yes, please, here are some thoughts:

  • I'm really not good at writing automated tests and benchmarks, any help would be appreciated. I keep testing manually and that's less then ideal (and it's mistake prone).

  • If we can write a Java wrapper for the C framework, it would be nice... but it could be as big a project as the whole gem, as a lot of minor details are implemented within the bridge between these two languages.

  • PRs or issues related to the C framework should be placed in the repository.

  • Bug reports and pull requests are welcome on GitHub at

  • If you love the project or thought the code was nice, maybe helped you in your own project, drop me a line. I'd love to know.


The gem is available as open source under the terms of the MIT License.

"I'm also writing a Ruby extension in C"

Really?! That's great!

We could all use some more documentation around the subject and having an eco-system for extension tidbits would be nice.

Here's a few things you can use from this project and they seem to be handy to have (and easy to port):

  • Iodine is using a Registry to keep dynamic Ruby objects that are owned by C-land from being collected by the garbage collector in Ruby-land...

    Some people use global Ruby arrays, adding and removing Ruby objects to the array, but that sounds like a performance hog to me.

    This one is a simple binary tree with a Ruby GC callback. Remember to initialize the Registry (Registry.init(owner)) so it's "owned" by some Ruby-land object, this allows it to bridge the two worlds for the GC's mark and sweep.

    I'm attaching it to one of iodine's library classes, just in-case someone adopts my code and decides the registry should be owned by the global Object class.

  • I was using a POSIX thread pool library (defer.h) until I realized how many issues Ruby has with non-Ruby threads... So now there's a Ruby-thread patch for this library at (rb-defer.c).

    Notice that all the new threads are free from the GVL - this allows true concurrency... but, you can't make Ruby API calls in that state.

    To perform Ruby API calls you need to re-enter the global lock (GVL), albeit temporarily, using rb_thread_call_with_gvl and rv_protect (gotta watch out from Ruby longjmp exceptions).

  • Since I needed to call Ruby methods while multi-threading and running outside the GVL, I wrote RubyCaller which let's me call an object's method and wraps all the rb_thread_call_with_gvl and rb_protect details in a secret hidden place I never have to see again. It also keeps track of the thread's state, so if we're already within the GVL, we won't enter it "twice" (which could crash Ruby sporadically).

These are nice code snippets that can be easily used in other extensions. They're easy enough to write, I guess, but I already did the legwork, so enjoy.