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middlegem is a Ruby gem that provides one-way middleware chain functionality. It aims to be simple and reliable. It might be a good fit for you if:

  • You want simplicity and reliability.
  • You don't need two-way middleware. middlegem does not allow processing both the "request" and the "response", for example. For that kind of functionality, I would recommend checking out ruby-middleware.
  • You want to explicitly define the order of your middlwares. middlegem encourages you to explicitly define your middlewares and the order they should run.


middlegem is a Ruby gem. If you use Bundler, you may install it by adding it to your Gemfile, like so:

gem 'middlegem'

And then execute:

$ bundle install

Or you may install it manually with:

$ gem install middlegem

middlegem has zero dependencies and requires very little setup to get started!

Key Concepts

middlegem is broken into three key parts: middlewares, middleware definitions, and middleware stacks.

Middlewares are the heart and soul of the gem. In essence, a middleware is a single transforming function that accepts input and produces output. In middlegem, any object that responds to the call method can be a middleware. By convention, however, middleware classes derive from Middlegem::Middleware.

Middlewares in middlegem are designed to operate on argument lists. This has two consequences:

  1. A middleware's call method should simply accept the arguments it expects to transform—no need to accept and "arguments array" and try to parse it!
  2. The call method must return an array. Because it is transforming an argument list, it must also return an argument list, i.e. an array.

While you can certainly use middlegem with a single input, always remember to return an array in your middleware call methods.

Midleware definitions are a key difference in middlegem from other middleware gems. They strive to solve a common problem with middlewares. Imagine, for example, that you have two middlewares: one that converts an input string to an integer, and another that multiplies that number by 10. Obviously, the conversion middleware must run first, or an error will occur. With a simple example like this, it is trivial to simply insert the middlewares in the right place at the right time. But as you begin adding more middlewares and—worse—begin allowing custom middlewares to be defined, things quickly become unmanageable! It becomes impossible to know exactly where a given middleware should be inserted in a middleware stack.

This is where middleware definitions come in. A middleware definition is essentially an object that determines 1) what middlewares are permitted in a middleware stack, and 2) in what order those middlewares should be run. Any object that implements a defined? method and a sort method can be avalid middleware definition, though by convention middleware definitions derive from Middlegem::Definition. The only middleware definition that currently ships with middlegem is Middlegem::ArrayDefinition, which allows you to define an ordered list of permitted middleware classes.

Finally, middleware stacks, represented by Middlegem::Stack, are chains of middlewares. Every Middlegem::Stack has a single middleware definition that determines how to run its middlewares. Note that Middlegem::Stack has no fancy methods for inserting middlewares at specific locations—it relies on Ruby's built-in methods. Instead, it allows ordering to be determined by the middleware definition.


Basic Usage

The easiest way to define middlewares is to create a class with a call method that optionally extends Middlegem::Middleware. In this example and the following ones, assume that these middlewares are defined:

class ParenthesesMiddleware << Middlegem::Middleware
  def call(input)

class BracketsMiddleware << Middlegem::Middleware
  def call(input)

class BracesMiddleware << Middlegem::Middleware
  def call(input)

class MultiplierMiddleware << Middlegem::Middleware
  attr_accessor :multiplier

  def initialize(multiplier)
    @multiplier = multiplier

  def call(num)
    [num * multiplier]

Now you'll need to define your middleware. If you're using Rails, initializers are usually a good place to do this. The easiest way to create a middleware definition is using Middlegem::ArrayDefinition, which allows you to specify an array of middleware classes. For example:

DEFINITION = Middlegem::ArrayDefinition.new([

Notice that the MultiplierMiddleware is at the top, because it must be given a number, and the others are arranged in "mathematical" order. Now, we can create a middleware stack with our definition.

stack = Middlegem::Stack.new(DEFINITION)

And add some middlewares, however you like:

stack.middlewares = [BracketsMiddleware]
stack.middlewares += [MultiplierMiddleware, BracesMiddleware]
stack.middlewares << ParenthesesMiddleware

Finally, we can call the stack with a number:

stack.call(10) # => ["{[(100)]}"]

Notice how the number is first multiplied, then given parentheses, then given brackets, then given braces, exactly as specified in the middleware definition.

Tie Resolvers

You may have noticed a problem here. What if multiple middleware instances of the same type are added to a stack. How will it know which to call? Take this code, for example, where procs are used as middleware:

DEFINITION = Middlegem::ArrayDefinition.new([Proc])

to_int = proc { |s| Integer(s) }
square = proc { |n| n*n }

stack = Middlegem::Stack.new(DEFINITION, middlewares: [

If stack.call('5') were run right now, the program would try to square '5', then convert it to an integer. Moreover, there is no way to specify which should come first—they are both procs, after all. For this reason, it is recommended that you keep all your middlewares in separate classes, so they can be defined easily. There are two potential solutions, however.

First, ArrayDefinition.new accepts an optional "tie resolver" that will be called in such cases. For example, let's say we have this middleware:

class AppendMiddleware
  attr_accessor :appended

  def initialize(appended)
    @appended = appended

  def call(input)
    [input + appended]

Obviously, the order of even individual AppendMiddlewares matters. "TAB" is a very differnt word from "BAT"! Imagining that we want the letters to be alphabetized, here is one potential solution:

DEFINITION = Middlegem::ArrayDefinition.new([AppendMiddleware], resolver: ->(ties) {
  if ties.count > 1 && ties.first.is_a? AppendMiddleware
    return ties.sort_by(&:appended)

stack = Middlegem::Stack.new(DEFINITION, middlewares: [

stack.call('') # => ['ABCDE']

As you can see, the resolver passed to ArrayDefinition.new will be called with an array of middleware whenever multiple middleware with the same class are encountered. The resolver is then expected to sort and return the array appropriately. In this case, we simply check whether the tied middlewares are AppendMiddlewares and sort them by their appended attribute if so.

While this works, the limitations quickly become obvious. Mainly, it requires a bunch of branching else/if or case/when structures in the resolver since that one resolver is called for all ties. While it may work for very simple use cases (such as preventing multiple instances of the same middleware at all), it is not feasible for anything more complicated.

For more complicated scenarios, it is instead recommended that you create your own implementation of Middlegem::Definition that allows ordering the middlewares in some other way. Perhaps you could set "priorities" on the middlewares, or organize them into "groups"—the possibilities with this method are limitless!


After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and the created tag, and push the .gem file to rubygems.org.


Bug reports and pull requests are welcome on GitHub at https://github.com/jacoblockard99/middlegem.


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