Middleware

Under the hood, Faraday uses a Rack-inspired middleware stack for making requests. Much of Faraday's power is unlocked with custom middleware. Some middleware is included with Faraday, and others are in external gems.

Here are some of the features that middleware can provide:

• authentication
• caching responses on disk or in memory
• cookies
• following redirects
• JSON encoding/decoding
• logging

To use these great features, create a Faraday::Connection with Faraday.new and add the correct middleware in a block. For example:

require 'faraday'

conn = Faraday.new do |f|
  f.request :json # encode req bodies as JSON
  f.response :logger # logs request and responses
  f.response :json # decode response bodies as JSON
  f.adapter :net_http # Use the Net::HTTP adapter
end
response = conn.get("http://httpbingo.org/get")

How it Works

A Faraday::Connection uses a Faraday::RackBuilder to assemble a Rack-inspired middleware stack for making HTTP requests. Each middleware runs and passes an Env object around to the next one. After the final middleware has run, Faraday will return a Faraday::Response to the end user.

The order in which middleware is stacked is important. Like with Rack, the first middleware on the list wraps all others, while the last middleware is the innermost one. If you want to use a custom adapter, it must therefore be last.

This is what makes things like the "retry middleware" possible. It doesn't really matter if the middleware was registered as a request or a response one, the only thing that matter is how they're added to the stack.

Say you have the following:

Faraday.new(...) do |conn|
  conn.request :authorization
  conn.response :json
  conn.response :parse_dates
end

This will result into a middleware stack like this:

authorization do
  # authorization request hook
  json do
    # json request hook
    parse_dates do
      # parse_dates request hook
      response = adapter.perform(request)
      # parse_dates response hook
    end
    # json response hook
  end
  # authorization response hook
end

In this example, you can see that parse_dates is the LAST middleware processing the request, and the FIRST middleware processing the response. This is why it's important for the adapter to always be at the end of the middleware list.

Using Middleware

Calling use is the most basic way to add middleware to your stack, but most middleware is conveniently registered in the request, response or adapter namespaces. All four methods are equivalent apart from the namespacing.

For example, the Faraday::Request::UrlEncoded middleware registers itself in Faraday::Request so it can be added with request. These two are equivalent:

# add by symbol, lookup from Faraday::Request,
# Faraday::Response and Faraday::Adapter registries
conn = Faraday.new do |f|
  f.request :url_encoded
  f.response :logger
  f.adapter :net_http
end

or:

# identical, but add the class directly instead of using lookups
conn = Faraday.new do |f|
  f.use Faraday::Request::UrlEncoded
  f.use Faraday::Response::Logger
  f.use Faraday::Adapter::NetHttp
end

This is also the place to pass options. For example:

conn = Faraday.new do |f|
  f.request :logger, bodies: true
end

DEFAULT_OPTIONS

DEFAULT_OPTIONS improve the flexibility and customizability of new and existing middleware. Class-level DEFAULT_OPTIONS and the ability to set these defaults at the application level compliment existing functionality in which options can be passed into middleware on a per-instance basis.

Using DEFAULT_OPTIONS

Using RaiseError as an example, you can see that DEFAULT_OPTIONS have been defined at the top of the class:

  DEFAULT_OPTIONS = { include_request: true }.freeze

These options will be set at the class level upon instantiation and referenced as needed within the class. From our same example:

  def response_values(env)
  ...
    return response unless options[:include_request]
  ...

If the default value provides the desired functionality, no further consideration is needed.

Setting Alternative Options per Application

In the case where it is desirable to change the default option for all instances within an application, it can be done by configuring the options in a /config/initializers file. For example:

# config/initializers/faraday_config.rb

Faraday::Response::RaiseError.default_options = { include_request: false }

After app initialization, all instances of the middleware will have the newly configured option(s). They can still be overridden on a per-instance bases (if handled in the middleware), like this:

  Faraday.new do |f|
    ...
    f.response :raise_error, include_request: true 
    ...
  end

Available Middleware

The following pages provide detailed configuration for the middleware that ships with Faraday:

• Authentication
• URL Encoding
• JSON Encoding/Decoding
• Instrumentation
• Logging
• Raising Errors

The Awesome Faraday project has a complete list of useful, well-maintained Faraday middleware. Middleware is often provided by external gems, like the faraday-retry gem.

Detailed Example

Here's a more realistic example:

Faraday.new(...) do |conn|
  # POST/PUT params encoder
  conn.request :url_encoded

  # Logging of requests/responses
  conn.response :logger

  # Last middleware must be the adapter
  conn.adapter :net_http
end

This request middleware setup affects POST/PUT requests in the following way:

1. Request::UrlEncoded encodes as "application/x-www-form-urlencoded" if not already encoded or of another type.
2. Response::Logger logs request and response headers, can be configured to log bodies as well.

Swapping middleware means giving the other priority. Specifying the "Content-Type" for the request is explicitly stating which middleware should process it.