Bandit is an HTTP server for Plug and WebSock apps.

Bandit is written entirely in Elixir and is built atop Thousand
Island. It can serve HTTP/1.x,
HTTP/2 and WebSocket clients over both HTTP and HTTPS. It is written with
correctness, clarity & performance as fundamental goals. It is the default HTTP
server for Phoenix since release 1.7.11 of the framework.

In ongoing automated performance
tests,
Bandit's HTTP/1.x engine is up to 4x faster than Cowboy depending on the number of concurrent
requests. When comparing HTTP/2 performance, Bandit is up to 1.5x faster than Cowboy. This is
possible because Bandit has been built from the ground up for use with Plug applications; this
focus pays dividends in both performance and also in the approachability of the code base.

Bandit also emphasizes correctness. Its HTTP/2 implementation scores 100% on the
h2spec suite in strict mode, and its
WebSocket implementation scores 100% on the
Autobahn test suite, both of
which run as part of Bandit's comprehensive CI suite. Extensive unit test,
credo, dialyzer, and performance regression test coverage round out a test suite
that ensures that Bandit is and will remain a platform you can count on.

Lastly, Bandit exists to demystify the lower layers of infrastructure code. In a world where
The New Thing is nearly always adding abstraction on top of abstraction, it's important to have
foundational work that is approachable & understandable by users above it in the stack.

Project Goals

• Implement comprehensive support for HTTP/1.0 through HTTP/2 & WebSockets (and
  beyond) backed by obsessive RFC literacy and automated conformance testing
• Aim for minimal internal policy and HTTP-level configuration. Delegate to Plug & WebSock as much as
  possible, and only interpret requests to the extent necessary to safely manage a connection
  & fulfill the requirements of safely supporting protocol correctness
• Prioritize (in order): correctness, clarity, performance. Seek to remove the mystery of
  infrastructure code by being approachable and easy to understand
• Along with our companion library Thousand
  Island, become the go-to HTTP
  & low-level networking stack of choice for the Elixir community by being
  reliable, efficient, and approachable

Project Status

• Complete support for Phoenix applications (WebSocket
  support requires Phoenix 1.7+)
• Complete support of the Plug API
• Complete support of the WebSock API
• Complete server support for HTTP/1.x as defined in RFC
  9112 & RFC
  9110
• Complete server support for HTTP/2 as defined in RFC
  9113 & RFC
  9110, comprehensively covered
  by automated h2spec conformance testing
• Support for HTTP content encoding compression on both HTTP/1.x and HTTP/2.
  gzip and deflate methods are supported per
  RFC9110§8.4.1.{2,3}
• Complete server support for WebSockets as defined in RFC
  6455, comprehensively covered by automated
  Autobahn conformance testing. Per-message
  compression as defined in RFC 7692 is also
  supported
• Extremely scalable and performant client handling at a rate up to 4x that of Cowboy for the same
  workload with as-good-or-better memory use

Any Phoenix or Plug app should work with Bandit as a drop-in replacement for
Cowboy; exceptions to this are errors (if you find one, please file an
issue!).

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Using Bandit With Phoenix

Bandit fully supports Phoenix. Phoenix applications which use WebSockets for
features such as Channels or LiveView require Phoenix 1.7 or later.

Using Bandit to host your Phoenix application couldn't be simpler:

1. Add Bandit as a dependency in your Phoenix application's mix.exs:

   elixirCopy
   {:bandit, "~> 1.8"}

2. Add the following adapter: line to your endpoint configuration in config/config.exs, as in the following example:

   elixirCopy
   # config/config.exs
   
   config :your_app, YourAppWeb.Endpoint,
     adapter: Bandit.PhoenixAdapter, # <---- ADD THIS LINE
     url: [host: "localhost"],
     render_errors: ...

3. That's it! You should now see messages at startup indicating that Phoenix is
   using Bandit to serve your endpoint, and everything should 'just work'. Note
   that if you have set any exotic configuration options within your endpoint,
   you may need to update that configuration to work with Bandit; see the
   Bandit.PhoenixAdapter
   documentation for more information.

Using Bandit With Plug Applications

Using Bandit to host your own Plug is very straightforward. Assuming you have
a Plug module implemented already, you can host it within Bandit by adding
something similar to the following to your application's Application.start/2
function:

elixirCopy
# lib/my_app/application.ex

defmodule MyApp.Application do
  use Application

  def start(_type, _args) do
    children = [
      {Bandit, plug: MyApp.MyPlug}
    ]

    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

For less formal usage, you can also start Bandit using the same configuration
options via the Bandit.start_link/1 function:

elixirCopy
# Start an http server on the default port 4000, serving MyApp.MyPlug
Bandit.start_link(plug: MyPlug)

Configuration

A number of options are defined when starting a server. The complete list is
defined by the t:Bandit.options/0 type.

Setting up an HTTPS Server

By far the most common stumbling block encountered when setting up an HTTPS
server involves configuring key and certificate data. Bandit is comparatively
easy to set up in this regard, with a working example looking similar to the
following:

elixirCopy
# lib/my_app/application.ex

defmodule MyApp.Application do
  use Application

  def start(_type, _args) do
    children = [
      {Bandit,
       plug: MyApp.MyPlug,
       scheme: :https,
       certfile: "/absolute/path/to/cert.pem",
       keyfile: "/absolute/path/to/key.pem"}
    ]

    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

WebSocket Support

If you're using Bandit to run a Phoenix application as suggested above, there is
nothing more for you to do; WebSocket support will 'just work'.

If you wish to interact with WebSockets at a more fundamental level, the
WebSock and
WebSockAdapter libraries
provides a generic abstraction for WebSockets (very similar to how Plug is
a generic abstraction on top of HTTP). Bandit fully supports all aspects of
these libraries.

Receiving messages in your Plug process: A word of warning

The Plug specification is concerned only with the shape of the c:Plug.init/1
and c:Plug.call/2 functions; it says nothing about the process model that
underlies the call, nor about how the Plug function should respond to any
messages it may receive. Although it is occasionally necessary to receive
messages from within your Plug call, this must be done with caution as Bandit
makes extensive use of messaging internally, especially with HTTP/2 based
requests.

In particular, you must ensure that your code never receives messages that
match the patterns {:bandit, _} or {:plug_conn, :sent}. Any receive calls
you make should be appropriately guarded to ensure that these messages remain in
the process' mailbox for Bandit to process them when required.

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Implementation Details

Bandit primarily consists of three protocol-specific implementations, one each
for HTTP/1, HTTP/2 and WebSockets. Each of these implementations is
largely distinct from one another, and is described in its own README linked
above.

If you're just taking a casual look at Bandit or trying to understand how an
HTTP server works, the HTTP/1 implementation is likely the best place to
start exploring.

Contributing

Contributions to Bandit are very much welcome! Before undertaking any substantial work, please
open an issue on the project to discuss ideas and planned approaches so we can ensure we keep
progress moving in the same direction.

All contributors must agree and adhere to the project's Code of
Conduct.

Security disclosures should be handled per Bandit's published security policy.

Installation

Bandit is available in Hex. The package can be installed
by adding bandit to your list of dependencies in mix.exs:

elixirCopy
def deps do
  [
    {:bandit, "~> 1.8"}
  ]
end

Documentation can be found at https://bandit.hexdocs.pm.

License

MIT