Migrating from Cowboy 2.4 to 2.5

Cowboy 2.5 focused on making the test suites pass. A variety of new features, fixes and improvements have also been worked on.

Features added

  • Add option linger_timeout to control how long Cowboy will wait before closing the socket when shutting down the connection. This helps avoid the TCP reset problem HTTP/1.1 suffers from. The default is now 1000 ms.
  • It is now possible to stream a response body without using chunked transfer-encoding when the protocol is HTTP/1.1. To enable this behavior, simply pass the content-length header with the expected size when initiating the streamed response.
  • Update Ranch to 1.6.2
  • Update Cowlib to 2.6.0

Experimental features added

  • Websocket handlers now feature a commands-based interface. The return value from the callbacks can now take the form {Commands, State} where Commands can be frames to be sent or commands yet to be introduced. New commands will be available only through this new interface.
  • Add the {active, boolean()} Websocket handler command. It allows disabling reading from the socket when false is returned. true reenables reading from the socket.
  • Add the protocol option logger that allows configuring which logger module will be used. The logger module must follow the interface of the new logger module in Erlang/OTP 21, or be set to error_logger to keep the old behavior. A similar transport option exists in Ranch 1.6; both options are necessary to override Cowboy's default behavior completely.
  • Add the {log, Level, Format, Args} stream handler command. Making it a command rather than a direct call will simplify silencing particular log messages.

New functions

  • The function cowboy_req:stream_events/3 streams one or more text/event-stream events, encoding them automatically.
  • The functions cowboy_req:read_and_match_urlencoded_body/2,3 can be used to read, parse and match application/x-www-form-urlencoded request bodies, in a similar way to cowboy_req:match_qs/2.

Bugs fixed

  • Fix Erlang/OTP 21 warnings.
  • Ensure that the port number is always defined in the Req object. When it is not provided in the request, the default port number for the protocol being used will be set.
  • Ensure stream handlers can run after cowboy_stream_h.
  • Honor the SETTINGS_ENABLE_PUSH HTTP/2 setting: don't send PUSH frames to clients that disabled it.
  • Fix HTTP/2 settings_timeout option when the value is set to infinity.
  • HTTP/1.1 responses will no longer include a trailer header when the request had no te header.
  • HTTP/1.1 204 responses no longer send the transfer-encoding header when cowboy_req:stream_reply/2,3 is used to send a response.
  • Improve HTTP/1.1 keepalive handling to avoid processing requests that follow the final request that will receive a response.
  • Improve the validation of HTTP/1.1 absolute-form requests.
  • When the switch_protocol is used after a response was sent, Cowboy will no longer attempt to send the 101 informational response for the protocol upgrade. This caused a crash of the connection previously.
  • Errors that occur when a callback returned by content_types_provided does not exist have been improved.
  • Prevent annoying error logs when using sendfile in Erlang/OTP 20 and lower.
  • Add missing frame types to websocket_handle.
  • A test suite has been added for RFC8297 to ensure that 103 informational responses can be sent.
  • Numerous test cases have been fixed, improved or removed in order to make the test suites pass. Most of the failures were caused by broken tests.
  • Some misguiding or incorrect statements in the documentation have been removed or clarified.

Cowboy 2.5 User Guide

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