[appendix]
== Migrating from Cowboy 2.13 to 2.14

Cowboy 2.14 adds experimental support for HTTP/3
WebTransport based on the most recent draft. It
also has a new data delivery mechanism for HTTP/2
and HTTP/3 Websocket, providing better performance.

Cowboy 2.14 requires Erlang/OTP 24.0 or greater.

=== Features added

* The `relay` data delivery mechanism has been
  added to HTTP/2 and HTTP/3 protocols. Using
  this mechanism lets the Websocket protocol
  bypass stream handlers to forward data from
  the connection process to the Websocket
  session process, as well as better manage
  HTTP/2's flow control. This results in a
  noticeable performance improvement. This
  new mechanism can be used by all sub-protocols
  built on top of HTTP/2 or HTTP/3 such as
  Websocket or the upcoming HTTP/2 WebTransport.

* The `last_modified` callback of REST handlers
  now accepts `undefined` as a return value to
  allow conditionally providing a timestamp.

=== Experimental features added

* Experimental support for HTTP/3 WebTransport
  has been added, based on the most recent RFC
  drafts. The implementation should also be
  compatible with earlier drafts that are
  currently in use by some browsers. Both
  HTTP/3 and HTTP/3 WebTransport are disabled
  by default; to enable, the environment
  variable COWBOY_QUICER must be set at
  compile-time, and a number of options must
  be provided at run time, including
  `enable_connect_protocol`, `h3_datagram`,
  `wt_max_sessions` and for earlier drafts
  `enable_webtransport`. The test suite is
  the best place to get started at this time.

=== Optimisation-related changes

* The `dynamic_buffer` option introduced in
  the previous release has been tweaked to
  start at 512 bytes and have its value
  changed less abruptly. This is based on
  additional work done implementing the same
  feature in RabbitMQ.

* The static file handler will now use `raw`
  mode to read file information to avoid a
  bottleneck when querying the file server.

=== Bugs fixed

* It was possible for Websocket to fail to
  enable active mode again after it had been
  disabled. This has been fixed.