[appendix]
== Migrating from Cowboy 2.12 to 2.13

Cowboy 2.13 focuses on improving the performance of
Websocket, as well as the HTTP protocols. It also
contains a variety of new features and bug fixes.
In addition, Cowboy 2.13 is the first Cowboy version
that contains the experimental HTTP/3 support.

Cowboy 2.13 requires Erlang/OTP 24.0 or greater.

=== Features added

* The option `dynamic_buffer` has been added. When
  enabled, Cowboy will dynamically change the
  `buffer` socket option based on how much data
  it receives. It will start at 1024 bytes and
  go up to 131072 bytes by default. This applies
  to HTTP/1.1, HTTP/2 and Websocket. The performance
  gains are very important depending on the scenario.

* HTTP/1.1 and HTTP/2 now accept the `hibernate`
  option. When set the connection process will
  automatically hibernate to reduce memory usage
  at a small performance cost.

* The `protocols` and `alpn_default_protocol` protocol
  options have been added to control exactly which
  HTTP protocols are allowed over clear and TLS listeners.

* The Websocket `max_frame_size` option can now be
  set dynamically via the `set_options` command.
  This allows configuring a smaller max size and
  increase it after authentication or other checks.

* `cowboy_req:set_resp_headers` now accept lists of
  headers. This can be used to simplify passing
  headers coming from client applications such as
  Gun. Note that the set-cookie header cannot be
  provided using this function.

* `cowboy_rest` now always sets the allow header.

* Update Ranch to 1.8.1.

* Update Cowlib to 2.14.0.

* When using Hex.pm, version check requirements will
  now be relaxed. Cowboy will accept any Ranch version
  from 1.8.0 to 2.2.0 as well as future 2.x versions.
  Similarly, any Cowlib 2.x version from 2.14.0 will
  be accepted.

=== Experimental features added

* Experimental support for HTTP/3 has been added,
  including Websocket over HTTP/3. HTTP/3 support
  is disabled by default; to enable, the environment
  variable COWBOY_QUICER must be set at compile-time.

=== Features deprecated

* The `inactivity_timeout` option is now deprecated
  for all protocols. It is de facto ignored when
  `hibernate` is enabled.

=== Optimisation-related changes

* The behavior of the `idle_timeout` timer has been
  changed for HTTP/2 and Websocket. Cowboy used to
  reset the timer on every data packet received from
  the socket. Now Cowboy will check periodically
  whether new data was received in the interval.

* URI and query string hex encoding and decoding has
  been optimised.

* Websocket UTF-8 validation of text frames has been
  optimised.

* Websocket unmasking has been optimised.

=== Bugs fixed

* HTTP/1.1 upgrade to HTTP/2 is now disabled over TLS,
  as HTTP/2 over TLS must be negotiated via ALPN.

* `cowboy_req:filter_cookies` could miss valid cookies.
  It has been corrected.

* HTTP/1.1 could get to a state where it would stop
  receiving data from the socket, or buffer the data
  without processing it, and the connection eventually
  time out. This has been fixed.

* Websocket did not compress zero-length frames properly.
  This resulted in decompression errors in the client.
  This has been corrected.

* Websocket compression will now be disabled when only
  the server sets `client_max_window_bits`, as otherwise
  decompression errors will occur.

* Websocket will now apply `max_frame_size` both to
  compressed frames as well as the uncompressed payload.
  Cowboy will stop decompressing when the limit is
  reached.

* Cowboy now properly handles exits of request processes
  that occurred externally (e.g. via `exit/2`).

* Invalid return values from `content_types_provided`
  could result in an atom sent to the socket, leading
  to a cryptic error message. The invalid value will
  now result in a better error message.