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[appendix]
== Migrating from Cowboy 2.7 to 2.8

Cowboy 2.8 contains many optimizations for all
protocols. HTTP/1.1 has received the largest
improvements and Cowboy will now be able to
handle noticeably more requests. Thanks to
the folks at Stressgrid for helping identify that
the performance was lower than it should have been
and for benchmarking my many changes and experiments.

Cowboy 2.8 also contains a small number of tweaks
and bug fixes. Cowboy 2.8 is the first Cowboy release,
ever, to be consistently green on all tested platforms.
This is mostly due to the reworking of some test cases,
but a few bugs were discovered and fixed in the process.

Cowboy 2.8 requires Erlang/OTP 22.0 or greater. It may
also work out of the box with Erlang/OTP 21.3 but this
was not tested and is not supported.

=== Features added

* Cowboy will now use `active,N` instead of `active,once`
  to receive data from the socket. This greatly improves
  the performance and allows Cowboy to process more
  requests, especially for HTTP/1.1. The `active_n`
  protocol option can be configured to change the
  `active,N` value. The default is 100 for all protocols.

* Add a `linger_timeout` option for HTTP/2. The default
  is 1000, or one second. This helps ensure that the
  final GOAWAY frame will be properly received by clients.

* The function `cowboy_req:parse_header/2,3` will now
  parse the headers `access-control-request-headers`,
  `access-control-request-method`, `content-encoding`,
  `content-language`, `max-forwards`, `origin`,
  `proxy-authorization` and `trailer`.

* A Performance chapter has been added to the guide.
  More content will be added in future releases.

* Update Cowlib to 2.9.1.

=== Experimental features added

* A `protocols` protocol option allows configuring which
  protocol will be used for clear listeners. Setting it
  to `[http2]` will disable HTTP/1.1 entirely. This feature
  will be extended in a future release.

=== Features modified

* The default value for HTTP/1.1's `max_keepalive` option
  has been increased. It now allows 1000 requests before
  gracefully closing the connection.

* The default value for HTTP/2's `max_received_frame_rate`
  option has been increased. It now allows 10000 frames every
  10 seconds.

* Cowboy will now accept whitespace in cookie names. This
  is in line with the recommended parsing algorithm for the
  upcoming cookie RFC update, and corresponds to what browsers
  are doing.

=== Bugs fixed

* The number of Transport:send/2 calls has been optimized
  for HTTP/2. Reducing the number of calls has a noticeable
  impact on the number of requests that can be processed.

* Trying to use `cowboy_req:reply/4` with a status code of
  204 or 304 and a non-empty response body will now result
  in a crash. Using `cowboy_req:stream_reply/2,3` with 204
  or 304 and then attempting to send a body will also result
  in a crash. These status codes disallow response bodies
  and trying to send one will break HTTP/1.1 framing.

* A crash has been fixed related to HTTP/1.1 pipelining.
  The bug was most likely introduced in Cowboy 2.6 when
  flow control was added for HTTP/1.1 request bodies.

* The HTTP/1.1 protocol code could get stuck because of flow
  control. This has been corrected.

* A crash has been fixed for HTTP/1.1. It occurred when
  a flow control update was requested (such as reading
  the request body) after the body was fully read.

* The timeout was incorrectly reset sometimes when a stream
  (a pair of request/response) terminated. This has been
  corrected.

* Handling of hibernation for Websocket has been improved.
  Websocket over HTTP/2 now supports hibernating. Stray
  messages no longer cancel hibernation.

* The `cowboy_compress_h` stream handler will now ignore
  malformed accept-encoding headers instead of crashing.

* The manual pages for `cowboy:start_clear(3)` and
  `cowboy:start_tls(3)` now mentions that some protocol
  options may be documented in the releevant stream
  handler.

* The manual page for `cowboy_req:parse_header(3)` was
  corrected. When an unsupported header is given the
  function crashes, it does not return an `undefined` tuple.

* The routing algorithm description in the user guide has
  been improved.

* The test suites are now consistently green on all tested
  platforms. Most of the test failures were caused by flaky
  tests. Avoiding the use of timeouts fixed most of them.
  A small number of tests had to be reworked.