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[appendix]
== Migrating from Cowboy 1.0 to 2.0

A lot has changed between Cowboy 1.0 and 2.0. The `cowboy_req`
interface in particular has seen a massive revamp. Hooks are
gone, their functionality can now be achieved via stream
handlers.

The documentation has seen great work, in particular the
manual. Each module and each function now has its own dedicated
manual page with full details and examples.

=== Compatibility

Compatibility with Erlang/OTP R16, 17 and 18 has been dropped.
Erlang/OTP 19.0 or above is required. It is non-trivial to
make Cowboy 2.0 work with older Erlang/OTP versions.

Cowboy 2.0 is not compatible with Cowlib versions older than
2.0. It should be compatible with Ranch 1.0 or above, however
it has not been tested with Ranch versions older than 1.4.

Cowboy 2.0 is tested on Arch Linux, Ubuntu, FreeBSD, Windows
and OSX. It is tested with every point release (latest patch
release) and also with HiPE on the most recent release.

Cowboy 2.0 now comes with Erlang.mk templates.

=== Features added

* The HTTP/2 protocol is now supported.

* Cowboy no longer uses only one process per connection.
  It now uses one process per connection plus one process
  per request by default. This is necessary for HTTP/2.
  There might be a slight drop in performance for HTTP/1.1
  connections due to this change.

* Cowboy internals have largely been reworked in order to
  support HTTP/2. This opened the way to stream handlers,
  which are a chain of modules that are called whenever
  something happens relating to a request/response.

* The `cowboy_stream_h` stream handler has been added.
  It provides most of Cowboy's default behavior.

* The `cowboy_compress_h` stream handler has been added.
  It compresses responses when possible. It's worth noting
  that it compresses in more cases than Cowboy 1.0 ever did.

* Because of the many changes in the internals of Cowboy,
  many options have been added or modified. Of note is that
  the Websocket options are now given per handler rather
  than for the entire listener.

* Websocket permessage-deflate compression is now supported
  via the `compress` option.

* Static file handlers will now correctly find files found
  in '.ez' archives.

* Constraints have been generalized and are now used not only
  in the router but also in some `cowboy_req` functions. Their
  interface has also been modified to allow for reverse
  operations and formatting of errors.

=== Features removed

* SPDY support has been removed. Use HTTP/2 instead.

* Hooks have been removed. Use xref:streams[stream handlers] instead.

* The undocumented `waiting_stream` hack has been removed.
  It allowed disabling chunked transfer-encoding for HTTP/1.1.
  It has no equivalent in Cowboy 2.0. Open a ticket if necessary.

* Sub protocols still exist, but their interface has largely changed
  and they are no longer documented for the time being.

=== Changed behaviors

* The handler behaviors have been renamed and are now `cowboy_handler`,
  `cowboy_loop`, `cowboy_rest` and `cowboy_websocket`.

* Plain HTTP, loop, REST and Websocket handlers have had their
  init and terminate callbacks unified. They now all use the
  `init/2` and `terminate/3` callbacks. The latter is now optional.
  The terminate reason has now been documented for all handlers.

* The tuple returned to switch to a different handler type has
  changed. It now takes the form `{Module, Req, State}` or
  `{Module, Req, State, Opts}`, where `Opts` is a map of options
  to configure the handler. The timeout and hibernate options
  must now be specified using this map, where applicable.

* All behaviors that used to accept `halt` or `shutdown` tuples
  as a return value no longer do so. The return value is now
  a `stop` tuple, consistent across Cowboy.

* Middlewares can no longer return an `error` tuple. They have
  to send the response and return a `stop` tuple instead.

* The `known_content_type` REST handler callback has been removed
  as it was found unnecessary.

* Websocket handlers have both the normal `init/2` and
  an optional `websocket_init/1` function. The reason for
  that exception is that the `websocket_*` callbacks execute
  in a separate process from the `init/2` callback, and it
  was therefore not obvious how timers or monitors should
  be setup properly. They are effectively initializing the
  handler before and after the HTTP/1.1 upgrade.

* Websocket handlers can now send frames directly from
  `websocket_init/1`. The frames will be sent immediately
  after the handshake.

* Websocket handler callbacks no longer receive the Req
  argument. The `init/2` callback still receives it and
  can be used to extract relevant information. The `terminate/3`
  callback, if implemented, may still receive the Req
  (see next bullet point).

* Websocket handlers have a new `req_filter` option that
  can be used to customize how much information should be
  discarded from the Req object after the handshake. Note
  that the Req object is only available in `terminate/3`
  past that point.

* Websocket handlers have their timeout default changed
  from infinity to 60 seconds.

=== New functions

* The `cowboy_req:scheme/1` function has been added.

* The `cowboy_req:uri/1,2` function has been added, replacing the
  less powerful functions `cowboy_req:url/1` and `cowboy_req:host_url/1`.

* The functions `cowboy_req:match_qs/2` and `cowboy_req:match_cookies/2`
  allow matching query string and cookies against constraints.

* The function `cowboy_req:set_resp_cookie/3` has been added to
  complement `cowboy_req:set_resp_cookie/4`.

* The functions `cowboy_req:resp_header/2,3` and `cowboy_req:resp_headers/1`
  have been added. They can be used to retrieve response headers
  that were previously set.

* The function `cowboy_req:set_resp_headers/2` has been added. It
  allows setting many response headers at once.

* The functions `cowboy_req:push/3,4` can be used to push resources
  for protocols that support it (by default only HTTP/2).

=== Changed functions

* The `cowboy:start_http/4` function was renamed to `cowboy:start_clear/3`.

* The `cowboy:start_https/4` function was renamed to `cowboy:start_tls/3`.

* Most, if not all, functions in the `cowboy_req` module have been modified.
  Please consult the changelog of each individual functions. The changes
  are mainly about simplifying and clarifying the interface. The Req is no
  longer returned when not necessary, maps are used wherever possible,
  and some functions have been renamed.

* The position of the `Opts` argument for `cowboy_req:set_resp_cookie/4`
  has changed to improve consistency. It is now the last argument.

=== Removed functions

* The functions `cowboy_req:url/1` and `cowboy_req:host_url/1` have been
  removed in favor of the new function `cowboy_req:uri/1,2`.

* The functions `cowboy_req:meta/2,3` and `cowboy_req:set_meta/3` have
  been removed. The Req object is now a public map, therefore they became
  unnecessary.

* The functions `cowboy_req:set_resp_body_fun/2,3` have been removed.
  For sending files, the function `cowboy_req:set_resp_body/2` can now
  take a sendfile tuple.

* Remove many undocumented functions from `cowboy_req`, including the
  functions `cowboy_req:get/2` and `cowboy_req:set/3`.

=== Other changes

* The correct percent-decoding algorithm is now used for path elements
  during routing. It will no longer decode `+` characters.

* The router will now properly handle path segments `.` and `..`.

* Routing behavior has changed for URIs containing latin1 characters.
  They are no longer allowed. URIs are expected to be in UTF-8 once
  they are percent-decoded.

* Clients that send multiple headers of the same name
  will have the values of those headers concatenated into a
  comma-separated list. This is of special importance in the
  case of the content-type header, as previously only the
  first value was used in the `content_types_accepted/2` step
  in REST handlers.

* Etag comparison in REST handlers has been fixed. Some requests may
  now fail when they succeeded in the past.

* The `If-*-Since` headers are now ignored in REST handlers if
  the corresponding `If*-Match` header exist. The former is
  largely a backward compatible header and this shouldn't create
  any issue. The new behavior follows the current RFCs more closely.

* The static file handler has been improved to handle more special
  characters on systems that accept them.