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## Appendix D. 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.

## D.1. 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.

## D.2. 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.

## D.3. 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.

## D.4. Bugs fixed

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

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