See these 0.16.x to 0.17.0 upgrade instructions to bring your existing apps up to speed.

• Enhancements
  • [Endpoint] Allow check_origin and force_ssl to be config in transports and fallback to endpoint config
  • [Transport] Log when check_origin fails
• Bug fixes
  • [Mix] Properly humanize names in the generator
• Deprecations
  • [Endpoint] render_errors: [default_format: "html"] is deprecated in favor of render_errors: [accepts: ["html"]]
• Backward incompatible changes
  • [Controller] The "format" param for overriding the accept header has been renamed to "_format" and is no longer injected into the params when parsing the Accept headers. Use get_format/1 to access the negotiated format.
  • [ChannelTest] In order to test channels, one must now explicitly create a socket and pass it to subscribe_and_join. For example, subscribe_and_join(MyChannel, "my_topic") should now become socket() |> subscribe_and_join(MyChannel, "my_topic") or socket("user:id", %{user_id: 13}) |> subscribe_and_join(MyChannel, "my_topic").