This is a place holder for whatever incompatible changes we are making.
New configuration defaults
A number of configurations have new defaults and/or semantics. Please make sure you verify your configs accordingly, and that you get expected behavior.
New default configurations for "ignore mismatch" configurations
The following configurations now all have a new default configuration value of "2":
The old defaults ("0") had a behavior where if the client sent any of the corresponding accept header, we would honor that. The effect could be poor cache hit ratio, and worse, high churn rate of objects in the cache. The fix would be to always ignore the clients various accept headers, which could have the side effect of serving the wrong object to the UA. As of Apache Traffic Server v4.1.0 and later, we've added this new configuration value of "2", which is a safer variation on "1". It will ignore the respective accept header from the client, but only if there is no Vary: from the origin.
New default for proxy.config.net.sock_option_flag_out
The new default enables TCP_NODELAY on all outgoing ("origin") connections.
New values for proxy.config.exec_thread.affinity
The value 1 is now for NUMA nodes. This is often 1:1 with sockets, but that is not guaranteed. The old values are all shifted up by one.
Changes to existing plugin APIs
A number of changes to the existing plugin APIs are made in this release. This implies that you at a minimum must recompile any custom plugins. And more likely than not, you will even have to make modifications to your code. There is a convenience tool part of the source code distribution, named apichecker.pl. You can run this against your source code, to get some guidelines of changes you might have to make.