These are the changes from Apache Traffic Server v3.2 to v4.0. If you are upgrading from v3.0 or earlier, please see the upgrade instructions to v3.2. Note that not all these changes might affect your configuration, but you are encouraged to verify that your configurations are in line with the changes in these upgrade documents.
Body Factory now used for Customizable User Response Pages
Body Factory is now enabled by default - the stock templates are located in the /usr/local/etc/trafficserver/body_factory/default folder. Please be aware of https://issues.apache.org/jira/browse/TS-2217 - Body Factory cannot be disabled and use of proxy.config.body_factory.enable_customizations = 0 will result in empty responses for errors.
Remap processor threads
We have eliminated the old configuration proxy.config.remap.use_remap_processor, and instead it's all controlled via the proxy.config.remap.num_remap_threads configuration. The default for the number of remap threads is now 0, which means that the remap processor threads feature is disabled. To enable the remap processors, set this configuration to a value greater than 0.
Jira Ticket: https://issues.apache.org/jira/browse/TS-846
HostDB cache format change
The hash function for the HostDB cache has changed. This does not require any configuration changes, but the first time running the new version, you will see a log message like
WARNING: header missing/corrupt: [hostdb.config] : reinitializing database.
Jira Ticket: https://issues.apache.org/jira/browse/TS-1925.
Default RAM cache size
The default RAM cache (if proxy.config.cache.ram_cache.size is set to -1) has been increased by a magnitude. A 128GB disk will now allocate 1.2G of RAM cache, whereas previously it would only allocate 128MB.
Jira Ticket: https://issues.apache.org/jira/browse/TS-1994.
New port configuration directives were introduced in Traffic Server 3.2. The backwards compatibility support for old configurations has been removed.
Jira Ticket: https://issues.apache.org/jira/browse/TS-2057.
SSL compression is now disabled by default, controlled via this configuration:
Jira Ticket: https://issues.apache.org/jira/browse/TS-1466
The following configurations are no longer supported (or necessary). They should be removed from your records.config if you have them in there.