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   distributed with this work for additional information
   regarding copyright ownership.  The ASF licenses this file
   to you under the Apache License, Version 2.0 (the
   "License"); you may not use this file except in compliance
   with the License.  You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing,
   software distributed under the License is distributed on an
   "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
   KIND, either express or implied.  See the License for the
   specific language governing permissions and limitations
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.. include:: ../../common.defs

.. _developer-debug-tags:

Debug Tags
**********

Use the API macro
``Dbg(ctl, const char *format_str, ...)`` to add
traces in your plugin. In this macro:

-  ``ctl`` is the Traffic Server parameter that enables Traffic Server
   to print out ``format_str``.  It is an instance of the class ``DbgCtl``.

-  ``...`` are variables for ``format_str`` in the standard ``printf``
   style.

The ``DbgCtl`` control is enabled when debug output is
enabled globally by configuration, and ``tag`` matches a configured
regular expression.

Run Traffic Server with the ``-Ttag`` option. For example, if the tag is
``my-plugin``, then the debug output goes to ``traffic.out.``\ See
below:

::

       traffic_server -T"my-plugin"

Sets the following variables in :file:`records.yaml` (in the Traffic Server
``config`` directory):

.. code-block:: yaml

   ts:
     diags:
       debug:
         enabled: 1
         tags: debug-tag-name


In this case, debug output goes to ``traffic.out``.

The statement ``"Starting my-plugin at <time>"`` appears whenever you
run Traffic Server with the ``my-plugin`` tag:

::

       traffic_server -T"my-plugin"

Example:

.. code-block:: cpp

       namespace
       {
       DbgCtl my_dbg_ctl{"my-plugin"}; // Non-local variable.
       }
       ...
       Dbg(my_dbg_ctl, "Starting my-plugin at %d", the_time);
       ...
       Dbg(my_dbg_ctl, "Later on in my-plugin");

Other Useful Internal Debug Tags
================================

Embedded in the base Traffic Server code are many debug tags for
internal debugging purposes. These can also be used to follow Traffic
Server behavior for testing and analysis.

The debug tag setting (``-T`` and ``proxy.config.diags.debug.tags``) is a
anchored regular expression (PCRE) against which the tag for a specific debug
message is matched. This means the value "http" matches debug messages
with the tag "http" but also "http\_seq" and "http\_trans". If you want
multiple tags then use a pipe symbol to separate the tags. For example
"http\_tproxy\|dns\|hostdb" will match any of the message tags
"http\_tproxy", "dns", "hostdb", or "dns\_srv" (but not "http\_trans"
nor "splitdns").

Some of the useful HTTP debug tags are:

-  ``http_hdrs`` - traces all incoming and outgoing HTTP headers.

-  ``http_trans`` - traces actions in an HTTP transaction.

-  ``http_seq`` - traces the sequence of the HTTP state machine.

-  ``http_tproxy`` - transparency related HTTP events

-  ``dns`` - DNS operations

-  ``hostdb`` - Host name lookup

-  ``iocore_net`` - Socket and low level IO (very voluminous)

-  ``socket`` - socket operations

-  ``ssl`` - SSL related events

-  ``cache`` - Cache operations (many subtags, examine the output to
   narrow the tag set)

-  ``cache_update`` - Cache updates including writes

-  ``cache_read`` - Cache read events.

-  ``dir_probe`` - Cache searches.

-  ``sdk`` - gives some warning concerning API usage.