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.. include:: ../../common.defs

.. _admin-plugins-xdebug:

XDebug Plugin
*************

The `XDebug` plugin allows HTTP clients to debug the operation of
the Traffic Server cache using the default ``X-Debug`` header. The plugin
is triggered by the presence of the ``X-Debug`` or the configured header in
the client request. The contents of this header should be the names of the
debug headers that are desired in the response. The `XDebug` plugin
will inject the desired headers into the client response.  In addition, a value of the
form ``fwd=n`` may appear in the ``X-Debug`` header, where ``n`` is a nonnegative
number.  If ``n`` is zero, the ``X-Debug`` header will be deleted.  Otherwise, ``n`` is
decremented by 1.  ``=n`` may be omitted, in which case the ``X-Debug`` header will
not be modified or deleted.

`XDebug` is a global plugin. It is installed by adding it to the
:file:`plugin.config` file. It currently takes a single, optional
configuration option, ``--header``. E.g.

    --header=ATS-My-Debug

This overrides the default ``X-Debug`` header name.


Debugging Headers
=================

The `XDebug` plugin is able to generate the following debugging headers:

Via
    If the ``Via`` header is requested, the `XDebug` plugin sets the
    :ts:cv:`proxy.config.http.insert_response_via_str` configuration variable
    to ``3`` for the request.

Diags
    If the ``Diags`` header is requested, the `XDebug` plugin enables the
    transaction specific diagnostics for the transaction. This also requires
    that :ts:cv:`proxy.config.diags.debug.enabled` is set to ``1``.

Probe
    All request and response headers are written to the response body. Because
    the body is altered, it disables writing to cache.
    In conjunction with the `fwd` tag, the response body will contain a
    chronological log of all headers for all transactions used for this
    response.

    Layout:

    - Request Headers from Client  -> Proxy A
    - Request Headers from Proxy A -> Proxy B
    - Original content body
    - Response Headers from Proxy B -> Proxy A
    - Response Headers from Proxy A -> Client

X-Cache-Key
    The ``X-Cache-Key`` header contains the URL that identifies the HTTP object in the
    Traffic Server cache. This header is particularly useful if a custom cache
    key is being used.

X-Cache
    The ``X-Cache`` header contains the results of any cache lookups.

    ==========  ===========
    Value       Description
    ==========  ===========
    none        No cache lookup was attempted.
    miss        The object was not found in the cache.
    hit-stale   The object was found in the cache, but it was stale.
    hit-fresh   The object was fresh in the cache.
    skipped     The cache lookup was skipped.
    ==========  ===========

    If a request goes through multiple proxies, each one appends its X-Cache header content
    the end of the existing X-Cache header. This is the same order as for the
    ``Via`` header.

X-Cache-Generation
  The cache generation ID for this transaction, as specified by the
  :ts:cv:`proxy.config.http.cache.generation` configuration variable.

X-Milestones
    The ``X-Milestones`` header contains detailed information about
    how long the transaction took to traverse portions of the HTTP
    state machine. The timing information is obtained from the
    :c:func:`TSHttpTxnMilestoneGet` API. Each milestone value is a
    fractional number of seconds since the beginning of the
    transaction.

X-Transaction-ID
    A unique transaction ID, which identifies this request / transaction. This
    matches the log field format that is also available, %<cruuid>.

X-Remap
    If the URL was remapped for a request, this header gives the *to* and *from* field from the line in remap.config that caused
    the URL to be remapped.

X-ParentSelection-Key
    The ``X-ParentSelection-Key`` header contains the URL that is used to
    determine parent selection for an object in the Traffic Server. This
    header is particularly useful if a custom parent selection key is
    being used.
