.. Licensed to the Apache Software Foundation (ASF) under one
   or more contributor license agreements.  See the NOTICE file
   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
   under the License.

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

.. _http-proxy-caching:

HTTP Proxy Caching
******************

HTTP proxy caching enables you to store copies of frequently-accessed web
objects (such as documents, images, and articles) and then serve this
information to users on demand. It improves performance and frees up
Internet bandwidth for other tasks.

.. toctree::
   :maxdepth: 2

Understanding HTTP Web Proxy Caching
====================================

Internet users direct their requests to web servers all over the
Internet. A caching server must act as a *web proxy server* so it can
serve those requests. After a web proxy server receives requests for web
objects, it either serves the requests or forwards them to the *origin
server* (the web server that contains the original copy of the
requested information). The Traffic Server proxy supports *explicit
proxy caching*, in which the user's client software must be configured
to send requests directly to the Traffic Server proxy. The following
overview illustrates how Traffic Server serves a request.

#. Traffic Server receives a client request for a web object.

#. Using the object address, Traffic Server tries to locate the
   requested object in its object database (*cache*).

#. If the object is in the cache, then Traffic Server checks to see if
   the object is fresh enough to serve. If it is fresh, then Traffic
   Server serves it to the client as a *cache hit* (see the figure
   below).

   .. figure:: /static/images/admin/cache_hit.jpg
      :align: center
      :alt: A cache hit

      A cache hit

#. If the data in the cache is stale, then Traffic Server connects to
   the origin server and checks if the object is still fresh (a
   :term:`revalidation`). If it is, then Traffic Server immediately sends
   the cached copy to the client.

#. If the object is not in the cache (a *cache miss*) or if the server
   indicates the cached copy is no longer valid, then Traffic Server
   obtains the object from the origin server. The object is then
   simultaneously streamed to the client and the Traffic Server local
   cache (see the figure below). Subsequent requests for the object can
   be served faster because the object is retrieved directly from cache.

   .. figure:: /static/images/admin/cache_miss.jpg
      :align: center
      :alt: A cache miss

      A cache miss

Caching is typically more complex than the preceding overview suggests.
In particular, the overview does not discuss how Traffic Server ensures
freshness, serves correct HTTP alternates, and treats requests for
objects that cannot or should not be cached. The following sections discuss
these issues in greater detail.

.. _ensuring-cached-object-freshness:

Ensuring Cached Object Freshness
================================

When Traffic Server receives a request for a web object, it first tries
to locate the requested object in its cache. If the object is in cache,
then Traffic Server checks to see if the object is fresh enough to
serve. For HTTP objects, Traffic Server supports optional
author-specified expiration dates. Traffic Server adheres to these
expiration dates; otherwise, it picks an expiration date based on how
frequently the object is changing and on administrator-chosen freshness
guidelines. Objects can also be revalidated by checking with the origin
server to see if an object is still fresh.

HTTP Object Freshness
---------------------

Traffic Server determines whether an HTTP object in the cache is fresh
by checking the following conditions in order:

-  **Checking the** ``Expires`` **or** ``max-age`` **header**

   Some HTTP objects contain ``Expires`` headers or ``max-age`` headers
   that explicitly define how long the object can be cached. Traffic
   Server compares the current time with the expiration time to
   determine if the object is still fresh.

-  **Checking the** ``Last-Modified`` **/** ``Date`` **header**

   If an HTTP object has no ``Expires`` header or ``max-age`` header,
   then Traffic Server can calculate a freshness limit using the
   following formula::

      freshness_limit = ( date - last_modified ) * 0.10

   where *date* is the date in the object's server response header
   and *last_modified* is the date in the ``Last-Modified`` header.
   If there is no ``Last-Modified`` header, then Traffic Server uses the
   date the object was written to cache. The value ``0.10`` (10 percent)
   can be increased or reduced to better suit your needs. Refer to
   `Modifying Aging Factor for Freshness Computations`_.

   The computed freshness limit is bound by a minimum and maximum value.
   Refer to `Setting Absolute Freshness Limits`_ for more information.

-  **Checking the absolute freshness limit**

   For HTTP objects that do not have ``Expires`` headers or do not have
   both ``Last-Modified`` and ``Date`` headers, Traffic Server uses a
   maximum and minimum freshness limit. Refer to
   `Setting Absolute Freshness Limits`_.

-  **Checking revalidate rules in** :file:`cache.config`

   Revalidate rules apply freshness limits to specific HTTP objects. You
   can set freshness limits for objects originating from particular
   domains or IP addresses, objects with URLs that contain specified
   regular expressions, objects requested by particular clients, and so
   on. Refer to :file:`cache.config`.

Modifying Aging Factor for Freshness Computations
-------------------------------------------------

If an object does not contain any expiration information, then Traffic
Server can estimate its freshness from the ``Last-Modified`` and
``Date`` headers. By default, Traffic Server stores an object for 10% of
the time that elapsed since it last changed. You can increase or reduce
the percentage according to your needs.

To modify the aging factor for freshness computations:

#. Change the value for :ts:cv:`proxy.config.http.cache.heuristic_lm_factor`.

#. Run the :option:`traffic_ctl config reload` command to apply the configuration changes.

Setting Absolute Freshness Limits
---------------------------------

Some objects do not have ``Expires`` headers or do not have both
``Last-Modified`` and ``Date`` headers. To control how long these
objects are considered fresh in the cache, specify an *absolute
freshness limit*.

To specify an absolute freshness limit:

#. Edit the variables :ts:cv:`proxy.config.http.cache.heuristic_min_lifetime`
   and :ts:cv:`proxy.config.http.cache.heuristic_max_lifetime` in
   :file:`records.config`.

#. Run the :option:`traffic_ctl config reload` command to apply the configuration changes.

Specifying Header Requirements
------------------------------

To further ensure freshness of the objects in the cache, configure
Traffic Server to cache only objects with specific headers. By default,
Traffic Server caches all objects (including objects with no headers);
you should change the default setting only for specialized proxy
situations. If you configure Traffic Server to cache only HTTP objects
with ``Expires`` or ``max-age`` headers, then the cache hit rate will be
noticeably reduced (since very few objects will have explicit expiration
information).

To configure Traffic Server to cache objects with specific headers:

#. Change the value for :ts:cv:`proxy.config.http.cache.required_headers`
   in :file:`records.config`.

#. Run the :option:`traffic_ctl config reload` command to apply the configuration changes.

Cache-Control Headers
---------------------

Even though an object might be fresh in the cache, clients or servers
often impose their own constraints that preclude retrieval of the object
from the cache. For example, a client might request that a object not
be retrieved from a cache, or if it does allow cache retrieval, then it
cannot have been cached for more than 10 minutes.

Traffic Server bases the servability of a cached object on ``Cache-Control``
headers that appear in both client requests and server responses. The following
``Cache-Control`` headers affect whether objects are served from cache:

-  The ``no-cache`` header, sent by clients, tells Traffic Server that
   it should not serve any objects directly from the cache. When present in a
   client request, Traffic Server will always obtain the object from the
   origin server. You can configure Traffic Server to ignore client
   ``no-cache`` headers. Refer to `Configuring Traffic Server to Ignore Client no-cache Headers`_
   for more information.

-  The ``max-age`` header, sent by servers, is compared to the object
   age. If the age is less than ``max-age``, then the object is fresh
   and can be served from the Traffic Server cache.

-  The ``min-fresh`` header, sent by clients, is an *acceptable
   freshness tolerance*. This means that the client wants the object to
   be at least this fresh. Unless a cached object remains fresh at least
   this long in the future, it is revalidated.

-  The ``max-stale`` header, sent by clients, permits Traffic Server to
   serve stale objects provided they are not too old. Some browsers
   might be willing to take slightly stale objects in exchange for
   improved performance, especially during periods of poor Internet
   availability.

Traffic Server applies ``Cache-Control`` servability criteria after HTTP
freshness criteria. For example, an object might be considered fresh but will
not be served if its age is greater than its ``max-age``.

Revalidating HTTP Objects
-------------------------

When a client requests an HTTP object that is stale in the cache,
Traffic Server revalidates the object. A *revalidation* is a query to
the origin server to check if the object is unchanged. The result of a
revalidation is one of the following:

-  If the object is still fresh, then Traffic Server resets its
   freshness limit and serves the object.

-  If a new copy of the object is available, then Traffic Server caches
   the new object (thereby replacing the stale copy) and simultaneously
   serves the object to the client.

-  If the object no longer exists on the origin server, then Traffic
   Server does not serve the cached copy.

-  If the origin server does not respond to the revalidation query, then
   Traffic Server serves the stale object along with a
   ``111 Revalidation Failed`` warning.

By default, Traffic Server revalidates a requested HTTP object in the
cache if it considers the object to be stale. Traffic Server evaluates
object freshness as described in `HTTP Object Freshness`_.
You can reconfigure how Traffic Server evaluates freshness by selecting
one of the following options:

*Traffic Server considers all HTTP objects in the cache to be stale:*
    Always revalidate HTTP objects in the cache with the origin server.

*Traffic Server considers all HTTP objects in the cache to be fresh:*
    Never revalidate HTTP objects in the cache with the origin server.

*Traffic Server considers all HTTP objects without* ``Expires`` *or* ``Cache-control`` *headers to be stale:*
    Revalidate all HTTP objects without ``Expires`` or ``Cache-Control`` headers.

To configure how Traffic Server revalidates objects in the cache, you
can set specific revalidation rules in :file:`cache.config`.

To configure revalidation options

#. Edit the variable :ts:cv:`proxy.config.http.cache.when_to_revalidate`
   in :file:`records.config`.

#. Run the :option:`traffic_ctl config reload` command to apply the configuration changes.

.. _pushing-content-into-the-cache:

Pushing Content into the Cache
==============================

Traffic Server supports the HTTP ``PUSH`` method of content delivery.
Using HTTP ``PUSH``, you can deliver content directly into the cache
without client requests.

Configuring Traffic Server for PUSH Requests
--------------------------------------------

Before you can deliver content into your cache using HTTP ``PUSH``, you
must configure Traffic Server to accept ``PUSH`` requests.

#. Edit :file:`ip_allow.yaml` to allow ``PUSH`` from the appropriate addresses.

#. Update :ts:cv:`proxy.config.http.push_method_enabled` in
   :file:`records.config`::

        CONFIG proxy.config.http.push_method_enabled INT 1

#. Run the command :option:`traffic_ctl config reload` to apply the configuration changes.

Understanding HTTP PUSH
-----------------------

``PUSH`` uses the HTTP 1.1 message format. The body of a ``PUSH``
request contains the response header and response body that you want to
place in the cache. The following is an example of a ``PUSH`` request::

   PUSH http://www.company.com HTTP/1.0
   Content-length: 84

   HTTP/1.0 200 OK
   Content-type: text/html
   Content-length: 17

   <HTML>
   a
   </HTML>

.. important::

   Your ``PUSH`` headers must include ``Content-length``, the value for which
   must include both headers and body byte counts. The value is not optional,
   and an improper (too large or too small) value will result in undesirable
   behavior.

Tools that will help manage pushing
-----------------------------------

Traffic Server comes with a Perl script for pushing, :program:`tspush`,
which can assist with understanding how to write scripts for pushing
content yourself.

Pinning Content in the Cache
============================

The *Cache Pinning Option* configures Traffic Server to keep certain
HTTP objects in the cache for a specified time. You can use this option
to ensure that the most popular objects are in cache when needed and to
prevent Traffic Server from deleting important objects. Traffic Server
observes ``Cache-Control`` headers and pins an object in the cache only
if it is indeed cacheable.

To set cache pinning rules:

#. Enable :ts:cv:`proxy.config.cache.permit.pinning` in :file:`records.config`::

        CONFIG proxy.config.cache.permit.pinning INT 1

#. Add a rule in :file:`cache.config` for each URL you want Traffic Server to
   pin in the cache. For example::

      url_regex=^https?://(www.)?apache.org/dev/ pin-in-cache=12h

#. Run the command :option:`traffic_ctl config reload` to apply the configuration changes.

Caching HTTP Objects
====================

When Traffic Server receives a request for a web object that is not in
the cache, it retrieves the object from the origin server and serves it
to the client. At the same time, Traffic Server checks if the object is
cacheable before storing it in its cache to serve future requests.

Traffic Server responds to caching directives from clients and origin
servers, as well as directives you specify through configuration options
and files.

Client Directives
-----------------

By default, Traffic Server does not cache objects with the following
request headers:

-  ``Authorization``

-  ``Cache-Control: no-store``

-  ``Cache-Control: no-cache``

   To configure Traffic Server to ignore this request header, refer to
   `Configuring Traffic Server to Ignore Client no-cache Headers`_.

-  ``Cookie`` (for text objects)

   By default, Traffic Server caches objects served in response to
   requests that contain cookies (even if the object is text). You can
   configure Traffic Server to not cache cookied content of any type,
   cache all cookied content, or cache cookied content that is of image
   type only. For more information, refer to `Caching Cookied Objects`_.

Configuring Traffic Server to Ignore Client no-cache Headers
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

By default, Traffic Server strictly observes client
``Cache-Control: no-cache`` directives. If a requested object contains a
``no-cache`` header, then Traffic Server forwards the request to the
origin server even if it has a fresh copy in cache. You can configure
Traffic Server to ignore client ``no-cache`` directives such that it
ignores ``no-cache`` headers from client requests and serves the object
from its cache.

#. Edit :ts:cv:`proxy.config.http.cache.ignore_client_no_cache` in
   :file:`records.config`. ::

        CONFIG proxy.config.http.cache.ignore_client_no_cache INT 1

#. Run the command :option:`traffic_ctl config reload` to apply the configuration changes.

Origin Server Directives
------------------------

By default, Traffic Server does not cache objects with the following response
headers:

-  ``Cache-Control: no-store``

-  ``Cache-Control: private``

-  ``WWW-Authenticate``

   To configure Traffic Server to ignore ``WWW-Authenticate`` headers,
   refer to `Configuring Traffic Server to Ignore WWW-Authenticate Headers`_.

-  ``Set-Cookie``

-  ``Cache-Control: no-cache``

   To configure Traffic Server to ignore ``no-cache`` headers, refer to
   `Configuring Traffic Server to Ignore Server no-cache Headers`_.

-  ``Expires`` header with a value of 0 (zero) or a past date.

Configuring Traffic Server to Ignore Server no-cache Headers
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

By default, Traffic Server strictly observes ``Cache-Control: no-cache``
directives. A response from an origin server with a ``no-cache`` header
is not stored in the cache and any previous copy of the object in the
cache is removed. If you configure Traffic Server to ignore ``no-cache``
headers, then Traffic Server also ignores ``no-store`` headers. The
default behavior of observing ``no-cache`` directives is appropriate
in most cases.

To configure Traffic Server to ignore server ``no-cache`` headers:

#. Edit :ts:cv:`proxy.config.http.cache.ignore_server_no_cache` in
   :file:`records.config`. ::

        CONFIG proxy.config.http.cache.ignore_server_no_cache INT 1

#. Run the command :option:`traffic_ctl config reload` to apply the configuration changes.

Configuring Traffic Server to Ignore WWW-Authenticate Headers
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

By default, Traffic Server does not cache objects that contain
``WWW-Authenticate`` response headers. The ``WWW-Authenticate`` header
contains authentication parameters the client uses when preparing the
authentication challenge response to an origin server.

When you configure Traffic Server to ignore origin server
``WWW-Authenticate`` headers, all objects with ``WWW-Authenticate``
headers are stored in the cache for future requests. However, the
default behavior of not caching objects with ``WWW-Authenticate``
headers is appropriate in most cases. Only configure Traffic Server to
ignore server ``WWW-Authenticate`` headers if you are knowledgeable
about HTTP 1.1.

To configure Traffic Server to ignore server ``WWW-Authenticate``
headers:

#. Edit :ts:cv:`proxy.config.http.cache.ignore_authentication` in
   :file:`records.config`. ::

        CONFIG proxy.config.http.cache.ignore_authentication INT 1

#. Run the command :option:`traffic_ctl config reload` to apply the configuration changes.

Configuration Directives
------------------------

In addition to client and origin server directives, Traffic Server
responds to directives you specify through configuration options and
files.

You can configure Traffic Server to do the following:

-  Not cache any HTTP objects. Refer to `Disabling HTTP Object Caching`_.

-  Cache *dynamic content*. That is, objects with URLs that end in ``.asp`` or
   contain a question mark (``?``), semicolon (``;``), or ``cgi``. For more
   information, refer to `Caching Dynamic Content`_.

-  Cache objects served in response to the ``Cookie:`` header. Refer to
   `Caching Cookied Objects`_.

-  Observe ``never-cache`` rules in :file:`cache.config`.

Disabling HTTP Object Caching
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

By default, Traffic Server caches all HTTP objects except those for
which you have set ``never-cache`` as :ref:`action rules <cache-config-format-action>`
in :file:`cache.config`. You can disable HTTP object caching so that all HTTP
objects are served directly from the origin server and never cached, as
detailed below.

To disable HTTP object caching manually:

#. Set :ts:cv:`proxy.config.http.cache.http` to ``0`` in :file:`records.config`. ::

        CONFIG proxy.config.http.cache.http INT 0

#. Run the command :option:`traffic_ctl config reload` to apply the configuration changes.

Caching Dynamic Content
~~~~~~~~~~~~~~~~~~~~~~~

A URL is considered dynamic if it ends in ``.asp`` or contains a
question mark (``?``), a semicolon (``;``), or ``cgi``. By
default, Traffic Server caches dynamic content. You can configure the
system to ignore dynamic looking content, although this is recommended
only if the content is truly dynamic, but fails to advertise so with
appropriate ``Cache-Control`` headers.

To configure Traffic Server's cache behaviour in regard to dynamic
content:

#. Edit :ts:cv:`proxy.config.http.cache.cache_urls_that_look_dynamic` in
   :file:`records.config`. To disable caching, set the variable to ``0``,
   and to explicitly permit caching use ``1``. ::

        CONFIG proxy.config.http.cache.cache_urls_that_look_dynamic INT 0

#. Run the command :option:`traffic_ctl config reload` to apply the configuration changes.

Caching Cookied Objects
~~~~~~~~~~~~~~~~~~~~~~~

.. XXX This should be extended to xml as well!

By default, Traffic Server caches objects served in response to requests
that contain cookies. This is true for all types of objects including
text. Traffic Server does not cache cookied text content because object
headers are stored along with the object, and personalized cookie header
values could be saved with the object. With non-text objects, it is
unlikely that personalized headers are delivered or used.

You can reconfigure Traffic Server to:

-  Not cache cookied content of any type.

-  Cache cookied content that is of image type only.

-  Cache all cookied content regardless of type.

To configure how Traffic Server caches cookied content:

#. Edit :ts:cv:`proxy.config.http.cache.cache_responses_to_cookies` in
   :file:`records.config`.

#. Run the command :option:`traffic_ctl config reload` to apply the configuration changes.

Forcing Object Caching
======================

You can force Traffic Server to cache specific URLs (including dynamic
URLs) for a specified duration, regardless of ``Cache-Control`` response
headers.

To force document caching:

#. Add a rule for each URL you want Traffic Server to pin to the cache
   :file:`cache.config`::

       url_regex=^https?://(www.)?apache.org/dev/ ttl-in-cache=6h

#. Run the command :option:`traffic_ctl config reload` to apply the configuration changes.

Caching HTTP Alternates
=======================

Some origin servers answer requests to the same URL with a variety of
objects. The content of these objects can vary widely, according to
whether a server delivers content for different languages, targets
different browsers with different presentation styles, or provides
different document formats (HTML, XML). Different versions of the same
object are termed *alternates* and are cached by Traffic Server based
on ``Vary`` response headers. You can also limit the number of
alternate versions of an object allowed in the cache.

Limiting the Number of Alternates for an Object
-----------------------------------------------

You can limit the number of alternates Traffic Server can cache per
object (the default is 3).

.. important::

   Large numbers of alternates can affect Traffic Server
   cache performance because all alternates have the same URL. Although
   Traffic Server can look up the URL in the index very quickly, it must
   scan sequentially through available alternates in the object store.

To alter the limit on the number of alternates:

#. Edit :ts:cv:`proxy.config.cache.limits.http.max_alts` in :file:`records.config`. ::

    CONFIG proxy.config.cache.limits.http.max_alts INT 5

#. Run the command :option:`traffic_ctl config reload` to apply the configuration changes.

.. _transaction-buffering-control:

Using Transaction Buffering Control
===================================

By default, I/O operations are run at full speed, as fast as either Traffic
Server, the network, or the cache can support. This can be problematic for
large objects if the client side connection is significantly slower. In such
cases the content will be buffered in ram while waiting to be sent to the
client. This could potentially also happen for ``POST`` requests if the client
connection is fast and the origin server connection slow. If very large objects
are being used this can cause the memory usage of Traffic Server to become
`very large <https://issues.apache.org/jira/browse/TS-1496>`_.

This problem can be ameliorated by controlling the amount of buffer space used
by a transaction. A high water and low water mark are set in terms of bytes
used by the transaction. If the buffer space in use exceeds the high water
mark, the connection is throttled to prevent additional external data from
arriving. Internal operations continue to proceed at full speed until the
buffer space in use drops below the low water mark and external data I/O is
re-enabled.

Although this is intended primarily to limit the memory usage of Traffic Server
it can also serve as a crude rate limiter by setting a buffer limit and then
throttling the client side connection either externally or via a transform.
This will cause the connection to the origin server to be limited to roughly
the client side connection speed.

Traffic Server does network I/O in large chunks (32K or so) and therefore the
granularity of transaction buffering control is limited to a similar precision.

The buffer size calculations include all elements in the transaction, including
any buffers associated with :ref:`transform plugins <developer-plugins-http-transformations>`.

Transaction buffering control can be enabled globally by using configuration
variables or by :c:func:`TSHttpTxnConfigIntSet` in a plugin.

================= ================================================== =======================================================
Value             Variable                                           :c:func:`TSHttpTxnConfigIntSet` key
================= ================================================== =======================================================
Enable buffering  :ts:cv:`proxy.config.http.flow_control.enabled`    :c:member:`TS_CONFIG_HTTP_FLOW_CONTROL_ENABLED`
Set high water    :ts:cv:`proxy.config.http.flow_control.high_water` :c:member:`TS_CONFIG_HTTP_FLOW_CONTROL_HIGH_WATER_MARK`
Set low water     :ts:cv:`proxy.config.http.flow_control.low_water`  :c:member:`TS_CONFIG_HTTP_FLOW_CONTROL_LOW_WATER_MARK`
================= ================================================== =======================================================

Be careful to always have the low water mark equal or less than the high water
mark. If you set only one, the other will be set to the same value.

If using :c:func:`TSHttpTxnConfigIntSet`, it must be called no later than
:c:data:`TS_HTTP_READ_RESPONSE_HDR_HOOK`.

.. _admin-configuration-reducing-origin-requests:

Reducing Origin Server Requests (Avoiding the Thundering Herd)
==============================================================

When an object can not be served from cache, the request will be proxied to the
origin server. For a popular object, this can result in many near simultaneous
requests to the origin server, potentially overwhelming it or associated
resources. There are several features in Traffic Server that can be used to
avoid this scenario.

.. _admin-config-read-while-writer:

Read While Writer
-----------------

When Traffic Server goes to fetch something from origin, and upon receiving
the response, any number of clients can be allowed to start serving the
partially filled cache object once background_fill_completed_threshold % of the
object has been received.

While some other HTTP proxies permit clients to begin reading the response
immediately upon the proxy receiving data from the origin server, ATS does not
begin allowing clients to read until after the complete HTTP response headers
have been read and processed. This is a side-effect of ATS making no
distinction between a cache refresh and a cold cache, which prevents knowing
whether a response is going to be cacheable.

As non-cacheable responses from an origin server are generally due to that
content being unique to different client requests, ATS will not enable
read-while-writer functionality until it has determined that it will be able
to cache the object.

The following settings must be made in :file:`records.config` to enable
read-while-writer functionality in ATS::

    CONFIG proxy.config.cache.enable_read_while_writer INT 1
    CONFIG proxy.config.http.background_fill_active_timeout INT 0
    CONFIG proxy.config.http.background_fill_completed_threshold FLOAT 0.000000
    CONFIG proxy.config.cache.max_doc_size INT 0

All four configurations are required, for the following reasons:

-  :ts:cv:`proxy.config.cache.enable_read_while_writer` being set to ``1`` turns
   the feature on, as it is off (``0``) by default.

.. _background_fill:

-  The background fill feature (both
   :ts:cv:`proxy.config.http.background_fill_active_timeout` and
   :ts:cv:`proxy.config.http.background_fill_completed_threshold`) should be
   allowed to kick in for every possible request. This is necessary in the event
   the writer (the first client session to request the object, which triggered
   ATS to contact the origin server) goes away. Another client session needs to
   take over the writer.

   As such, you should set the background fill timeouts and threshold to zero;
   this assures they never time out and are always allowed to kick in.

-  The :ts:cv:`proxy.config.cache.max_doc_size` should be unlimited (set to 0),
   since the object size may be unknown, and going over this limit would cause
   a disconnect on the objects being served.

Once these are enabled, you have something that is very close, but not quite
the same, to Squid's Collapsed Forwarding.

In addition to the above settings, the settings :ts:cv:`proxy.config.cache.read_while_writer.max_retries`
and :ts:cv:`proxy.config.cache.read_while_writer_retry.delay` allow to control the number
of retries TS attempts to trigger read-while-writer until the download of first fragment
of the object is completed::

    CONFIG proxy.config.cache.read_while_writer.max_retries INT 10

    CONFIG proxy.config.cache.read_while_writer_retry.delay INT 50


Open Read Retry Timeout
-----------------------

The open read retry configurations attempt to reduce the number of concurrent
requests to the origin for a given object. While an object is being fetched
from the origin server, subsequent requests would wait
:ts:cv:`proxy.config.http.cache.open_read_retry_time` milliseconds before
checking if the object can be served from cache. If the object is still being
fetched, the subsequent requests will retry
:ts:cv:`proxy.config.http.cache.max_open_read_retries` times. Thus, subsequent
requests may wait a total of (``max_open_read_retries`` x ``open_read_retry_time``)
milliseconds before establishing an origin connection of its own. For instance,
if they are set to ``5`` and ``10`` respectively, connections will wait up to
50ms for a response to come back from origin from a previous request, until
this request is allowed through.

.. important::

    These settings are inappropriate when objects are uncacheable. In those
    cases, requests for an object effectively become serialized. The subsequent
    requests would await at least ``open_read_retry_time`` milliseconds before
    being proxied to the origin.

It is advisable that this setting be used in conjunction with `Read While Writer`_
for big (those that take longer than (``max_open_read_retries`` x
``open_read_retry_time``) milliseconds to transfer) cacheable objects. Without
the read-while-writer settings enabled, while the initial fetch is ongoing, not
only would subsequent requests be delayed by the maximum time, but also, those
requests would result in unnecessary requests to the origin server.

Since ATS now supports setting these settings per-request or remap rule, you
can configure this to be suitable for your setup much more easily.

The configurations are (with defaults)::

    CONFIG proxy.config.http.cache.max_open_read_retries INT -1
    CONFIG proxy.config.http.cache.open_read_retry_time INT 10

The defaults are such that the feature is disabled and every connection is
allowed to go to origin without artificial delay. When enabled, you will try
``max_open_read_retries`` times, each with an ``open_read_retry_time`` timeout.

Open Write Fail Action
----------------------

In addition to the open read retry settings TS supports a new setting
:ts:cv:`proxy.config.http.cache.open_write_fail_action` that allows to further
reduce multiple concurrent requests hitting the origin for the same object by
either returning a stale copy, in case of hit-stale or an error in case of cache
miss for all but one of the requests.
