.. _vcl-built-in-subs:
Built in subroutines
====================
Various built-in subroutines are called during processing of client-
and backend requests as well as upon ``vcl.load`` and ``vcl.discard``.
See :ref:`reference-states` for a detailed graphical overview of the
states and how they relate to core code functions and VCL subroutines.
Subroutines always terminate with a ``return()`` on a keyword, which
determines how processing continues in the request processing state
machine.
The behaviour for ``return()`` keywords is identical or at least
similar across subroutines, so differences are only documented where
relevant.
common return keywords
----------------------
.. _fail:
``fail``
Transition to :ref:`vcl_synth` on the client side as for
``return(synth(503, "VCL Failed"))``, but with any request state
changes undone as if ``std.rollback()`` was called and forcing a
connection close.
Intended for fatal errors, for which only minimal error handling is
possible.
.. _synth:
``synth(status code, reason)``
Transition to :ref:`vcl_synth` with ``resp.status`` and
``resp.reason`` being preset to the arguments of ``synth()``.
.. _pass:
``pass``
Switch to pass mode. Control will eventually pass to
:ref:`vcl_pass`.
.. _pipe:
``pipe``
Switch to pipe mode. Control will eventually pass to
:ref:`vcl_pipe`.
.. _restart:
``restart``
Restart the transaction. Increases the ``req.restarts`` counter.
If the number of restarts is higher than the *max_restarts*
parameter, control is passed to :ref:`vcl_synth` as for
``return(synth(503, "Too many restarts"))``
For a restart, all modifications to ``req`` attributes are
preserved except for ``req.restarts`` and ``req.xid``, which need
to change by design.
client side
-----------
.. _vcl_recv:
vcl_recv
~~~~~~~~
Called at the beginning of a request, after the complete request has
been received and parsed, after a `restart` or as the result of an ESI
include.
Its purpose is to decide whether or not to serve the request, possibly
modify it and decide on how to process it further. A backend hint may
be set as a default for the backend processing side.
The `vcl_recv` subroutine may terminate with calling ``return()`` on one
of the following keywords:
``fail``
see `fail`_
``synth(status code, reason)``
see `synth`_
``restart``
see `restart`_
``pass``
see `pass`_
``pipe``
see `pipe`_
``hash``
Continue processing the object as a potential candidate for
caching. Passes the control over to :ref:`vcl_hash`.
``purge``
Purge the object and it's variants. Control passes through
:ref:`vcl_hash` to :ref:`vcl_purge`.
``vcl(label)``
Switch to vcl labelled *label*. This will continue vcl processing
in this vcl's :ref:`vcl_recv` as if it was the active vcl.
See the :ref:`varnishadm(1)` ``vcl.label`` command.
.. _vcl_pipe:
vcl_pipe
~~~~~~~~
Called upon entering pipe mode. In this mode, the request is passed on
to the backend, and any further data from both the client and backend
is passed on unaltered until either end closes the
connection. Basically, Varnish will degrade into a simple TCP proxy,
shuffling bytes back and forth. For a connection in pipe mode, no
other VCL subroutine will ever get called after `vcl_pipe`.
The `vcl_pipe` subroutine may terminate with calling ``return()`` with one
of the following keywords:
``fail``
see `fail`_
``synth(status code, reason)``
see `synth`_
``pipe``
Proceed with pipe mode.
.. _vcl_pass:
vcl_pass
~~~~~~~~
Called upon entering pass mode. In this mode, the request is passed
on to the backend, and the backend's response is passed on to the
client, but is not entered into the cache. Subsequent requests
submitted over the same client connection are handled normally.
The `vcl_pass` subroutine may terminate with calling ``return()`` with one
of the following keywords:
``fail``
see `fail`_
``synth(status code, reason)``
see `synth`_
``restart``
see `restart`_
``fetch``
Proceed with pass mode - initiate a backend request.
.. _vcl_hash:
vcl_hash
~~~~~~~~
Called after `vcl_recv` to create a hash value for the request. This is
used as a key to look up the object in Varnish.
The `vcl_hash` subroutine may terminate with calling ``return()`` with one
of the following keywords:
``fail``
see `fail`_
``lookup``
Look up the object in cache.
Control passes to :ref:`vcl_purge` when coming from a ``purge``
return in `vcl_recv`.
Otherwise control passes to the next subroutine depending on the
result of the cache lookup:
* a hit: pass to :ref:`vcl_hit`
* a miss or a hit on a hit-for-miss object (an object with
``obj.uncacheable == true``): pass to :ref:`vcl_miss`
* a hit on a hit-for-pass object (for which ``pass(DURATION)`` had been
previously returned from ``vcl_backend_response``): pass to
:ref:`vcl_pass`
.. _vcl_purge:
vcl_purge
~~~~~~~~~
Called after the purge has been executed and all its variants have been evicted.
The `vcl_purge` subroutine may terminate with calling ``return()`` with one
of the following keywords:
``fail``
see `fail`_
``synth(status code, reason)``
see `synth`_
``restart``
see `restart`_
.. _vcl_miss:
vcl_miss
~~~~~~~~
Called after a cache lookup if the requested document was not found in
the cache or if :ref:`vcl_hit` returned ``fetch``.
Its purpose is to decide whether or not to attempt to retrieve the
document from the backend. A backend hint may be set as a default for
the backend processing side.
The `vcl_miss` subroutine may terminate with calling ``return()`` with one
of the following keywords:
``fail``
see `fail`_
``synth(status code, reason)``
see `synth`_
``restart``
see `restart`_
``pass``
see `pass`_
``fetch``
Retrieve the requested object from the backend. Control will
eventually pass to `vcl_backend_fetch`.
.. _vcl_hit:
vcl_hit
~~~~~~~
Called when a cache lookup is successful. The object being hit may be
stale: It can have a zero or negative `ttl` with only `grace` or
`keep` time left.
The `vcl_hit` subroutine may terminate with calling ``return()``
with one of the following keywords:
``fail``
see `fail`_
``synth(status code, reason)``
see `synth`_
``restart``
see `restart`_
``pass``
see `pass`_
``miss``
Synchronously refresh the object from the backend despite the
cache hit. Control will eventually pass to :ref:`vcl_miss`.
``deliver``
Deliver the object. If it is stale, a background fetch to refresh
it is triggered.
.. _vcl_deliver:
vcl_deliver
~~~~~~~~~~~
Called before any object except a `vcl_synth` result is delivered to the client.
The `vcl_deliver` subroutine may terminate with calling ``return()`` with one
of the following keywords:
``fail``
see `fail`_
``synth(status code, reason)``
see `synth`_
``restart``
see `restart`_
``deliver``
Deliver the object to the client.
.. _vcl_synth:
vcl_synth
~~~~~~~~~
Called to deliver a synthetic object. A synthetic object is generated
in VCL, not fetched from the backend. Its body may be constructed using
the ``synthetic()`` function.
A `vcl_synth` defined object never enters the cache, contrary to a
:ref:`vcl_backend_error` defined object, which may end up in cache.
The subroutine may terminate with calling ``return()`` with one of the
following keywords:
``fail``
see `fail`_
``restart``
see `restart`_
``deliver``
Directly deliver the object defined by `vcl_synth` to the client
without calling `vcl_deliver`.
Backend Side
------------
.. _vcl_backend_fetch:
vcl_backend_fetch
~~~~~~~~~~~~~~~~~
Called before sending the backend request. In this subroutine you
typically alter the request before it gets to the backend.
The `vcl_backend_fetch` subroutine may terminate with calling
``return()`` with one of the following keywords:
``fail``
see `fail`_
``fetch``
Fetch the object from the backend.
``abandon``
Abandon the backend request. Unless the backend request was a
background fetch, control is passed to :ref:`vcl_synth` on the
client side with ``resp.status`` preset to 503.
``error(status code, reason)``
Transition to :ref:`vcl_backend_error` with ``beresp.status`` and
``beresp.reason`` being preset to the arguments of ``error()`` if
arguments are provided.
Before calling `vcl_backend_fetch`, Varnish core prepares the `bereq`
backend request as follows:
* Unless the request is a `pass`,
* set ``bereq.method`` to ``GET`` and ``bereq.proto`` to
``HTTP/1.1`` and
* set ``bereq.http.Accept_Encoding`` to ``gzip`` if
:ref:`ref_param_http_gzip_support` is enabled.
* If there is an existing cache object to be revalidated, set
``bereq.http.If-Modified-Since`` from its ``Last-Modified`` header
and/or set ``bereq.http.If-None-Match`` from its ``Etag`` header
* Set ``bereq.http.X-Varnish`` to the current transaction id (`vxid`)
These changes can be undone or modified in `vcl_backend_fetch` before
the backend request is issued.
In particular, to cache non-GET requests, ``req.method`` needs to be
saved to a header or variable in :ref:`vcl_recv` and restored to
``bereq.method``. Notice that caching non-GET requests typically also
requires changing the cache key in :ref:`vcl_hash` e.g. by also
hashing the request method and/or request body.
HEAD request can be satisfied from cached GET responses.
.. _vcl_backend_response:
vcl_backend_response
~~~~~~~~~~~~~~~~~~~~
Called after the response headers have been successfully retrieved from
the backend.
The `vcl_backend_response` subroutine may terminate with calling
``return()`` with one of the following keywords:
``fail``
see `fail`_
``deliver``
For a 304 response, create an updated cache object.
Otherwise, fetch the object body from the backend and initiate
delivery to any waiting client requests, possibly in parallel
(streaming).
``retry``
Retry the backend transaction. Increases the `retries` counter.
If the number of retries is higher than *max_retries*,
control will be passed to :ref:`vcl_backend_error`.
``abandon``
Abandon the backend request. Unless the backend request was a
background fetch, control is passed to :ref:`vcl_synth` on the
client side with ``resp.status`` preset to 503.
``pass(duration)``
Mark the object as a hit-for-pass for the given duration. Subsequent
lookups hitting this object will be turned into passed transactions,
as if ``vcl_recv`` had returned ``pass``.
``error(status code, reason)``
Transition to :ref:`vcl_backend_error` with ``beresp.status`` and
``beresp.reason`` being preset to the arguments of ``error()`` if
arguments are provided.
304 handling
~~~~~~~~~~~~
For a 304 response, varnish core code amends ``beresp`` before calling
`vcl_backend_response`:
* If the gzip status changed, ``Content-Encoding`` is unset and any
``Etag`` is weakened
* Any headers not present in the 304 response are copied from the
existing cache object. ``Content-Length`` is copied if present in
the existing cache object and discarded otherwise.
* The status gets set to 200.
`beresp.was_304` marks that this conditional response processing has
happened.
Note: Backend conditional requests are independent of client
conditional requests, so clients may receive 304 responses no matter
if a backend request was conditional.
beresp.ttl / beresp.grace / beresp.keep
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Before calling `vcl_backend_response`, core code sets ``beresp.ttl``
based on the response status and the response headers ``Age``,
``Cache-Control`` or ``Expires`` and ``Date`` as follows:
* If present and valid, the value of the ``Age`` header is effectively
deduced from all ttl calculations.
* For status codes 200, 203, 204, 300, 301, 304, 404, 410 and 414:
* If ``Cache-Control`` contains an ``s-maxage`` or ``max-age`` field
(in that order of preference), the ttl is set to the respective
non-negative value or 0 if negative.
* Otherwise, if no ``Expires`` header exists, the default ttl is
used.
* Otherwise, if ``Expires`` contains a time stamp before ``Date``,
the ttl is set to 0.
* Otherwise, if no ``Date`` header is present or the ``Date`` header
timestamp differs from the local clock by no more than the
`clock_skew` parameter, the ttl is set to
* 0 if ``Expires`` denotes a past timestamp or
* the difference between the local clock and the ``Expires``
header otherwise.
* Otherwise, the ttl is set to the difference between ``Expires``
and ``Date``
* For status codes 302 and 307, the calculation is identical except
that the default ttl is not used and -1 is returned if neither
``Cache-Control`` nor ``Expires`` exists.
* For all other status codes, ttl -1 is returned.
``beresp.grace`` defaults to the `default_grace` parameter.
For a non-negative ttl, if ``Cache-Control`` contains a
``stale-while-revalidate`` field value, ``beresp.grace`` is
set to that value if non-negative or 0 otherwise.
``beresp.keep`` defaults to the `default_keep` parameter.
.. _vcl_backend_error:
vcl_backend_error
~~~~~~~~~~~~~~~~~
This subroutine is called if we fail the backend fetch or if
*max_retries* has been exceeded.
A synthetic object is generated in VCL, whose body may be constructed
using the ``synthetic()`` function.
The `vcl_backend_error` subroutine may terminate with calling ``return()``
with one of the following keywords:
``fail``
see `fail`_
``deliver``
Deliver and possibly cache the object defined in
`vcl_backend_error` **as if it was fetched from the backend**, also
referred to as a "backend synth".
``retry``
Retry the backend transaction. Increases the `retries` counter.
If the number of retries is higher than *max_retries*,
:ref:`vcl_synth` on the client side is called with ``resp.status``
preset to 503.
vcl.load / vcl.discard
----------------------
.. _vcl_init:
vcl_init
~~~~~~~~
Called when VCL is loaded, before any requests pass through it.
Typically used to initialize VMODs.
The `vcl_init` subroutine may terminate with calling ``return()``
with one of the following keywords:
``ok``
Normal return, VCL continues loading.
``fail``
Abort loading of this VCL.
.. _vcl_fini:
vcl_fini
~~~~~~~~
Called when VCL is discarded only after all requests have exited the VCL.
Typically used to clean up VMODs.
The `vcl_fini` subroutine may terminate with calling ``return()``
with one of the following keywords:
``ok``
Normal return, VCL will be discarded.
Henceforth, whatever our philosopher says about Matter will apply to extension and to extension alone. It cannot be apprehended by sight, nor by hearing, nor by smell, nor by taste, for it is neither colour, nor sound, nor odour, nor juice. Neither can it be touched, for it is not a body, but it becomes corporeal on being blended with sensible qualities. And, in a later essay, he describes it as receiving all things and letting them depart again without retaining the slightest trace of their presence.483 Why then, it may be asked, if Plotinus meant extension, could he not say so at once, and save us all this trouble in hunting out his meaning? There were very good reasons why he should not. In the first place, he wished to express himself, so far as possible, in Aristotelian phraseology, and this was incompatible with the reduction of Matter to extension. In the next place, the idea of an infinite void had been already appropriated by the Epicureans, to whose system he was bitterly opposed. And, finally, the extension of ordinary327 experience had not the absolute generality which was needed in order to bring Matter into relation with that ultimate abstraction whence, like everything else, it has now to be derived. That the millionaire was genuine, ¡°in person and not a caricature,¡± as Dick put it, was evident. Both the nurse, his relative, and his wife, were chatting with him as Jeff delivered the heavy packed ball made up of the gum. 233 "I guess not," said Landor, tolerantly, as he turned[Pg 106] his horse over to his orderly; "but, anyway," he added to Ellton, "we had a picnic¡ªof a sort." Si, unable to think of anything better, went with him. The train had stopped on a switch, and seemed likely to rust fast to the rails, from the way other trains were going by in both directions. The bridge gang, under charge of a burly, red-faced young Englishman, was in the rear car, with their tools, equipments, bedding and cooking utensils. THE DEACON HAS SOME EXPERIENCES WITH THE QUADRUPED. "You are not within a mile of the truth. I know it. Look here: I believe that is Gen. Rosecrans's own cow. She's gone, and I got an order to look around for her. I've never seen her, but from the description given me I believe that's she. Who brought her here?" "Deacon, these brothers and sisters who have come here with me to-night are, like myself, deeply interested in the moral condition of the army, where we all have sons or kinsmen. Now, can't you sit right there and tell us of your observations and experiences, as a Christian man and father, from day to day, of every day that you were down there? Tell us everything, just as it happened each day, that we may be able to judge for ourselves." HAS AN ENCOUNTER WITH THE PROVOST-MARSHAL. "Wonder which one o' them is the 200th Injianny's?" said Si to Shorty. "And your mother, and Harry?" The daughter must be the girl who was talking to him now. She sat on a little stool by the fire, and had brought out some sewing. "Over at Grandturzel¡ªcan't see wot's burning from here. Git buckets and come!" These things, however, gave little concern to the worthy who commanded the Kentish division. Tyler, though an excellent blacksmith, possessed few of the qualities requisite for forming a good general. Provided there was no very sensible diminution in the number of his followers, he cared not a straw for the score or two who, after quarrelling, or perhaps fighting, withdrew in such disgust that they vowed rather to pay the full tax for ever than submit to the insolence of the rebels. One man could fight as well as another, reasoned he; and, provided he was obeyed, what mattered it by whom. Dick went and Tom came¡ªit was sure to be all one in the end. But this burst of indignation soon passed away, and upon the suggestion of the prudent Sir Robert Hailes, he sent an evasive answer, with a command that the Commons should attend him at Windsor on the Sunday following. That it was a stratagem to gain entrance to the Tower, was the opinion of several, but, after much discussion, it was decided that the man should be admitted, and that the monk should be exhibited merely to intimidate the rebels, until the result of this promised communication should be known. HoMEŮͬÐÔÁµcbcb
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