..
Copyright (c) 2012-2015 Varnish Software AS
SPDX-License-Identifier: BSD-2-Clause
See LICENSE file for full text of license
.. _users-guide-compression:
Compression
-----------
In Varnish 3.0 we introduced native support for compression, using gzip
encoding. *Before* 3.0, Varnish would never compress objects.
In Varnish 4.0 compression defaults to "on", meaning that it tries to
be smart and do the sensible thing.
If you don't want Varnish tampering with the encoding you can disable
compression all together by setting the parameter `http_gzip_support` to
false. Please see man :ref:`varnishd(1)` for details.
Default behaviour
~~~~~~~~~~~~~~~~~
The default behaviour is active when the `http_gzip_support` parameter
is set to "on" and neither `beresp.do_gzip` nor `beresp.do_gunzip` are
used in VCL.
Unless returning from `vcl_recv` with `pipe` or `pass`, Varnish
modifies `req.http.Accept-Encoding`: if the client supports gzip
`req.http.Accept-Encoding` is set to "gzip", otherwise the header is
removed.
Unless the request is a `pass`, Varnish sets `bereq.http.Accept-Encoding`
to "gzip" before `vcl_backend_fetch` runs, so the header can be changed
in VCL.
If the server responds with gzipped content it will be stored in memory
in its compressed form and `Accept-Encoding` will be added to the
`Vary` header.
To clients supporting gzip, compressed content is delivered unmodified.
For clients not supporting gzip, compressed content gets decompressed
on the fly while delivering it. The `Content-Encoding` response header
gets removed and any `Etag` gets weakened (by prepending "W/").
For Vary Lookups, `Accept-Encoding` is ignored.
Compressing content if backends don't
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
You can tell Varnish to compress content before storing it in cache in
`vcl_backend_response` by setting `beresp.do_gzip` to "true", like this::
sub vcl_backend_response {
if (beresp.http.content-type ~ "text") {
set beresp.do_gzip = true;
}
}
With `beresp.do_gzip` set to "true", Varnish will make the following
changes to the headers of the resulting object before inserting it in
the cache:
* set `obj.http.Content-Encoding` to "gzip"
* add "Accept-Encoding" to `obj.http.Vary`, unless already present
* weaken any `Etag` (by prepending "W/")
Generally, Varnish doesn't use much CPU so it might make more sense to
have Varnish spend CPU cycles compressing content than doing it in your
web- or application servers, which are more likely to be CPU-bound.
Please make sure that you don't try to compress content that is
uncompressable, like JPG, GIF and MP3 files. You'll only waste CPU cycles.
Uncompressing content before entering the cache
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
You can also uncompress content before storing it in cache by setting
`beresp.do_gunzip` to "true". One use case for this feature is to work
around badly configured backends uselessly compressing already compressed
content like JPG images (but fixing the misbehaving backend is always
the better option).
With `beresp.do_gunzip` set to "true", Varnish will make the following
changes to the headers of the resulting object before inserting it in
the cache:
* remove `obj.http.Content-Encoding`
* weaken any `Etag` (by prepending "W/")
.. XXX pending closing #940: remove any "Accept-Encoding" from `obj.http.Vary`
GZIP and ESI
~~~~~~~~~~~~
If you are using Edge Side Includes (ESI) you'll be happy to note that
ESI and GZIP work together really well. Varnish will magically decompress
the content to do the ESI-processing, then recompress it for efficient
storage and delivery.
Turning off gzip support
~~~~~~~~~~~~~~~~~~~~~~~~
When the `http_gzip_support` parameter is set to "off", Varnish does
not do any of the header alterations documented above, handles `Vary:
Accept-Encoding` like it would for any other `Vary` value and ignores
`beresp.do_gzip` and `beresp.do_gunzip`.
A random outburst
~~~~~~~~~~~~~~~~~
Poul-Henning Kamp has written :ref:`phk_gzip` which talks a bit more
about how the implementation works.
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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