..
Copyright (c) 2014-2020 Varnish Software AS
SPDX-License-Identifier: BSD-2-Clause
See LICENSE file for full text of license
.. _users-guide-handling_misbehaving_servers:
Grace mode and keep
-------------------
Sometimes you want Varnish to serve content that is somewhat stale
instead of waiting for a fresh object from the backend. For example,
if you run a news site, serving a main page that is a few seconds old
is not a problem if this gives your site faster load times.
In Varnish this is achieved by using `grace mode`. A related idea
is `keep`, which is also explained here.
Grace mode
~~~~~~~~~~
When several clients are requesting the same page Varnish will send
one request to the backend and place the others on hold while fetching
one copy from the backend. In some products this is called request
coalescing and Varnish does this automatically.
If you are serving thousands of hits per second the queue of waiting
requests can get huge. There are two potential problems - one is a
thundering herd problem - suddenly releasing a thousand threads to
serve content might send the load sky high. Secondly - nobody likes to
wait.
Setting an object's `grace` to a positive value tells Varnish that it
should serve the object to clients for some time after the TTL has
expired, while Varnish fetches a new version of the object. The default
value is controlled by the runtime parameter ``default_grace``.
Keep
~~~~
Setting an object's `keep` tells Varnish that it should keep an object
in the cache for some additional time. The reasons to set `keep` is to
use the object to construct a conditional GET backend request (with
If-Modified-Since: and/or ?f-None-Match: headers), allowing the
backend to reply with a 304 Not Modified response, which may be more
efficient on the backend and saves re-transmitting the unchanged body.
The values are additive, so if grace is 10 seconds and keep is 1 minute,
then objects will survive in cache for 70 seconds after the TTL has
expired.
Setting grace and keep
~~~~~~~~~~~~~~~~~~~~~~
We can use VCL to make Varnish keep all objects for 10 minutes beyond
their TTL with a grace period of 2 minutes::
sub vcl_backend_response {
set beresp.grace = 2m;
set beresp.keep = 8m;
}
The effect of grace and keep
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For most users setting the default grace and/or a suitable grace for
each object is enough. The default VCL will do the right thing and
behave as described above. However, if you want to customize how
Varnish behaves, then you should know some of the details on how this
works.
When ``sub vcl_recv`` ends with ``return (hash)`` (which is the
default behavior), Varnish will look for a matching object in its
cache. Then, if it only found an object whose TTL has run out, Varnish
will consider the following:
* Is there already an ongoing backend request for the object?
* Is the object within the `grace period`?
Then, Varnish reacts using the following rules:
* If the `grace period` has run out and there is no ongoing backend
request, then ``sub vcl_miss`` is called immediately, and the object
will be used as a 304 candidate.
* If the `grace period` has run out and there is an ongoing backend
request, then the request will wait until the backend request
finishes.
* If there is no backend request for the object, one is scheduled.
* Assuming the object will be delivered, ``sub vcl_hit`` is called
immediately.
Note that the backend fetch happens asynchronously, and the moment the
new object is in it will replace the one we've already got.
If you do not define your own ``sub vcl_hit``, then the default one is
used. It looks like this::
sub vcl_hit {
return (deliver);
}
Note that the condition ``obj.ttl + obj.grace > 0s`` will (in ``sub
vcl_hit``) always evaluate to true. In earlier versions (6.0.0 and
earlier), this was not the case, and a test in the builtin VCL was
necessary to make sure that "keep objects" (objects in the cache where
both TTL and grace had run out) would not be delivered to the clients.
In the current version, when there are only "keep objects" available,
``sub vcl_miss`` will be called, and a fetch for a new object will be
initiated.
Misbehaving servers
~~~~~~~~~~~~~~~~~~~
A key feature of Varnish is its ability to shield you from misbehaving
web- and application servers.
If you have enabled :ref:`users-guide-advanced_backend_servers-health`
you can check if the backend is sick and modify the behavior when it
comes to grace. This can done in the following way::
sub vcl_backend_response {
set beresp.grace = 24h;
// no keep - the grace should be enough for 304 candidates
}
sub vcl_recv {
if (std.healthy(req.backend_hint)) {
// change the behavior for healthy backends: Cap grace to 10s
set req.grace = 10s;
}
}
In the example above, the special variable ``req.grace`` is set. The
effect is that, when the backend is healthy, objects with grace above
10 seconds will have an `effective` grace of 10 seconds. When the
backend is sick, the default VCL kicks in, and the long grace is used.
Additionally, you might want to stop cache insertion when a backend fetch
returns an ``5xx`` error::
sub vcl_backend_response {
if (beresp.status >= 500 && bereq.is_bgfetch) {
return (abandon);
}
}
Summary
~~~~~~~
Grace mode allows Varnish to deliver slightly stale content to clients
while getting a fresh version from the backend. The result is faster
load times at lower cost.
It is possible to limit the grace during lookup by setting
``req.grace`` and then change the behavior when it comes to
grace. Often this is done to change the `effective` grace depending on
the health of the backend.
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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