..
Copyright (c) 2021 Varnish Software AS
SPDX-License-Identifier: BSD-2-Clause
See LICENSE file for full text of license
.. role:: ref(emphasis)
.. _vcl-probe(7):
=========
VCL-probe
=========
---------------------------------
Configuring Backend Health Probes
---------------------------------
:Manual section: 7
.. _reference-vcl_probes:
Backend health probes
---------------------
Varnish can be configured to periodically send a request to test if a
backend is answering and thus "healthy".
Probes can be configured per backend::
backend foo {
[...]
.probe = {
[...]
}
}
They can be named and shared between backends::
probe light {
[...]
}
backend foo {
.probe = light;
}
backend bar {
.probe = light;
}
Or a ``default`` probe can be defined, which applies to all backends
without a specific ``.probe`` configured::
probe default {
[...]
}
The basic syntax is the same as for backends::
probe name {
.attribute1 = value;
.attribute2 = "value";
[...]
}
There are no mandatory attributes, they all have defaults.
Attribute ``.url``
------------------
The URL to query. Defaults to ``/``::
.url = "/health-probe";
Attribute ``.request``
----------------------
Can be used to specify a full HTTP/1.1 request to be sent::
.request = "GET / HTTP/1.1"
"Host: example.com"
"X-Magic: We're fine with this."
"Connection: close";
Each of the strings will have ``CRLF`` appended and a final HTTP
header block terminating ``CRLF`` will be appended as well.
Because connection shutdown is part of the health check,
``Connection: close`` is mandatory.
Attribute ``.expected_response``
--------------------------------
The expected HTTP status, defaults to ``200``::
.expected_response = 418;
Attribute ``.expect_close``
---------------------------
Whether or not to expect the backend to close the underlying connection.
Accepts ``true`` or ``false``, defaults to ``true``::
.expect_close = false;
Warning: when the backend does not close the connection,
setting ``expect_close`` to ``false`` makes probe tasks wait until
they time out before inspecting the response.
Attribute ``.timeout``
----------------------
How fast the probe must succeed, default is two seconds::
.timeout = 10s;
Attribute ``.interval``
-----------------------
Time between probes, default is five seconds::
.interval = 1m;
The backend health shift register
---------------------------------
Backend health probes uses a 64 stage shift register to remember the
most recent health probes and to evaluate the total health of the backend.
In the CLI, a good backend health status looks like this:
.. code-block:: text
varnish> backend.list -p boot.backend
Backend name Admin Probe Health Last change
boot.backend probe 5/5 healthy Wed, 13 Jan 2021 10:31:50 GMT
Current states good: 5 threshold: 4 window: 5
Average response time of good probes: 0.000793
Oldest ================================================== Newest
4444444444444444444444444444444444444444444444444444444444444444 Good IPv4
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX Good Xmit
RRRRRRRRRRRRRRRRRRRRRRR----RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR Good Recv
HHHHHHHHHHHHHHHHHHHHHHH--------HHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH Happy
Starting from the bottom, the last line shows that this backend has been
declared "Happy" for most the 64 health probes, but there were some
trouble some while ago.
However, in this case the ``.window`` is configured to five, and the
``.threshold`` is set to four, so at this point in time, the backend
is considered fully healthy.
An additional ``.initial`` fills that many "happy" entries in the
shift register when the VCL is loaded, so that backends can quickly
become healthy, even if their health is normally considered over
many samples::
.interval = 1s;
.window = 60;
.threshold = 45;
.initial = 43;
This backend will be considered healthy if three out of four health
probes in the last minute were good, but it becomes healthy as soon
as two good probes have happened after the VCL was loaded.
The default values are:
* ``.window`` = 8
* ``.threshold`` = 3
* ``.initial`` = one less than ``.threshold``
Note that the default ``.initial`` means that the backend will be marked
unhealthy until the first probe response come back successful.
This means that for backends created on demand (by vmods) cannot use the
default value for ``.initial``, as the freshly created backend would very
likely still be unhealthy when the backend request happens.
SEE ALSO
========
* :ref:`varnishd(1)`
* :ref:`vcl(7)`
* :ref:`vcl-backend(7)`
* :ref:`vmod_directors(3)`
* :ref:`vmod_std(3)`
HISTORY
=======
VCL was developed by Poul-Henning Kamp in cooperation with Verdens
Gang AS, Redpill Linpro and Varnish Software. This manual page is
written by Per Buer, Poul-Henning Kamp, Martin Blix Grydeland,
Kristian Lyngst?l, Lasse Karstensen and others.
COPYRIGHT
=========
This document is licensed under the same license as Varnish
itself. See LICENSE for details.
* Copyright (c) 2006 Verdens Gang AS
* Copyright (c) 2006-2021 Varnish Software AS
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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