.. _users-guide-backend_servers:
Backend servers
---------------
Varnish has a concept of "backend" or "origin" servers. A backend
server is the server providing the content Varnish will accelerate.
Our first task is to tell Varnish where it can find its backends. Start
your favorite text editor and open the relevant VCL file.
Somewhere in the top there will be a section that looks a bit like this.::
# backend default {
# .host = "127.0.0.1";
# .port = "8080";
# }
We remove the comment markings in this text stanza making the it look like.::
backend default {
.host = "127.0.0.1";
.port = "8080";
}
Now, this piece of configuration defines a backend in Varnish called
*default*. When Varnish needs to get content from this backend it will
connect to port 8080 on localhost (127.0.0.1).
Varnish can have several backends defined you can even join
several backends together into clusters of backends for load balancing
purposes.
backends can also be empty or 'none' with the following syntax.::
backend default none;
Multiple backends
-----------------
At some point you might need Varnish to cache content from several
servers. You might want Varnish to map all the URL into one single
host or not. There are lot of options.
Lets say we need to introduce a Java application into out PHP web
site. Lets say our Java application should handle URL beginning with
`/java/`.
We manage to get the thing up and running on port 8000. Now, lets have
a look at the `default.vcl`.::
backend default {
.host = "127.0.0.1";
.port = "8080";
}
We add a new backend.::
backend java {
.host = "127.0.0.1";
.port = "8000";
}
Now we need tell Varnish where to send the difference URL. Lets look at `vcl_recv`.::
sub vcl_recv {
if (req.url ~ "^/java/") {
set req.backend_hint = java;
} else {
set req.backend_hint = default;
}
}
It's quite simple, really. Lets stop and think about this for a
moment. As you can see you can define how you choose backends based on
really arbitrary data. You want to send mobile devices to a different
backend? No problem. ``if (req.http.User-agent ~ /mobile/) ..`` should do the
trick.
Without an explicit backend selection, varnish will continue using
the `default` backend. If there is no backend named `default`, the
first backend found in the vcl will be used as the default backend.
Backends and virtual hosts in Varnish
-------------------------------------
Varnish fully supports virtual hosts. They might however work in a somewhat
counter-intuitive fashion since they are never declared
explicitly. You set up the routing of incoming HTTP requests in
`vcl_recv`. If you want this routing to be done on the basis of virtual
hosts you just need to inspect `req.http.host`.
You can have something like this::
sub vcl_recv {
if (req.http.host ~ "foo.com") {
set req.backend_hint = foo;
} elsif (req.http.host ~ "bar.com") {
set req.backend_hint = bar;
}
}
Note that the first regular expressions will match "foo.com",
"www.foo.com", "zoop.foo.com" and any other host ending in "foo.com". In
this example this is intentional but you might want it to be a bit
more tight, maybe relying on the ``==`` operator in stead, like this::
sub vcl_recv {
if (req.http.host == "foo.com" || req.http.host == "www.foo.com") {
set req.backend_hint = foo;
}
}
.. _users-guide-advanced_backend_servers-directors:
Directors
---------
You can also group several backend into a group of backends. These
groups are called directors. This will give you increased performance
and resilience.
You can define several backends and group them together in a
director. This requires you to load a VMOD, a Varnish module, and then to
call certain actions in `vcl_init`.::
import directors; # load the directors
backend server1 {
.host = "192.168.0.10";
}
backend server2 {
.host = "192.168.0.10";
}
sub vcl_init {
new bar = directors.round_robin();
bar.add_backend(server1);
bar.add_backend(server2);
}
sub vcl_recv {
# send all traffic to the bar director:
set req.backend_hint = bar.backend();
}
This director is a round-robin director. This means the director will
distribute the incoming requests on a round-robin basis. There is
also a *random* director which distributes requests in a, you guessed it,
random fashion. If that is not enough, you can also write your own director
(see :ref:`ref-writing-a-director`).
But what if one of your servers goes down? Can Varnish direct all the
requests to the healthy server? Sure it can. This is where the Health
Checks come into play.
.. _users-guide-advanced_backend_servers-health:
Health checks
-------------
Lets set up a director with two backends and health checks. First let
us define the backends::
backend server1 {
.host = "server1.example.com";
.probe = {
.url = "/";
.timeout = 1s;
.interval = 5s;
.window = 5;
.threshold = 3;
}
}
backend server2 {
.host = "server2.example.com";
.probe = {
.url = "/";
.timeout = 1s;
.interval = 5s;
.window = 5;
.threshold = 3;
}
}
What is new here is the ``probe``. In this example Varnish will check the
health of each backend every 5 seconds, timing out after 1 second. Each
poll will send a GET request to /. If 3 out of the last 5 polls succeeded
the backend is considered healthy, otherwise it will be marked as sick.
Refer to the :ref:`reference-vcl_probes` section in the
:ref:`vcl(7)` documentation for more information.
Now we define the 'director'::
import directors;
sub vcl_init {
new vdir = directors.round_robin();
vdir.add_backend(server1);
vdir.add_backend(server2);
}
You use this `vdir` director as a backend_hint for requests, just like
you would with a simple backend. Varnish will not send traffic to hosts
that are marked as unhealthy.
Varnish can also serve stale content if all the backends are down. See
:ref:`users-guide-handling_misbehaving_servers` for more information on
how to enable this.
Please note that Varnish will keep health probes running for all loaded
VCLs. Varnish will coalesce probes that seem identical - so be careful
not to change the probe config if you do a lot of VCL loading. Unloading
the VCL will discard the probes. For more information on how to do this
please see ref:`reference-vcl-director`.
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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