..
Copyright (c) 2015-2019 Varnish Software AS
SPDX-License-Identifier: BSD-2-Clause
See LICENSE file for full text of license
.. _ref-writing-a-director:
%%%%%%%%%%%%%%%%%%
Writing a Director
%%%%%%%%%%%%%%%%%%
Varnish already provides a set of general-purpose directors, and since Varnish
4, it is bundled in the built-in :ref:`vmod_directors(3)`. Writing a director
boils down to writing a VMOD, using the proper data structures and APIs. Not
only can you write your own director if none of the built-ins fit your needs,
but since Varnish 4.1 you can even write your own backends.
Backends can be categorized as such:
- static: native backends declared in VCL
- dynamic: native backends created by VMODs
- custom: backends created and fully managed by VMODs
Backends vs Directors
=====================
The intuitive classification for backend and director is an endpoint for the
former and a loadbalancer for the latter, but the actual implementation is a bit
more subtle. VMODs can accept backend arguments and return backends in VCL (see
:ref:`ref-vmod-vcl-c-types`), but the underlying C type is ``struct director``
aka the ``VCL_BACKEND`` typedef.
Under the hood director is a generic concept, and a backend is a kind of
director.
The line between the two is somewhat blurry at this point, let's look at some
code instead::
// VRT interface from vrt.h
struct vdi_methods {
unsigned magic;
#define VDI_METHODS_MAGIC 0x4ec0c4bb
const char *type;
vdi_http1pipe_f *http1pipe;
vdi_healthy_f *healthy;
vdi_resolve_f *resolve;
vdi_gethdrs_f *gethdrs;
vdi_getip_f *getip;
vdi_finish_f *finish;
vdi_event_f *event;
vdi_release_f *release;
vdi_destroy_f *destroy;
vdi_panic_f *panic;
vdi_list_f *list;
};
struct director {
unsigned magic;
#define DIRECTOR_MAGIC 0x3336351d
void *priv;
char *vcl_name;
struct vcldir *vdir;
struct lock *mtx;
};
A director can be summed up as:
- being of a specific ``type`` with a set of operations which is
identical for all instances of that particular type
- some instance specific attributes such as a ``vcl_name``
and ``type``\ -specific private data
The difference between a *load balancing* director and a *backend*
director is mainly the functions they will implement.
The fundamental steps towards a director implementation are:
- implement the required functions
- fill a ``struct vdi_methods`` with the name of your director type
and your function pointers
Existence of a ``healthy`` callback signifies that the director has
some means of dynamically determining its health state.
- in your constructor or other initialization routine, allocate and
initialize your director-specific configuration state (aka private
data) and call ``VRT_AddDirector()`` with your ``struct
vdi_methods``, the pointer to your state and a printf format for the
name of your director instance
- implement methods or functions returning ``VCL_BACKEND``
- in your destructor or other finalizer, call ``VRT_DelDirector()``
- implement a ``destroy`` callback to destroy the actual director
private state. It will be called when all references to the director
are gone, until then the private state must remain intact and
``vdi_methods`` functions callable (but they may return errors).
While vmods can implement functions returning directors,
:ref:`ref-vmod-vcl-c-objects` are usually a more natural
representation with vmod object instances being or referring to the
director private data.
Load Balancing Directors
========================
As in :ref:`vmod_directors(3)`, you can write directors that will group
backends sharing the same role, and pick them according to a strategy. If you
need more than the built-in strategies (round-robin, hash, ...), even though
they can be stacked, it is always possible to write your own.
In this case you simply need to implement the ``resolve`` function for the
director. Directors are walked until a leaf director is found. A leaf director
doesn't have a ``resolve`` function and is used to actually make the backend
request, just like the backends you declare in VCL.
*load balancing* directors use ``VRT_Assign_Backend()`` to take
references to other directors. They *must* implement a ``release``
callback which has to release all references to other directors and
ensure that none are gained after it returns.
Static Directors
================
As opposed to dynamic backends covered below, directors which are
guaranteed to have VCL lifetime (that is, they do not get destroyed
before the VCL goes cold) can call ``VRT_StaticDirector()`` to avoid
reference counting overhead.
Dynamic Backends
================
If you want to speak HTTP/1 over TCP or UDS, but for some reason VCL
does not fit the bill, you can instead reuse the whole backend
facility. It allows you for instance to add and remove backends
on-demand without the need to reload your
VCL. You can then leverage your provisioning system.
Consider the following snippet::
backend default {
.host = "localhost";
}
The VCL compiler turns this declaration into a ``struct
vrt_backend``. When the VCL is loaded, Varnish calls
``VRT_new_backend`` (or rather ``VRT_new_backend_clustered`` for VSM
efficiency) in order to create the director. Varnish doesn't expose
its data structure for actual backends, only the director abstraction
and dynamic backends are built just like static backends, one *struct*
at a time. You can get rid of the ``struct vrt_backend`` as soon as
you have the ``struct director``.
A (dynamic) backend can't exceed its VCL's lifespan, because native
backends are *owned* by VCLs. Though a dynamic backend can't outlive
its VCL, it can be deleted any time with ``VRT_delete_backend``. The
VCL will delete the remaining backends once discarded, you don't need
to take care of it.
Reference counting is used to ensure that backends which are no longer
referenced are destroyed.
Finally, Varnish will take care of event propagation for *all* native backends,
but dynamic backends can only be created when the VCL is warm. If your backends
are created by an independent thread (basically outside of VCL scope) you must
subscribe to VCL events and watch for VCL state (see
:ref:`ref-vmod-event-functions`). Varnish will panic if you try to create a
backend on a cold VCL, and ``VRT_new_backend`` will return ``NULL`` if the VCL
is cooling. You are also encouraged to comply with the
:ref:`ref_vcl_temperature` in general.
.. _ref-writing-a-director-loadbalancer:
Health Probes
=============
It is possible in a VCL program to query the health of a director (see
:ref:`std.healthy()`). A director can report its health if it implements the
``healthy`` function, it is otherwise always considered healthy.
Unless you are making a dynamic backend, you need to take care of the
health probes yourselves. For *load balancing* directors, being
healthy typically means having at least one healthy underlying backend
or director.
For dynamic backends, it is just a matter of assigning the ``probe`` field in
the ``struct vrt_backend``. Once the director is created, the probe definition
too is no longer needed. It is then Varnish that will take care of the health
probe and disable the feature on a cold VCL (see
:ref:`ref-vmod-event-functions`).
Instead of initializing your own probe definition, you can get a ``VCL_PROBE``
directly built from VCL (see :ref:`ref-vmod-vcl-c-types`).
Custom Backends
===============
If you want to implement a custom backend, have a look at how Varnish
implements native backends. It is the canonical implementation, and
though it provides other services like connection pooling or
statistics, it is essentially a director which state is a ``struct
backend``. Varnish native backends currently speak HTTP/1 over TCP or
UDS, and as such, you need to make your own custom backend if you want
Varnish to do otherwise such as connect over UDP or speak a different
protocol.
If you want to leverage probes declarations in VCL, which have the advantage of
being reusable since they are only specifications, you can. However, you need
to implement the whole probing infrastructure from scratch.
You may also consider making your custom backend compliant with regards to the
VCL state (see :ref:`ref-vmod-event-functions`).
If you are implementing the `gethdrs` method of your backend (i.e. your
backend is able to generate a backend response to be manipulated in
`vcl_backend_response`), you will want to log the response code, protocol and
the various headers it'll create for easier debugging. For this, you can look
at the `VSL*` family of functions, listed in `cache/cache.h`.
Data structure considerations
-----------------------------
When you are creating a custom backend, you may want to provide the semantics
of the native backends. In this case, instead of repeating the redundant fields
between data structures, you can use the macros ``VRT_BACKEND_FIELDS`` and
``VRT_BACKEND_PROBE_FIELDS`` to declare them all at once. This is the little
dance Varnish uses to copy data between the ``struct vrt_backend`` and its
internal data structure for example.
The copy can be automated with the macros ``VRT_BACKEND_HANDLE`` and
``VRT_BACKEND_PROBE_HANDLE``. You can look at how they can be used in the
Varnish code base.
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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