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 <div class="section" id="vmod-varnish-modules">
<h1>VMOD - Varnish Modules<a class="headerlink" href="#vmod-varnish-modules" title="Permalink to this headline">¶</a></h1>
For all you can do in VCL, there are things you can not do.
Look an IP number up in a database file for instance.
VCL provides for inline C code, and there you can do everything,
but it is not a convenient or even readable way to solve such
problems.
This is where VMODs come into the picture: A VMOD is a shared
library with some C functions which can be called from VCL code.
For instance:
<div class="highlight-default"><div class="highlight"><pre>import std;

sub vcl_deliver {
 set resp.http.foo = std.toupper(req.url);
}
</pre></div>
</div>
The &quot;std&quot; vmod is one you get with Varnish, it will always be there
and we will put &quot;boutique&quot; functions in it, such as the &quot;toupper&quot;
function shown above. The full contents of the &quot;std&quot; module is
documented in vmod_std(3).
This part of the manual is about how you go about writing your own
VMOD, how the language interface between C and VCC works, where you
can find contributed VMODs etc. This explanation will use the &quot;std&quot;
VMOD as example, having a Varnish source tree handy may be a good
idea.
<div class="section" id="vmod-directory">
<h2>VMOD Directory<a class="headerlink" href="#vmod-directory" title="Permalink to this headline">¶</a></h2>
The VMOD directory is an up-to-date compilation of maintained
extensions written for Varnish Cache:
<blockquote>
<div><a class="reference external" href="https://www.varnish-cache.org/vmods">https://www.varnish-cache.org/vmods</a></div></blockquote>
</div>
<div class="section" id="the-vmod-vcc-file">
<h2>The vmod.vcc file<a class="headerlink" href="#the-vmod-vcc-file" title="Permalink to this headline">¶</a></h2>
The interface between your VMOD and the VCL compiler (&quot;VCC&quot;) and the
VCL runtime (&quot;VRT&quot;) is defined in the vmod.vcc file which a python
script called &quot;vmodtool.py&quot; turns into thaumaturgically challenged C
data structures that does all the hard work.
The std VMODs vmod.vcc file looks somewhat like this:
<div class="highlight-default"><div class="highlight"><pre>$Module std 3
$Event event_function
$Function STRING toupper(STRING_LIST)
$Function STRING tolower(STRING_LIST)
$Function VOID set_ip_tos(INT)
</pre></div>
</div>
The first line gives the name of the module and the manual section where
the documentation will reside.
The second line specifies an optional &quot;Event&quot; function, which will be
called whenever a VCL program which imports this VMOD is loaded or
transitions to any of the warm, active, cold or discarded states.
More on this below.
The next three lines define three functions in the VMOD, along with the
types of the arguments, and that is probably where the hardest bit of
writing a VMOD is to be found, so we will talk about that at length in
a moment.
Notice that the third function returns VOID, that makes it a &quot;procedure&quot;
in VCL lingo, meaning that it cannot be used in expressions, right side
of assignments and such. Instead it can be used as a primary action,
something functions which return a value can not:
<div class="highlight-default"><div class="highlight"><pre>sub vcl_recv {
 std.set_ip_tos(32);
}
</pre></div>
</div>
Running vmodtool.py on the vmod.vcc file, produces a &quot;vcc_if.c&quot; and
&quot;vcc_if.h&quot; files, which you must use to build your shared library
file.
Forget about vcc_if.c everywhere but your Makefile, you will never
need to care about its contents, and you should certainly never
modify it, that voids your warranty instantly.
But vcc_if.h is important for you, it contains the prototypes for
the functions you want to export to VCL.
For the std VMOD, the compiled vcc_if.h file looks like this:
<div class="highlight-default"><div class="highlight"><pre>struct vmod_priv;

VCL_STRING vmod_toupper(VRT_CTX, const char *, ...);
VCL_STRING vmod_tolower(VRT_CTX, const char *, ...);
VCL_VOID vmod_set_ip_tos(VRT_CTX, VCL_INT);

vmod_event_f event_function;
</pre></div>
</div>
Those are your C prototypes. Notice the <code class="docutils literal">vmod_</code> prefix on the
function names.
<div class="section" id="named-arguments-and-default-values">
<h3>Named arguments and default values<a class="headerlink" href="#named-arguments-and-default-values" title="Permalink to this headline">¶</a></h3>
The basic vmod.vcc function declaration syntax introduced above makes all
arguments mandatory for calls from vcl - which implies that they need
to be given in order.
Naming the arguments as in:
<div class="highlight-default"><div class="highlight"><pre>$Function BOOL match_acl(ACL acl, IP ip)
</pre></div>
</div>
allows calls from VCL with named arguments in any order, for example:
<div class="highlight-default"><div class="highlight"><pre>if (debug.match_acl(ip=client.ip, acl=local)) { # ...
</pre></div>
</div>
Named arguments also take default values, so for this example from
the debug vmod:
<div class="highlight-default"><div class="highlight"><pre>$Function STRING argtest(STRING one, REAL two=2, STRING three=&quot;3&quot;,
 STRING comma=&quot;,&quot;, INT four=4)
</pre></div>
</div>
only argument <cite>one</cite> is required, so that all of the following are
valid invocations from vcl:
<div class="highlight-default"><div class="highlight"><pre>debug.argtest(&quot;1&quot;, 2.1, &quot;3a&quot;)
debug.argtest(&quot;1&quot;, two=2.2, three=&quot;3b&quot;)
debug.argtest(&quot;1&quot;, three=&quot;3c&quot;, two=2.3)
debug.argtest(&quot;1&quot;, 2.4, three=&quot;3d&quot;)
debug.argtest(&quot;1&quot;, 2.5)
debug.argtest(&quot;1&quot;, four=6);
</pre></div>
</div>
The C interface does not change with named arguments and default
values, arguments remain positional and defaul values appear no
different to user specified values.
<cite>Note</cite> that default values have to be given in the native C-type
syntax, see below. As a special case, <code class="docutils literal">NULL</code> has to be given as <code class="docutils literal">0</code>.
</div>
</div>
<div class="section" id="vcl-and-c-data-types">
<h2>VCL and C data types<a class="headerlink" href="#vcl-and-c-data-types" title="Permalink to this headline">¶</a></h2>
VCL data types are targeted at the job, so for instance, we have data
types like &quot;DURATION&quot; and &quot;HEADER&quot;, but they all have some kind of C
language representation. Here is a description of them.
All but the PRIV and STRING_LIST types have typedefs: VCL_INT, VCL_REAL,
etc.
<dl class="docutils">
<dt>ACL</dt>
<dd>C-type: <code class="docutils literal">const struct vrt_acl *</code>
A type for named ACLs declared in VCL.
</dd>
<dt>BACKEND</dt>
<dd>C-type: <code class="docutils literal">const struct director *</code>
A type for backend and director implementations. See
<a class="reference internal" href="directors.html#ref-writing-a-director">Writing a Director</a>.
</dd>
<dt>BLOB</dt>
<dd>C-type: <code class="docutils literal">const struct vmod_priv *</code>
An opaque type to pass random bits of memory between VMOD
functions.
</dd>
<dt>BOOL</dt>
<dd>C-type: <code class="docutils literal">unsigned</code>
Zero means false, anything else means true.
</dd>
<dt>BYTES</dt>
<dd>C-type: <code class="docutils literal">double</code>
Unit: bytes.
A storage space, as in 1024 bytes.
</dd>
<dt>DURATION</dt>
<dd>C-type: <code class="docutils literal">double</code>
Unit: seconds.
A time interval, as in 25 seconds.
</dd>
<dt>ENUM</dt>
<dd>vcc syntax: ENUM { val1, val2, ... }
vcc example: <code class="docutils literal">ENUM { one, two, three } number=&quot;one&quot;</code>
C-type: <code class="docutils literal">const char *</code>
Allows values from a set of constant strings. <cite>Note</cite> that the
C-type is a string, not a C enum.
</dd>
<dt>HEADER</dt>
<dd>C-type: <code class="docutils literal">const struct gethdr_s *</code>
These are VCL compiler generated constants referencing a
particular header in a particular HTTP entity, for instance
<code class="docutils literal">req.http.cookie</code> or <code class="docutils literal">beresp.http.last-modified</code>. By passing
a reference to the header, the VMOD code can both read and write
the header in question.
If the header was passed as STRING, the VMOD code only sees
the value, but not where it came from.
</dd>
<dt>HTTP</dt>
<dd>C-type: <code class="docutils literal">struct http *</code>
TODO
</dd>
<dt>INT</dt>
<dd>C-type: <code class="docutils literal">long</code>
A (long) integer as we know and love them.
</dd>
<dt>IP</dt>
<dd>C-type: <code class="docutils literal">const struct suckaddr *</code>
This is an opaque type, see the <code class="docutils literal">include/vsa.h</code> file for
which primitives we support on this type.
</dd>
<dt>PRIV_CALL</dt>
<dd>See <a class="reference internal" href="#ref-vmod-private-pointers">Private Pointers</a> below.</dd>
<dt>PRIV_TASK</dt>
<dd>See <a class="reference internal" href="#ref-vmod-private-pointers">Private Pointers</a> below.</dd>
<dt>PRIV_TOP</dt>
<dd>See <a class="reference internal" href="#ref-vmod-private-pointers">Private Pointers</a> below.</dd>
<dt>PRIV_VCL</dt>
<dd>See <a class="reference internal" href="#ref-vmod-private-pointers">Private Pointers</a> below.</dd>
<dt>PROBE</dt>
<dd>C-type: <code class="docutils literal">const struct vrt_backend_probe *</code>
A named standalone backend probe definition.
</dd>
<dt>REAL</dt>
<dd>C-type: <code class="docutils literal">double</code>
A floating point value.
</dd>
<dt>STRING</dt>
<dd>C-type: <code class="docutils literal">const char *</code>
A NUL-terminated text-string.
Can be NULL to indicate a nonexistent string, for instance in:
<div class="highlight-default"><div class="highlight"><pre>mymod.foo(req.http.foobar);
</pre></div>
</div>
If there were no &quot;foobar&quot; HTTP header, the vmod_foo()
function would be passed a NULL pointer as argument.
When used as a return value, the producing function is
responsible for arranging memory management. Either by
freeing the string later by whatever means available or
by using storage allocated from the client or backend
workspaces.
</dd>
<dt>STEVEDORE</dt>
<dd>C-type: <code class="docutils literal">const struct stevedore *</code>
A storage backend.
</dd>
<dt>STRING_LIST</dt>
<dd>C-type: <code class="docutils literal">const char *, ...</code>
A multi-component text-string. We try very hard to avoid
doing text-processing in Varnish, and this is one way we
to avoid that, by not editing separate pieces of a string
together to one string, unless we have to.
Consider this contrived example:
<div class="highlight-default"><div class="highlight"><pre>set req.http.foo = std.toupper(req.http.foo + req.http.bar);
</pre></div>
</div>
The usual way to do this, would be be to allocate memory for
the concatenated string, then pass that to <code class="docutils literal">toupper()</code> which in
turn would return another freshly allocated string with the
modified result. Remember: strings in VCL are <code class="docutils literal">const</code>, we
cannot just modify the string in place.
What we do instead, is declare that <code class="docutils literal">toupper()</code> takes a &quot;STRING_LIST&quot;
as argument. This makes the C function implementing <code class="docutils literal">toupper()</code>
a vararg function (see the prototype above) and responsible for
considering all the <code class="docutils literal">const char *</code> arguments it finds, until the
magic marker &quot;vrt_magic_string_end&quot; is encountered.
Bear in mind that the individual strings in a STRING_LIST can be
NULL, as described under STRING, that is why we do not use NULL
as the terminator.
Right now we only support STRING_LIST being the last argument to
a function, we may relax that at a latter time.
If you don't want to bother with STRING_LIST, just use STRING
and make sure your workspace_client and workspace_backend params
are big enough.
</dd>
<dt>TIME</dt>
<dd>C-type: <code class="docutils literal">double</code>
Unit: seconds since UNIX epoch.
An absolute time, as in 1284401161.
</dd>
<dt>VOID</dt>
<dd>C-type: <code class="docutils literal">void</code>
Can only be used for return-value, which makes the function a VCL
procedure.
</dd>
</dl>
</div>
<div class="section" id="private-pointers">
<h2>Private Pointers<a class="headerlink" href="#private-pointers" title="Permalink to this headline">¶</a></h2>
It is often useful for library functions to maintain local state,
this can be anything from a precompiled regexp to open file descriptors
and vast data structures.
The VCL compiler supports the following private pointers:
<ul class="simple">
<li><code class="docutils literal">PRIV_CALL</code> &quot;per call&quot; private pointers are useful to cache/store
state relative to the specific call or its arguments, for instance a
compiled regular expression specific to a regsub() statement or a
simply caching the last output of some expensive lookup.</li>
<li><code class="docutils literal">PRIV_TASK</code> &quot;per task&quot; private pointers are useful for state that
applies to calls for either a specific request or a backend
request. For instance this can be the result of a parsed cookie
specific to a client. Note that <code class="docutils literal">PRIV_TASK</code> contexts are separate
for the client side and the backend side, so use in
<code class="docutils literal">vcl_backend_*</code> will yield a different private pointer from the
one used on the client side.</li>
<li><code class="docutils literal">PRIV_TOP</code> &quot;per top-request&quot; private pointers live for the
duration of one request and all its ESI-includes. They are only
defined for the client side. When used from backend VCL subs, a NULL
pointer will be passed.</li>
<li><code class="docutils literal">PRIV_VCL</code> &quot;per vcl&quot; private pointers are useful for such global
state that applies to all calls in this VCL, for instance flags that
determine if regular expressions are case-sensitive in this vmod or
similar. The <code class="docutils literal">PRIV_VCL</code> object is the same object that is passed
to the VMOD's event function.</li>
</ul>
The way it works in the vmod code, is that a <code class="docutils literal">struct vmod_priv *</code> is
passed to the functions where one of the <code class="docutils literal">PRIV_*</code> argument types is
specified.
This structure contains three members:
<div class="highlight-default"><div class="highlight"><pre>typedef void vmod_priv_free_f(void *);
struct vmod_priv {
 void *priv;
 int len;
 vmod_priv_free_f *free;
};
</pre></div>
</div>
The &quot;priv&quot; element can be used for whatever the vmod code wants to
use it for, it defaults to a NULL pointer.
The &quot;len&quot; element is used primarily for BLOBs to indicate its size.
The &quot;free&quot; element defaults to NULL, and it is the modules responsibility
to set it to a suitable function, which can clean up whatever the &quot;priv&quot;
pointer points to.
When a VCL program is discarded, all private pointers are checked
to see if both the &quot;priv&quot; and &quot;free&quot; elements are non-NULL, and if
they are, the &quot;free&quot; function will be called with the &quot;priv&quot; pointer
as the only argument.
In the common case where a private data structure is allocated with
malloc would look like this:
<div class="highlight-default"><div class="highlight"><pre>if (priv-&gt;priv == NULL) {
 priv-&gt;priv = calloc(1, sizeof(struct myfoo));
 AN(priv-&gt;priv);
 priv-&gt;free = free; /* free(3) */
 mystate = priv-&gt;priv;
 mystate-&gt;foo = 21;
 ...
} else {
 mystate = priv-&gt;priv;
}
if (foo &gt; 25) {
 ...
}
</pre></div>
</div>
The per-call vmod_privs are freed before the per-vcl vmod_priv.
</div>
<div class="section" id="event-functions">
<h2>Event functions<a class="headerlink" href="#event-functions" title="Permalink to this headline">¶</a></h2>
VMODs can have an &quot;event&quot; function which is called when a VCL which
imports the VMOD is loaded or discarded. This corresponds to the
<code class="docutils literal">VCL_EVENT_LOAD</code> and <code class="docutils literal">VCL_EVENT_DISCARD</code> events, respectively.
In addition, this function will be called when the VCL temperature is
changed to cold or warm, corresponding to the <code class="docutils literal">VCL_EVENT_COLD</code> and
<code class="docutils literal">VCL_EVENT_WARM</code> events.
The first argument to the event function is a VRT context.
The second argument is the vmod_priv specific to this particular VCL,
and if necessary, a VCL specific VMOD &quot;fini&quot; function can be attached
to its &quot;free&quot; hook.
The third argument is the event.
If the VMOD has private global state, which includes any sockets or files
opened, any memory allocated to global or private variables in the C-code etc,
it is the VMODs own responsibility to track how many VCLs were loaded or
discarded and free this global state when the count reaches zero.
VMOD writers are strongly encouraged to release all per-VCL resources for a
given VCL when it emits a <code class="docutils literal">VCL_EVENT_COLD</code> event. You will get a chance to
reacquire the resources before the VCL becomes active again and be notified
first with a <code class="docutils literal">VCL_EVENT_WARM</code> event. Unless a user decides that a given VCL
should always be warm, an inactive VMOD will eventually become cold and should
manage resources accordingly.
An event function must return zero upon success. It is only possible to fail
an initialization with the <code class="docutils literal">VCL_EVENT_LOAD</code> or <code class="docutils literal">VCL_EVENT_WARM</code> events.
Should such a failure happen, a <code class="docutils literal">VCL_EVENT_DISCARD</code> or <code class="docutils literal">VCL_EVENT_COLD</code>
event will be sent to the VMODs that succeeded to put them back in a cold
state. The VMOD that failed will not receive this event, and therefore must
not be left half-initialized should a failure occur.
If your VMOD is running an asynchronous background job you can hold a reference
to the VCL to prevent it from going cold too soon and get the same guarantees
as backends with ongoing requests for instance. For that, you must acquire the
reference by calling <code class="docutils literal">VRT_ref_vcl</code> when you receive a <code class="docutils literal">VCL_EVENT_WARM</code> and
later calling <code class="docutils literal">VRT_rel_vcl</code> once the background job is over. Receiving a
<code class="docutils literal">VCL_EVENT_COLD</code> is your cue to terminate any background job bound to a VCL.
You can find an example of VCL references in vmod-debug:
<div class="highlight-default"><div class="highlight"><pre>priv_vcl-&gt;vclref = VRT_ref_vcl(ctx, &quot;vmod-debug&quot;);
...
VRT_rel_vcl(&amp;ctx, &amp;priv_vcl-&gt;vclref);
</pre></div>
</div>
In this simplified version, you can see that you need at least a VCL-bound data
structure like a <code class="docutils literal">PRIV_VCL</code> or a VMOD object to keep track of the reference
and later release it. You also have to provide a description, it will be printed
to the user if they try to warm up a cooling VCL:
<div class="highlight-default"><div class="highlight"><pre>$ varnishadm vcl.list
available auto/cooling 0 vcl1
active auto/warm 0 vcl2

$ varnishadm vcl.state vcl1 warm
Command failed with error code 300
Failed &lt;vcl.state vcl1 auto&gt;
Message:
 VCL vcl1 is waiting for:
 - vmod-debug
</pre></div>
</div>
In the case where properly releasing resources may take some time, you can
opt for an asynchronous worker, either by spawning a thread and tracking it, or
by using Varnish's worker pools.
</div>
<div class="section" id="when-to-lock-and-when-not-to-lock">
<h2>When to lock, and when not to lock<a class="headerlink" href="#when-to-lock-and-when-not-to-lock" title="Permalink to this headline">¶</a></h2>
Varnish is heavily multithreaded, so by default VMODs must implement
their own locking to protect shared resources.
When a VCL is loaded or unloaded, the event and priv-&gt;free are
run sequentially all in a single thread, and there is guaranteed
to be no other activity related to this particular VCL, nor are
there init/fini activity in any other VCL or VMOD at this time.
That means that the VMOD init, and any object init/fini functions
are already serialized in sensible order, and won't need any locking,
unless they access VMOD specific global state, shared with other VCLs.
Traffic in other VCLs which also import this VMOD, will be happening
while housekeeping is going on.
</div>
<div class="section" id="updating-vmods">
<h2>Updating VMODs<a class="headerlink" href="#updating-vmods" title="Permalink to this headline">¶</a></h2>
A compiled VMOD is a shared library file which Varnish dlopen(3)'s
using flags RTLD_NOW | RTLD_LOCAL.
As a general rule, once a file is opened with dlopen(3) you should
never modify it, but it is safe to rename it and put a new file
under the name it had, which is how most tools installs and updates
shared libraries.
However, when you call dlopen(3) with the same filename multiple
times it will give you the same single copy of the shared library
file, without checking if it was updated in the meantime.
This is obviously an oversight in the design of the dlopen(3) library
function, but back in the late 1980s nobody could imagine why a
program would ever want to have multiple different versions of the
same shared library mapped at the same time.
Varnish does that, and therefore you must restart the worker process
before Varnish will discover an updated VMOD.
If you want to test a new version of a VMOD, while being able to
instantly switch back to the old version, you will have to install
each version with a distinct filename or in a distinct subdirectory
and use <code class="docutils literal">import foo from &quot;...&quot;;</code> to reference it in your VCL.
We're not happy about this, but have found no sensible workarounds.
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<li><a class="reference internal" href="#">VMOD - Varnish Modules</a><ul>
<li><a class="reference internal" href="#vmod-directory">VMOD Directory</a></li>
<li><a class="reference internal" href="#the-vmod-vcc-file">The vmod.vcc file</a><ul>
<li><a class="reference internal" href="#named-arguments-and-default-values">Named arguments and default values</a></li>
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<li><a class="reference internal" href="#vcl-and-c-data-types">VCL and C data types</a></li>
<li><a class="reference internal" href="#private-pointers">Private Pointers</a></li>
<li><a class="reference internal" href="#event-functions">Event functions</a></li>
<li><a class="reference internal" href="#when-to-lock-and-when-not-to-lock">When to lock, and when not to lock</a></li>
<li><a class="reference internal" href="#updating-vmods">Updating VMODs</a></li>
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