Create and delete Varnish backends at runtime


Varnish Module to dynamically create and delete backends

Manual section: 3


import backend_dyn [from "path"] ;

BOOL backend_dyn.create(STRING name, STRING host, [STRING port,]
                        [PROBE probe,] [STRING host_header,]
                        [DURATION connect_timeout,]
                        [DURATION first_byte_timeout,]
                        [DURATION between_bytes_timeout,]
                        [INT max_connections,] [INT proxy_header])

BACKEND backend_dyn.by_name(STRING name)

BOOL backend_dyn.delete(BACKEND be)

STRING backend_dyn.version()


This Varnish module (VMOD) provides functions to create and delete backends at runtime. This means that backends can be created at any time while Varnish is running without reloading VCL, and that backends created by the VMOD can also be deleted by the VMOD at runtime.

Backends created by the VMOD are the same as backends defined by static declarations with the backend keyword, and they have the same configuration parameters with the same default values. Once created, a backend can be added to a director or assigned as a value of req.backend_hint or bereq.backend. Backends created by the VMOD are accessed by the finder method by_name, using the name string assigned at creation.

Once deleted, a backend is no longer in use by Varnish -- it is not selected by any director to which it may have been added, and is no longer returned by by_name.


import directors;

probe p { .url = "/"; }

sub vcl_init {
    # Backends can be created at VCL initialization.
    # The required parameters set a backend name and its host.
    # create() returns false if backend creation was unsuccessful.
    if (!backend_dyn.create(name="be1", host="be1host.com")) {

    # Dynamic backends may be added to directors.
    # by_name() returns the backend created with the given name.
    new rr = directors.round_robin();

    # Backend creation supports all of the optional configuration
    # parameters available to static backend declarations; these
    # have the same default values as for a static declaration if
    # left unspecified.
    if (!backend_dyn.create(name="be1", host="be1host.com",
                            port="8080", probe=p,
                            connect_timeout=1s, first_byte_timeout=2s,
                            max_connections=100, proxy_header=1)) {

    # Backends can also be deleted at VCL initialization.
    # delete() returns false if backend deletion was unsuccessful.
    # A deleted backend is no longer selected by any director to
    # which it was previously added.
    if (!backend_dyn.delete(backend_dyn.by_name("be1"))) {

sub vcl_recv {
    # Backends can be created and deleted in any VCL subroutine.
    if (req.method == "PUT") {
        if (!backend_dyn.create(name="be2", host="be2host.com")) {
    if (req.method == "DELETE") {
        if (!backend_dyn.delete(backend_dyn.by_name("be2"))) {
    set req.backend_hint = backend_dyn.by_name("be2");

sub vcl_backend_fetch {
    set bereq.backend = backend_dyn.by_name("be2");


  • STRING version()


BOOL create(PRIV_VCL, STRING name, STRING host, STRING port="", PROBE probe=0, STRING host_header="", DURATION connect_timeout=0, DURATION first_byte_timeout=0, DURATION between_bytes_timeout=0, INT max_connections=0, INT proxy_header=0)

Create a backend with the given configuration. The parameters correspond to the configuration fields of a static backend declaration. Further restrictions on required and optional parameters are given below.

create() returns false if backend creation was unsuccessful, true on success. After successful creation, the backend appears in the output of the CLI command backend.list, and statistics in the VBE.* namespace appear in the output of varnishstat. Its health can be set with the CLI command backend.set_health.

A backend created by the VMOD is "owned" by the VCL instance in which it is created, and is not available in any other VCL loaded by the Varnish process. The backend is deleted when its VCL is discarded, and it enters the cooldown state when its VCL is set to cold; in particular, no more health probes for that backend are performed, if any were defined, when the backend goes cold.

The parameters may be specified as named parameters in any order. If used without parameter names, they MUST appear strictly in the order given in the function signature shown above.

Required Parameters

The parameters name and host MUST be non-empty strings. name MUST contain only alphanumeric characters or the underscore. It MAY NOT be a name already used for another backend created by the VMOD in the current instance of VCL, unless the backend with the duplicate name has been deleted.

host MUST be either an IP address (IPv4 or IPv6) or a host name that can be resolved when create is called. A host name may resolve to both an IPv4 and IPv6 address, but for each address family, it MUST resolve to exactly one address.

Optional Parameters

port MUST be a numeric port number, or a TCP service name listed in /etc/services. The default is "80".

probe MUST specify a probe defined in a static declaration (or possibly a probe returned by another VMOD). If left unspecified, the backend has no probe.

host_header MUST be a non-empty legal value for the Host header, and is sent as the Host header in backend requests. By default, the value of the host parameter is used for Host.

Each of connect_timeout, first_byte_timeout and between_bytes_timeout MUST be a VCL duration, such as 0.5s, 1m, etc. If a timeout is set for a specific backend request (using bereq.connect_timeout, etc.), then that timeout overrides the setting in the backend definition. If any of these parameters are left unspecified, then the timeout "hierarchy" is followed as for standard Varnish -- a bereq.* timeout is used if set, otherwise the corresponding global parameter is used (-p connect_timeout, etc.).

max_connections MUST be an integer. By default, there is no upper bound for the connections to a backend.

proxy_header MUST be 0, 1 or 2. If 1 or 2, then that version of the PROXY protocol is used with the backend; if 0, then the PROXY protocol is not used. Default is 0.


if (!backend_dyn.create(name="be", host="myhost.com")) {
        return(synth(500, "Backend creation failed"));
if (!backend_dyn.create(name="app", host="",
                        port="8080", probe=p,
                        connect_timeout=1s, first_byte_timeout=2s,
                        between_bytes_timeout=1s)) {
        return(synth(500, "Backend creation failed"));



Return the backend created by the VMOD with the given name in the current instance of VCL. name MUST be the name of a backend previously created with create() in the current VCL.

by_name() cannot return a statically declared backend, a backend deleted by delete(), a backend owned by another VCL, or a backend created by another VMOD.

The function returns NULL if no backend with the given name exists (or has been deleted). If this happens when setting the value of req.backend_hint or bereq.backend, the result is typically a 503 response with the FetchError "No backend found".

XXX: Document dir.add_backend(NULL)


set req.backend_hint = backend_dyn.by_name("be");
set bereq.backend = backend_dyn.by_name("be");



Delete the backend created by the VMOD with the given name in the current instance of VCL. name MUST be the name of a backend previously created with create() in the current VCL.

After deletion, a backend is considered unhealthy and hence is o longer selected for new backend requests. It enters a "cooldown" state of at least 60 seconds, during which it may complete any requests that had already begun when it was deleted. Any current backend requests MUST be completed during this time. The VBE.* statistics for the backend continue to appear in the output of varnishstat during cooldown, and MAIN.n_backend is not decremented until cooldown elapses.

The backend no longer appears in the output of backend.list after deletion, and its health cannot be set by backend.set_health. It is not returned by by_name(), and cannot be used to set the value of req.backend_hint or bereq.backend. If the backend had been previously added to a director, it is no longer selected by the director after deletion.

delete() cannot delete a statically declared backend, a backend that has already been deleted, a backend owned by another VCL, or a backend created by another VMOD.

delete() returns false if deletion was unsuccessful, true otherwise.


if (!backend_dyn.delete(backend_dyn.by_name("be"))) {
        return(synth(500, "Backend delete failed"));


STRING version()

Returns the version string for this vmod.


import std;
std.log("Using VMOD backend_dyn version " + backend_dyn.version());


If create() is called in vcl_init and backend creation fails, then the VCL program will fail to load, and the VCC compiler will emit an error message.

If create() is called in any other VCL subroutine and an error occurs, then an error message will be written to the Varnish log using the tag VCL_Error, and the function returns false.

by_name() and delete() only fail if the specified backend was not created by the VMOD in the current VCL, or if it was already deleted. There is no error message.


This VMOD requires Varnish 6.0, to be released in September 2017 (or the current trunk). See the source repository for versions of the VMOD that are compatible with other versions of Varnish.


See INSTALL.rst in the source repository.


create() prevents the use of duplicate backend names created by this VMOD in the current instance of VCL, but it cannot prevent the re-use of a backend name created by other means, for example by a static declaration or another VMOD. Varnish has no problem using backends with duplicate names, since they are represented internally as distinct objects. But if two backends have the same name, they appear as duplicate entries in the output of backend.list; and if one of them is sick while the other is healthy, it is impossible to tell which is which. Also, VBE.* stats are maintained for only one of the backends with that name. So you should take care not to create backends with names used elsewhere in the VCL.

by_name() searches for backends in a linear list, and is locked out of the search if create() or delete() are executing concurrently. If you have a large number of dynamic backends, it is probably more efficient to add them to directors (even a director that has only one backend) than to set req.backend_hint or bereq.backend using by_name(), since a director does not execute the linear search.

It is in the nature of dynamic backends that they are not preserved when Varnish stops or when the VCL is discarded. While dynamic backends make it possible to change the backend configuration without reloading VCL, you may nevertheless want to update your backend configuration in VCL sources on the file system, so that your backends are not "lost" on restart.



This document is licensed under the same conditions as the
libvmod-backend_dyn project. See LICENSE for details.

Author: Geoffrey Simmons <geoffrey.simmons@uplex.de>