Varnish Directors Module

Manual section



import directors [from “path”] ;


vmod_directors enables backend load balancing in Varnish.

The module implements load balancing techniques, and also serves as an example on how one could extend the load balancing capabilities of Varnish.

To enable load balancing you must import this vmod (directors).

Then you define your backends. Once you have the backends declared you can add them to a director. This happens in executed VCL code. If you want to emulate the previous behavior of Varnish 3.0 you can just initialize the directors in vcl_init, like this:

sub vcl_init {
    new vdir = directors.round_robin();

As you can see there is nothing keeping you from manipulating the directors elsewhere in VCL. So, you could have VCL code that would add more backends to a director when a certain URL is called.

Note that directors can use other directors as backends.



new OBJ = round_robin()

Create a round robin director.

This director will pick backends in a round robin fashion.


new vdir = directors.round_robin();


VOID round_robin.add_backend(BACKEND)

Add a backend to the round-robin director.


vdir.add_backend(backend1); vdir.add_backend(backend2);


VOID round_robin.remove_backend(BACKEND)

Remove a backend from the round-robin director.


vdir.remove_backend(backend1); vdir.remove_backend(backend2);


BACKEND round_robin.backend()

Pick a backend from the director.


set req.backend_hint = vdir.backend();


new OBJ = fallback()

Create a fallback director.

A fallback director will try each of the added backends in turn, and return the first one that is healthy.


new vdir = directors.fallback();


VOID fallback.add_backend(BACKEND)

Add a backend to the director.

Note that the order in which this is done matters for the fallback director.


vdir.add_backend(backend1); vdir.add_backend(backend2);


VOID fallback.remove_backend(BACKEND)

Remove a backend from the director.


vdir.remove_backend(backend1); vdir.remove_backend(backend2);


BACKEND fallback.backend()

Pick a backend from the director.


set req.backend_hint = vdir.backend();


new OBJ = random()

Create a random backend director.

The random director distributes load over the backends using a weighted random probability distribution. The “testable” random generator in varnishd is used, which enables deterministic tests to be run (See: d00004.vtc).


new vdir = directors.random();


VOID random.add_backend(BACKEND, REAL)

Add a backend to the director with a given weight.

Each backend backend will receive approximately 100 * (weight / (sum(all_added_weights))) per cent of the traffic sent to this director.


# 2/3 to backend1, 1/3 to backend2. vdir.add_backend(backend1, 10.0); vdir.add_backend(backend2, 5.0);


VOID random.remove_backend(BACKEND)

Remove a backend from the director.


vdir.remove_backend(backend1); vdir.remove_backend(backend2);


BACKEND random.backend()

Pick a backend from the director.


set req.backend_hint = vdir.backend();


new OBJ = hash()

Create a hashing backend director.

The director chooses the backend server by computing a hash/digest of the string given to .backend().

Commonly used with client.ip or a session cookie to get sticky sessions.


new vdir = directors.hash();


VOID hash.add_backend(BACKEND, REAL)

Add a backend to the director with a certain weight.

Weight is used as in the random director. Recommended value is 1.0 unless you have special needs.


vdir.add_backend(backend1, 1.0); vdir.add_backend(backend2, 1.0);


VOID hash.remove_backend(BACKEND)

Remove a backend from the director.


vdir.remove_backend(backend1); vdir.remove_backend(backend2);


BACKEND hash.backend(STRING)

Pick a backend from the backend director.

Use the string or list of strings provided to pick the backend.


# pick a backend based on the cookie header from the client set req.backend_hint = vdir.backend(req.http.cookie);


new OBJ = shard()

Create a shard director.

Note that the shard director needs to be configured using at least one shard.add_backend() call(s) followed by a shard.reconfigure() call before it can hand out backends.


The shard director selects backends by a key, which can be provided directly or derived from strings. For the same key, the shard director will always return the same backend, unless the backend configuration or health state changes. Conversely, for differing keys, the shard director will likely choose different backends. In the default configuration, unhealthy backends are not selected.

The shard director resembles the hash director, but its main advantage is that, when the backend configuration or health states change, the association of keys to backends remains as stable as possible.

In addition, the rampup and warmup features can help to further improve user-perceived response times.


This basic technique allows for numerous applications like optimizing backend server cache efficiency, Varnish clustering or persisting sessions to servers without keeping any state, and, in particular, without the need to synchronize state between nodes of a cluster of Varnish servers:

  • Many applications use caches for data objects, so, in a cluster of application servers, requesting similar objects from the same server may help to optimize efficiency of such caches.

    For example, sharding by URL or some id component of the url has been shown to drastically improve the efficiency of many content management systems.

  • As special case of the previous example, in clusters of Varnish servers without additional request distribution logic, each cache will need store all hot objects, so the effective cache size is approximately the smallest cache size of any server in the cluster.

    Sharding allows to segregate objects within the cluster such that each object is only cached on one of the servers (or on one primary and one backup, on a primary for long and others for short etc…). Effectively, this will lead to a cache size in the order of the sum of all individual caches, with the potential to drastically increase efficiency (scales by the number of servers).

  • Another application is to implement persistence of backend requests, such that all requests sharing a certain criterion (such as an IP address or session ID) get forwarded to the same backend server.

When used with clusters of varnish servers, the shard director will, if otherwise configured equally, make the same decision on all servers. In other words, requests sharing a common criterion used as the shard key will be balanced onto the same backend server(s) no matter which Varnish server handles the request.

The drawbacks are:

  • the distribution of requests depends on the number of requests per key and the uniformity of the distribution of key values. In short, while this technique may lead to much better efficiency overall, it may also lead to less good load balancing for specific cases.

  • When a backend server becomes unavailable, every persistence technique has to reselect a new backend server, but this technique will also switch back to the preferred server once it becomes healthy again, so when used for persistence, it is generally less stable compared to stateful techniques (which would continue to use a selected server for as long as possible (or dictated by a TTL)).


When .reconfigure() is called, a consistent hashing circular data structure gets built from hash values of “ident%d” (default ident being the backend name) for each backend and for a running number from 1 to n (n is the number of replicas). Hashing creates the seemingly random order for placement of backends on the consistent hashing ring.

When .backend() is called, a load balancing key gets generated unless provided. The smallest hash value in the circle is looked up that is larger than the key (searching clockwise and wrapping around as necessary). The backend for this hash value is the preferred backend for the given key.

If a healthy backend is requested, the search is continued linearly on the ring as long as backends found are unhealthy or all backends have been checked. The order of these “alternative backends” on the ring is likely to differ for different keys. Alternative backends can also be selected explicitly.

On consistent hashing see:

Error Reporting

Failing methods should report errors to VSL with the Error tag, so when configuring the shard director, you are advised to check:

varnishlog -I Error:^shard


VOID shard.set_warmup(REAL probability=0.0)

Set the default warmup probability. See the warmup parameter of shard.backend().

Default: 0.0 (no warmup)


VOID shard.set_rampup(DURATION duration=0)

Set the default rampup duration. See rampup parameter of shard.backend().

Default: 0s (no rampup)


BOOL shard.add_backend(PRIV_TASK, BACKEND backend, STRING ident=0, DURATION rampup=973279260)

Add a backend backend to the director.

ident: Optionally specify an identification string for this backend, which will be hashed by shard.reconfigure() to construct the consistent hashing ring. The identification string defaults to the backend name.

ident allows to add multiple instances of the same backend.

rampup: Optionally specify a rampup time for this backend. Defaults to the director rampup time.

NOTE: Backend changes need to be finalized with shard.reconfigure() and are only supported on one shard director at a time.


BOOL shard.remove_backend(PRIV_TASK, BACKEND backend=0, STRING ident=0)

Remove backend(s) from the director. Either backend or ident must be specified. ident removes a specific instance. If backend is given without ident, all instances of this backend are removed.

NOTE: Backend changes need to be finalized with shard.reconfigure() and are only supported on one shard director at a time.


BOOL shard.clear(PRIV_TASK)

Remove all backends from the director.

NOTE: Backend changes need to be finalized with shard.reconfigure() and are only supported on one shard director at a time.


BOOL shard.reconfigure(PRIV_TASK, INT replicas=67, ENUM {CRC32,SHA256,RS} alg="SHA256")

Reconfigure the consistent hashing ring to reflect backend changes.

This method must be called at least once before the director can be used.


INT shard.key(STRING string, ENUM {CRC32,SHA256,RS} alg="SHA256")

Utility method to generate a sharding key for use with the shard.backend() method by hashing string with hash algorithm alg.


BACKEND shard.backend(ENUM {HASH,URL,KEY,BLOB} by="HASH", INT key=0, BLOB key_blob=0, INT alt=0, REAL warmup=-1, BOOL rampup=1, ENUM {CHOSEN,IGNORE,ALL} healthy="CHOSEN")

Lookup a backend on the consistent hashing ring.

This documentation uses the notion of an order of backends for a particular shard key. This order is deterministic but seemingly random as determined by the consistent hashing algorithm and is likely to differ for different keys, depending on the number of backends and the number of replicas. In particular, the backend order referred to here is _not_ the order given when backends are added.

  • by how to determine the sharding key

    default: HASH

    • HASH:

      • when called in backend context: Use the varnish hash value as set by vcl_hash

      • when called in client content: hash req.url

    • URL: hash req.url / bereq.url

    • KEY: use the key argument

    • BLOB: use the key_blob argument

    • key lookup key with by=KEY

      the shard.key() function may come handy to generate a sharding key from custom strings.

    • key_blob lookup key with by=BLOB

      Currently, this uses the first 4 bytes from the given blob in network byte order (big endian), left-padded with zeros for blobs smaller than 4 bytes.

  • alt alternative backend selection

    default: 0

    Select the alt-th alternative backend for the given key.

    This is particularly useful for retries / restarts due to backend errors: By setting alt=req.restarts or alt=bereq.retries with healthy=ALL, another server gets selected.

    The rampup and warmup features are only active for alt==0

  • rampup slow start for servers which just went healthy

    default: true

    If alt==0 and the chosen backend is in its rampup period, with a probability proportional to the fraction of time since the backup became healthy to the rampup period, return the next alternative backend, unless this is also in its rampup period.

    The default rampup interval can be set per shard director using the set_rampup() method or specifically per backend with the set_backend() method.

  • warmup probabilistic alternative server selection

    possible values: -1, 0..1

    default: -1

    -1: use the warmup probability from the director definition

    Only used for alt==0: Sets the ratio of requests (0.0 to 1.0) that goes to the next alternate backend to warm it up when the preferred backend is healthy. Not active if any of the preferred or alternative backend are in rampup.

    warmup=0.5 is a convenient way to spread the load for each key over two backends under normal operating conditions.

  • healthy

    default: CHOSEN

    • CHOSEN: Return a healthy backend if possible.

      For alt==0, return the first healthy backend or none.

      For alt > 0, ignore the health state of backends skipped for alternative backend selection, then return the next healthy backend. If this does not exist, return the last healthy backend of those skipped or none.

    • IGNORE: Completely ignore backend health state

      Just return the first or alt-th alternative backend, ignoring health state. Ignore rampup and warmup.

    • ALL: Check health state also for alternative backend selection

      For alt > 0, return the alt-th alternative backend of all those healthy, the last healthy backend found or none.


VOID shard.debug(INT)

intentionally undocumented


Development of a previous version of the shard director was partly sponsored by Deutsche Telekom AG - Products & Innovation.

Development of this version of the shard director was partly sponsored by BILD GmbH & Co KG.