Decider: Distributed Feature Flags
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dcdr (decider)

Distributed Feature Flags


This is pre-release software. The Consul backend support has been used in production at VSCO for a year, but the Etcd backend should be considered experimental.


Decider is a feature flag system with adaptable backends. It supports both percentile and boolean flags for controlled infrastructure rollouts and kill switches. Decider is built to be adaptable to any backing datastore. At the moment, Consul, Etcd, and Redis are supported. ZooKeeper support is planned.

Decider has four major components.

  • Client for use within your Go applications
  • Server for accessing features over HTTP
  • Watcher observes change in the datastore and writes them to disk
  • CLI for managing features, watches, and starting the server

Each of these components are comprised of lower level libraries that you can use to suit your system's specific needs.

About Feature Flags

Feature flags have many use cases and there are many implementations. With Decider, the three supported types of flags are boolean, percentile, and scalar. For our purposes at VSCO, these have been enough to handle our needs.

Boolean Flags

An example use case for a boolean flag would be an API kill switch that could alleviate load for a backing database.

disable-load-heavy-api-endpoint => true

Your code would look something like this.

if dcdr.IsAvailable("disable-load-heavy-api-endpoint") {
	// DB is having a bad day, please check back later
} else {
	// All good, go about your day

Percentile Flags

Percentiles work much the same way but allow you to stress features or new infrastructure with a percentage of the request volume.

A common use case for percentile features would be testing out a new backend store with dual write percentage.

rollout-new-fancy-db-dual-write => 0.1
// Handle the write to the existing store

if dcdr.IsAvailableForID("rollout-new-fancy-db-dual-write", user.Id) {
	// If the `user.Id` falls into 10% of requests do the dual write


Scalars have an added bonus: you may use their float64 values as scalars in certain cases.

Here, we'll use the float value to scale the wait time for DB inserts between 0-1000ms.

 daemon-db-insert-wait-ms => 0.1
// waitMS would be 1000*0.1 => 100
waitMS := dcdr.ScaleValue("daemon-db-insert-wait-ms", 0, 1000)
time.Sleep(waitMS * time.Millisecond)

Read more on how to use the Client.


Feature flags and remote configuration are hard problems to solve in the general sense. Most organizations will have many corner cases unique to their own infrastructure and policies that are cumbersome to solve in an abstract way. Decider is an extracted set of flexible libraries that we at VSCO have developed over the past year that have worked well for us in solving these issues.

This package does not set out to provide features like authentication or ACLs, but it does aim to provide enough of the tooling and libraries so that you can do so yourself.


In order to allow for expanding use cases and to avoid naming collisions, Decider provides arbitrary scoping of feature flags. An example use case would be providing separate features sets according to country code or mobile platform. Additionally, multiple Decider instances can be run within a cluster with separate namespaces and key sets by configuring config.hcl.

Audit Trail

Due to the sensitive nature of configuration management, knowing the who, what, and when of changes can be very important. Decider uses git to handle this responsibility. By easily specifying a git repository and its origin in config.hcl, Decider will export your keyspace as a JSON file and then commit and push the changeset to the specified origin. Of course, this is all optional if you enjoy living dangerously.


It's nice to know when changes are happening. Decider can be configured to emit Statsd events when changes occur. Custom event tags can be sent as well if your collector supports them. Included in this package is a DataDog adapter with Event and Tag support. Custom stats can also be configured by supplying a custom stats.IFace implementation.


git clone
cd dcdr

Getting Started


The dcdr CLI has comprehensive help system for all commands.

dcdr help [command]" for more information about a command.

Setting Features

Features have several fields that are accessible via set command.

	-n, --name="flag_name"
		the name of the flag to set
	-v, --value=0.0-1.0 or true|false
		the value of the flag
	-c, --comment="flag description"
		an optional comment or description
	-s, --scope="users/beta"
		an optional scope to nest the flag within


dcdr set -n new-feature -v 0.1 -c "some new feature" -s user-groups/beta

The above command sets the key dcdr/features/user-groups/beta/new-feature equal to 0.1 and commits the update to the audit repo.

Listing Features

Listing features can be filtered by a given scope and prefix. Any further fanciness can be handled by piping the output to grep or less.

	-p, --prefix="<flag-prefix>"
		List only flags with matching prefix.
	-s, --scope="<flag-scope>"
		List only flags within a scope.


dcdr list -p new -s user-groups/beta

Deleting Features

Features are removed using the dcdr delete command and take a name and scope parameters. If no scope is provided the default scope is assumed. Once deleted and if you have a repository configured, Decider will commit the changeset and push it to origin.

	-p, --prefix="<flag-prefix>"
		Name of the flag to delete
	-s, --scope="<flag-scope>"
		Optional scope to delete the flag from


dcdr delete -n another-feature -s user-groups/beta

Starting the Watcher

The watch command is central to how Decider features are distributed to nodes in a cluster. It observes the configured namespace and writes a JSON file containing the exported structure to the Server:OutputPath.

By default the Decider configuration and watch path are located in /etc/dcdr. If this path does not exist you will need to create it.

 sudo mkdir /etc/dcdr
 sudo chown `whoami` /etc/dcdr

You can override this location by setting the DCDR_CONFIG_DIR environment variable. More on configuration can be found here.

Tying the room together

TL;DR Using Docker Compose

Decider has many moving parts that require orchestration in order to do a proper demonstration of how they all work together. So thanks to Docker Compose we can bundle this up quite easily. This example uses a Consul backend and starts a dcdr watch and dcdr server for you.

Building & running the images


This starts a Consul agent for the backend, a Decider server, and a Decider Watcher.

With these services running you can now interact with the dcdr CLI via docker-compose exec from another terminal window through the dcdr_server container.

command to add/modify a feature flag:

$ docker-compose exec dcdr_server dcdr set -n feat-foo -v true
[dcdr] set flag 'dcdr/features/default/feat-foo'

result logs from running docker-compose log:

dcdr_watch_1   | [dcdr] 2018/03/13 18:25:48 wrote changes to: /etc/dcdr/decider.json

command to fetch feature flags from Decider server API:

$ docker-compose exec dcdr_server curl
  "dcdr": {
    "info": {},
    "features": {
      "feat-foo": true

result logs from running docker-compose log:

dcdr_server_1  | - - [13/Mar/2018:18:36:51 +0000] "GET /dcdr.json HTTP/1.1" 200 102 "" "curl/7.52.1"

Now you can update features, CURL the results from the server, and see the changes update in realtime.

From Scratch

This example uses the Consul backend. If you need instructions for getting Consul installed, check their Getting Started page.

Let's start a development consul agent with an empty feature set and see how this all works together. For simplicity we can use the default Decider configuration without a git repository or stats.

consul agent -bind "" -dev

This will start a local Consul agent ready to accept connections on Decider should now be able to connect to this instance and set features.

Set some features

# check that we can talk to the local agent
~  → dcdr list
[dcdr] no feature flags found in namespace: dcdr

# set a feature into the 'default' scope.
~  → dcdr set -n example-feature -v false
[dcdr] set flag 'dcdr/features/default/example-feature'

# set a feature into the 'user-groups/beta' scope.
~  → dcdr set -n example-feature -v true -s user-groups/beta
[dcdr] set flag 'dcdr/features/user-groups/beta/example-feature'

~  → dcdr list

Name             Type     Value  Comment  Scope             Updated By
example-feature  boolean  false           default           chrisb
example-feature  boolean  true            user-groups/beta  chrisb

Here we have set the feature example-feature into two separate scopes. In the 'default' scope the value is false and in the 'user-groups/beta' scope it has been set to true.

Start the watcher and observe changes
# start the watcher
~  → dcdr watch
[dcdr] 2016/03/09 17:56:17.250948 watching namespace: dcdr
[dcdr] 2016/03/09 17:56:17.365362 wrote changes to /etc/dcdr/decider.json

The watcher is now observing your «Namespace» and writing all changes to the Server:OutputPath default /etc/dcdr/decider.json.

Decider Server

The easiest way to view your feature flags is with dcdr server. This is a bare bones implementation of how to access features over HTTP. There is no authentication, so unless your use case is for internal access only you should include the server package into a new project and assemble your own. The server is built with gorilla/mux router and is extensible by adding additional middleware. Read more on custom servers here.

# start the server
~  → dcdr server
[dcdr] started watching /etc/dcdr/decider.json
[dcdr] 2016/03/09 18:03:46.211150 serving /dcdr.json on :8000

The server is now running on :8000 and features can be accessed by curling :8000/dcdr.json. In order to access your scopes the server accepts a x-dcdr-scopes header. This is a comma-delimited, priority-ordered list of scopes. Meaning that the scopes should provided with the highest priority first. For now we only have one scope so let's start simple.

# curl with no scopes
~  → curl -s :8000/dcdr.json
  "info": {},
  "dcdr": {
    "example-feature": false

Here we see that the default value of false is returned. The info key is where information like the current SHA of the repository would be if one was configured. Next, if we add the scope header we can access our scoped values.

~  → curl -sH "x-dcdr-scopes: user-groups/beta" :8000/dcdr.json
  "dcdr": {
    "info": {},
    "features": {
      "example-feature": true

Using the Go client

Included in this package is a Go client. By default this client uses the same config.hcl for its configuration. You may also provide custom your own custom configuration as well using config.Config and the client.New method. For this example we will assume the defaults are still in place and that the features from the above example have been set.

Require and initialize the client

import ""

// Initialize a client with the default configuration
client, err := client.NewDefault()

if err != nil {

Checking feature flags

The client has three main methods for interacting with flags IsAvailable(feature string). IsAvailableForID(feature string, id uint64), and ScaleValue(feature string, min float64, max float64).


This method is for checking boolean features or 'kill switches'. If a percentile feature is passed to this method it will always return false. So don't do that.

# set the default feature
dcdr set -n example-feature -v false
client, err := client.NewDefault()

if err != nil {

// example-feature would be false
if client.IsAvailable("example-feature") {
	fmt.Println("example-feature enabled")
} else {
	fmt.Println("example-feature disabled")

This example initializes a new Client and begins watching the 'default' feature scope. It then checks the example-feature and runs the appropriate code path given the current value of the feature.

So what about scopes?

To initialize a Decider Client into a given set of scopes use the WithScopes(scopes ...string) method. This method creates a new Client with an underlying feature set that has the provided scope values merged onto the default set. If a feature does not exist in any of the provided scopes the client will fallback to the 'default' scope to find the value. If the feature does not exist in any scope the client simply returns false.

# set the scoped feature
dcdr set -n example-feature -v true -s user-groups/beta
client, err := client.NewDefault()
scopedClient := client.WithScopes("user-groups/beta")

if err != nil {

// example-feature would be true
if scopedClient.IsAvailable("example-feature") {
	fmt.Println("example-feature enabled")
} else {
	fmt.Println("example-feature disabled")


# set a feature that does not exist in user-groups/beta
dcdr set -n another-feature -v true
client, err := client.NewDefault()
scopedClient := client.WithScopes("user-groups/beta")

if err != nil {

// another-feature would be true
if scopedClient.IsAvailable("another-feature") {
	fmt.Println("another-feature enabled")
} else {
	fmt.Println("another-feature disabled")


This method is used when a feature needs to be rolled out to only a percentage of requests. Functionally IsAvailableForID works exactly as IsAvailable with the exception of its id argument. Both the feature and id arguments are joined to generate a uint64 using hash/crc32. Which when combined with the float64 value of feature can compute into what percentile a given request falls.

See the Client#withinPercentile method for more details.

Using percentiles

# set a feature to 50%
dcdr set -n new-feature-rollout -v 0.5
client, err := client.NewDefault()

if err != nil {

userId := unint64(5)

// new-feature-rollout would be true
if client.IsAvailableForID("new-feature-rollout", userId) {
	fmt.Println("new-feature-rollout enabled")
} else {
	fmt.Println("new-feature-rollout disabled")


ScaleValue uses the same float64 values as IsAvailableForID but in this case these values are used to obtain a new value scaled between a min and a max.

For instance:

Given the feature db-insert-wait-ms => 0.5. When provided to the ScaleValue method would result in the following.

for {
	insertWaitMs := dcdr.ScaleValue("db-insert-wait-ms", 0, 1000)
	time.Sleep(insertWaitMs * time.Millisecond) // waits for 500ms


Building a custom Server

Exposing your feature flags to the open internet would be a terrible idea in most cases. The default server will work fine as long as access is restricted to internal network clients but what if we want to allow access to mobile devices? Since there are entirely too many auth strategies to cover and we are kind of lazy, Decider Server allows you to add middleware to customize its behavior to suit your authentication needs.

Extending with middleware

Below is an example of how to do authentication very poorly. However, if you look close enough you can imagine exactly where you might add a DB lookup for an OAuth token or something similar. The Use method takes variadic server.Middleware as a param to allow chainable customizations.

// Middleware helper type for handlers that receive a `Client`.
type Middleware func(client.IFace) func(http.Handler) http.Handler
const AuthorizationHeader = "Authorization"

// MockAuth example authentication middleware.
// Checks for any value in the http Authorization header.
// If no value is found a 401 status is sent.
func MockAuth(c client.IFace) func(http.Handler) http.Handler {
  return func(h http.Handler) http.Handler {
    fn := func(w http.ResponseWriter, r *http.Request) {
      if r.Header.Get(AuthorizationHeader) != "" {
        h.ServeHTTP(w, r)
      } else {
    return http.HandlerFunc(fn)

func main() {
	// Create a new Server and Client
	srv, err := server.NewDefault()

    if err != nil {

	// Add the MockAuth to the middleware chain

	// Begin serving on :8000
	// curl -sH "Authorization: authorized" :8000/dcdr.json

Here is a bit more useful example. This ScopedCountryCode middleware takes the X-Country header and appends it to x-dcdr-scopes. We then use the country-code/<cc> scope to look up per country feature flags from the K/V store.

const CountryCodeHeader = "X-Country"
const DcdrScopesHeader = "x-dcdr-scopes"

func ScopedCountryCode(c client.IFace) func(http.Handler) http.Handler {
	return func(h http.Handler) http.Handler {
		fn := func(w http.ResponseWriter, r *http.Request) {
			cc := strings.ToLower(r.Header.Get(CountryCodeHeader))

			if cc != "" {
				// Check for existing scopes and append 'country-code/xx'
				scopes := strings.Split(r.Header.Get(DcdrScopesHeader), ",")
				scopes = append(scopes, fmt.Sprintf("country-codes/%s", cc))
				r.Header.Set(DcdrScopesHeader, strings.Join(scopes, ","))

			h.ServeHTTP(w, r)

		return http.HandlerFunc(fn)

A full working example can be found in server/demo/main.go.


All configuration lives in config.hcl. By default Decider looks for this file in /etc/dcdr/config.hcl. You will need to create the /etc/dcdr directory. Your permissions depending on the machine may differ but to get started locally do the following.

 sudo mkdir /etc/dcdr
 sudo chown `whoami` /etc/dcdr
 dcdr init

The default config path can also be overriden by setting the DCDR_CONFIG_PATH environment variable to a location of your choosing.

Running dcdr init will create a default config file for you if one does not already exist. Once you have edited this file with your, statsd, and git repo configurations you can view this info by running the dcdr info command.

To create a new repository from scratch. Configure the config.hcl file with your RepoPath and RepoURL and then run dcdr init --create. This will create the repo add an empty JSON file and attempt to push it to the specified origin.

Example config.hcl

Username = "twoism"
Namespace = "dcdr"

Storage = "consul" // etcd, redis

Consul {
  Address = ""

//Redis {
//  Address = ":6379"

//Etcd {
//  Endpoints = [""]

Watcher {
  OutputPath = "/etc/dcdr/decider.json"

Server {
  JsonRoot = "dcdr"
  Endpoint = "/dcdr.json"

Git {
  RepoURL = ""
  RepoPath = "/etc/dcdr/audit"

Stats {
  Namespace = "dcdr"
  Host = ""
  Port = 8126