Featurevisor Go SDK

This is a port of Featurevisor Javascript SDK v3.x to Go, providing a way to evaluate feature flags, variations, and variables in your Go applications.

This SDK is compatible with Featurevisor v3.0 projects (and also v2 datafiles).

See example application here.

Table of contents

• Installation
• Public API
• Initialization
• Evaluation types
• Context
  • Setting initial context
  • Setting after initialization
  • Replacing existing context
  • Manually passing context
• Check if enabled
• Getting variation
• Getting variables
  • Type specific methods
• Getting all evaluations
• Sticky
  • Initialize with sticky
  • Set sticky afterwards
• Setting datafile
  • Merging by default
  • Replacing
  • Loading datafiles on demand
  • Updating datafile
  • Interval-based update
• Diagnostics
  • Levels
  • Handler
• Events
  • datafile_set
  • context_set
  • sticky_set
  • error
• Evaluation details
• Modules
  • Defining a module
  • Registering modules
• Child instance
• Close
• CLI usage
  • Test
  • Benchmark
  • Assess distribution
• Development of this package
  • Setting up
  • Running tests
  • Releasing
• License

Installation

In your Go application, install the SDK using Go modules:

go get github.com/featurevisor/featurevisor-go

Public API

The main runtime API is featurevisor.CreateFeaturevisor():

f := featurevisor.CreateFeaturevisor(featurevisor.FeaturevisorOptions{
    Datafile: datafileContent,
})

Most applications only need CreateFeaturevisor, the Featurevisor instance type, and FeaturevisorOptions. Public extension and observability types include FeaturevisorModule, FeaturevisorDiagnostic, and the datafile model types.

Initialization

The SDK can be initialized by passing datafile content directly:

package main

import (
    "io"
    "net/http"

    "github.com/featurevisor/featurevisor-go"
)

func main() {
    datafileURL := "https://cdn.yoursite.com/datafile.json"

    resp, err := http.Get(datafileURL)
    if err != nil {
        panic(err)
    }
    defer resp.Body.Close()

    datafileBytes, err := io.ReadAll(resp.Body)
    if err != nil {
        panic(err)
    }

    var datafileContent featurevisor.DatafileContent
    if err := datafileContent.FromJSON(string(datafileBytes)); err != nil {
        panic(err)
    }

    f := featurevisor.CreateFeaturevisor(featurevisor.FeaturevisorOptions{
        Datafile: datafileContent,
    })
}

Evaluation types

We can evaluate 3 types of values against a particular feature:

• Flag (bool): whether the feature is enabled or not
• Variation (string): the variation of the feature (if any)
• Variables: variable values of the feature (if any)

These evaluations are run against the provided context.

Context

Contexts are attribute values that we pass to SDK for evaluating features against.

Think of the conditions that you define in your segments, which are used in your feature's rules.

They are plain maps:

context := featurevisor.Context{
    "userId": "123",
    "country": "nl",
    // ...other attributes
}

Context can be passed to SDK instance in various different ways, depending on your needs:

Setting initial context

You can set context at the time of initialization:

import (
    "github.com/featurevisor/featurevisor-go"
)

f := featurevisor.CreateFeaturevisor(featurevisor.FeaturevisorOptions{
    Context: featurevisor.Context{
        "deviceId": "123",
        "country":  "nl",
    },
})

This is useful for values that don't change too frequently and available at the time of application startup.

Setting after initialization

You can also set more context after the SDK has been initialized:

f.SetContext(featurevisor.Context{
    "userId": "234",
})

This will merge the new context with the existing one (if already set).

Replacing existing context

If you wish to fully replace the existing context, you can pass true in second argument:

f.SetContext(featurevisor.Context{
    "deviceId": "123",
    "userId":   "234",
    "country":  "nl",
    "browser":  "chrome",
}, true) // replace existing context

Manually passing context

You can optionally pass additional context manually for each and every evaluation separately, without needing to set it to the SDK instance affecting all evaluations:

context := featurevisor.Context{
    "userId": "123",
    "country": "nl",
}

isEnabled := f.IsEnabled("my_feature", context)
variation := f.GetVariation("my_feature", context)
variableValue := f.GetVariable("my_feature", "my_variable", context)

When manually passing context, it will merge with existing context set to the SDK instance before evaluating the specific value.

Further details for each evaluation types are described below.

Check if enabled

Once the SDK is initialized, you can check if a feature is enabled or not:

featureKey := "my_feature"

isEnabled := f.IsEnabled(featureKey)

if isEnabled {
    // do something
}

You can also pass additional context per evaluation:

isEnabled := f.IsEnabled(featureKey, featurevisor.Context{
    // ...additional context
})

Getting variation

If your feature has any variations defined, you can evaluate them as follows:

featureKey := "my_feature"

variation := f.GetVariation(featureKey)

if variation != nil && *variation == "treatment" {
    // do something for treatment variation
} else {
    // handle default/control variation
}

Additional context per evaluation can also be passed:

variation := f.GetVariation(featureKey, featurevisor.Context{
    // ...additional context
})

Getting variables

Your features may also include variables, which can be evaluated as follows:

variableKey := "bgColor"

bgColorValue := f.GetVariable("my_feature", variableKey)

Additional context per evaluation can also be passed:

bgColorValue := f.GetVariable("my_feature", variableKey, featurevisor.Context{
    // ...additional context
})

Type specific methods

Next to generic GetVariable() methods, there are also type specific methods available for convenience:

f.GetVariableBoolean(featureKey, variableKey, context)
f.GetVariableString(featureKey, variableKey, context)
f.GetVariableInteger(featureKey, variableKey, context)
f.GetVariableDouble(featureKey, variableKey, context)
f.GetVariableArray(featureKey, variableKey, context)
f.GetVariableObject(featureKey, variableKey, context)
f.GetVariableJSON(featureKey, variableKey, context)

Type specific methods do not coerce values. GetVariableInteger() returns nil for the string "1", and GetVariableBoolean() returns nil for the string "true".

For typed arrays/objects, use Into methods with pointer outputs:

var items []string
_ = f.GetVariableArrayInto(featureKey, variableKey, context, &items)

var cfg MyConfig
_ = f.GetVariableObjectInto(featureKey, variableKey, context, &cfg)

context and OverrideOptions are optional and can be passed before the output pointer.

Getting all evaluations

You can get evaluations of all features available in the SDK instance:

allEvaluations := f.GetAllEvaluations(featurevisor.Context{})

fmt.Printf("%+v\n", allEvaluations)
// {
//   myFeature: {
//     enabled: true,
//     variation: "control",
//     variables: {
//       myVariableKey: "myVariableValue",
//     },
//   },
//
//   anotherFeature: {
//     enabled: true,
//     variation: "treatment",
//   }
// }

This is handy especially when you want to pass all evaluations from a backend application to the frontend.

Sticky

For the lifecycle of the SDK instance in your application, you can set some features with sticky values, meaning that they will not be evaluated against the fetched datafile:

Sticky values belong to an SDK or child instance. Evaluation options do not accept sticky overrides; create a child with SpawnOptions{Sticky: ...} when a child needs its own sticky state.

Initialize with sticky

import (
    "github.com/featurevisor/featurevisor-go"
)

f := featurevisor.CreateFeaturevisor(featurevisor.FeaturevisorOptions{
    Sticky: &featurevisor.StickyFeatures{
        "myFeatureKey": {
            Enabled: true,
            // optional
            Variation: func() *featurevisor.VariationValue {
                v := featurevisor.VariationValue("treatment")
                return &v
            }(),
            Variables: map[string]interface{}{
                "myVariableKey": "myVariableValue",
            },
        },
        "anotherFeatureKey": {
            Enabled: false,
        },
    },
})

Once initialized with sticky features, the SDK will look for values there first before evaluating the targeting conditions and going through the bucketing process.

Set sticky afterwards

You can also set sticky features after the SDK is initialized:

f.SetSticky(featurevisor.StickyFeatures{
    "myFeatureKey": {
        Enabled: true,
        Variation: func() *featurevisor.VariationValue {
            v := featurevisor.VariationValue("treatment")
            return &v
        }(),
        Variables: map[string]interface{}{
            "myVariableKey": "myVariableValue",
        },
    },
    "anotherFeatureKey": {
        Enabled: false,
    },
}, true) // replace existing sticky features (false by default)

Setting datafile

You may also initialize the SDK without passing datafile, and set it later on:

f.SetDatafile(datafileContent)

SetDatafile accepts either parsed featurevisor.DatafileContent or a raw JSON string.

Merging by default

By default, SetDatafile(datafile) merges the incoming datafile with the SDK instance's existing datafile:

• incoming Features and Segments override matching keys
• existing Features and Segments that are missing from the incoming datafile are kept
• Revision, SchemaVersion, and FeaturevisorVersion are taken from the incoming datafile

This means you can call SetDatafile more than once with different datafiles, and the SDK instance accumulates their features and segments together.

Replacing

Pass true as the second argument to replace the stored datafile entirely:

f.SetDatafile(datafileContent, true) // replace existing datafile

Loading datafiles on demand

Because merging is the default, a single SDK instance can start with a small datafile and load more datafiles later as your application needs them, instead of downloading every feature upfront.

This pairs well with targets, where each target produces a smaller datafile for a specific part of your application:

f := featurevisor.CreateFeaturevisor(featurevisor.FeaturevisorOptions{})

func loadDatafile(target string) {
    url := fmt.Sprintf("https://cdn.yoursite.com/production/featurevisor-%s.json", target)
    resp, err := http.Get(url)
    if err != nil {
        return
    }
    defer resp.Body.Close()

    datafileBytes, err := io.ReadAll(resp.Body)
    if err != nil {
        return
    }

    f.SetDatafile(string(datafileBytes))
}

loadDatafile("products")

// later, when the user reaches checkout
loadDatafile("checkout")

Updating datafile

You can set the datafile as many times as you want in your application, which will result in emitting a datafile_set event that you can listen and react to accordingly.

The triggers for setting the datafile again can be:

• periodic updates based on an interval (like every 5 minutes), or
• reacting to:
  • a specific event in your application (like a user action), or
  • an event served via websocket or server-sent events (SSE)

Interval-based update

Here's an example of using interval-based update:

import (
    "time"
    "io"
    "net/http"

    "github.com/featurevisor/featurevisor-go"
)

func updateDatafile(f *featurevisor.Featurevisor, datafileURL string) {
    ticker := time.NewTicker(5 * time.Minute)
    defer ticker.Stop()

    for range ticker.C {
        resp, err := http.Get(datafileURL)
        if err != nil {
            continue
        }
        defer resp.Body.Close()

        datafileBytes, err := io.ReadAll(resp.Body)
        if err != nil {
            continue
        }

        var datafileContent featurevisor.DatafileContent
        if err := datafileContent.FromJSON(string(datafileBytes)); err != nil {
            continue
        }

        f.SetDatafile(datafileContent)
    }
}

// Start the update goroutine
go updateDatafile(f, datafileURL)

Diagnostics

By default, Featurevisor reports diagnostics to the console for info level and above with a [Featurevisor] prefix.

Levels

Available diagnostic levels are fatal, error, warn, info, and debug.

Set the level during initialization or update it afterwards:

logLevel := featurevisor.LogLevelDebug
f := featurevisor.CreateFeaturevisor(featurevisor.FeaturevisorOptions{
    LogLevel: &logLevel,
})

f.SetLogLevel(featurevisor.LogLevelInfo)

Handler

Use OnDiagnostic to send structured diagnostics to your observability system:

f := featurevisor.CreateFeaturevisor(featurevisor.FeaturevisorOptions{
    LogLevel: &logLevel,
    OnDiagnostic: func(diagnostic featurevisor.FeaturevisorDiagnostic) {
        fmt.Println(diagnostic.Level, diagnostic.Code, diagnostic.Message)
    },
})

Modules can also subscribe to diagnostics or report their own from Setup via the provided module API.

Every diagnostic has Level, Code, Message, and an object-shaped Details map. Optional Module, ModuleName, and OriginalError fields describe provenance. Evaluation metadata belongs in Details.

Diagnostic handlers are isolated from SDK behavior. A panic in a handler does not stop other handlers or evaluations.

Events

Featurevisor SDK implements a simple event emitter that allows you to listen to events that happen in the runtime.

You can listen to these events that can occur at various stages in your application:

datafile_set

unsubscribe := f.On(featurevisor.EventNameDatafileSet, func(details featurevisor.EventDetails) {
    revision := details["revision"]               // new revision
    previousRevision := details["previousRevision"]
    revisionChanged := details["revisionChanged"] // true if revision has changed

    // list of feature keys that have new updates,
    // and you should re-evaluate them
    features := details["features"]

    // handle here
})

// stop listening to the event
unsubscribe()

The features array will contain keys of features that have either been:

• added, or
• updated, or
• removed

compared to the previous datafile content that existed in the SDK instance.

context_set

unsubscribe := f.On(featurevisor.EventNameContextSet, func(details featurevisor.EventDetails) {
    replaced := details["replaced"] // true if context was replaced
    context := details["context"]   // the new context

    fmt.Println("Context set")
})

sticky_set

unsubscribe := f.On(featurevisor.EventNameStickySet, func(details featurevisor.EventDetails) {
    replaced := details["replaced"] // true if sticky features got replaced
    features := details["features"] // list of all affected feature keys

    fmt.Println("Sticky features set")
})

error

unsubscribe := f.On(featurevisor.EventNameError, func(details featurevisor.EventDetails) {
    diagnostic := details["diagnostic"]
    fmt.Println(diagnostic)
})

The error event is emitted for diagnostics whose level is error.

Evaluation details

Besides logging with debug level enabled, you can also get more details about how the feature variations and variables are evaluated in the runtime against given context:

// flag
evaluation := f.EvaluateFlag(featureKey, context)

// variation
evaluation := f.EvaluateVariation(featureKey, context)

// variable
evaluation := f.EvaluateVariable(featureKey, variableKey, context)

The returned object will always contain the following properties:

• FeatureKey: the feature key
• Reason: the reason how the value was evaluated

And optionally these properties depending on whether you are evaluating a feature variation or a variable:

• BucketValue: the bucket value between 0 and 100,000
• RuleKey: the rule key
• Error: the error object
• Enabled: if feature itself is enabled or not
• Variation: the variation object
• VariationValue: the variation value
• VariableKey: the variable key
• VariableValue: the variable value
• VariableSchema: the variable schema
• VariableOverrideIndex: index of matched variable override when applicable

Modules

Modules allow you to intercept the evaluation process and customize it further as per your needs.

Defining a module

A module is a simple struct with a recommended unique Name and optional functions:

If Setup panics, the module is not registered. Featurevisor removes subscriptions created during setup, reports module_setup_error, and calls Close when present.

import (
    "github.com/featurevisor/featurevisor-go"
)

myCustomModule := &featurevisor.FeaturevisorModule{
    // recommended for diagnostics and removal
    Name: "my-custom-module",

    // rest of the properties below are all optional per module
    Setup: func(api featurevisor.FeaturevisorModuleApi) {
        api.ReportDiagnostic(featurevisor.FeaturevisorModuleReportedDiagnostic{
            Level:   featurevisor.LogLevelInfo,
            Code:    "module_ready",
            Message: "Module is ready",
        })
    },

    // before evaluation
    Before: func(options featurevisor.EvaluateOptions) featurevisor.EvaluateOptions {
        // update context before evaluation
        if options.Context == nil {
            options.Context = featurevisor.Context{}
        }
        options.Context["someAdditionalAttribute"] = "value"
        return options
    },

    // after evaluation
    After: func(evaluation featurevisor.Evaluation, options featurevisor.EvaluateOptions) featurevisor.Evaluation {
        if evaluation.Reason == "error" {
            // log error
            return evaluation
        }
        return evaluation
    },

    // configure bucket key
    BucketKey: func(options featurevisor.ConfigureBucketKeyOptions) featurevisor.BucketKey {
        // return custom bucket key
        return options.BucketKey
    },

    // configure bucket value (between 0 and 100,000)
    BucketValue: func(options featurevisor.ConfigureBucketValueOptions) featurevisor.BucketValue {
        // return custom bucket value
        return options.BucketValue
    },

    Close: func() {
        // clean up module resources
    },
}

Registering modules

You can register modules at the time of SDK initialization:

import (
    "github.com/featurevisor/featurevisor-go"
)

f := featurevisor.CreateFeaturevisor(featurevisor.FeaturevisorOptions{
    Modules: []*featurevisor.FeaturevisorModule{
        myCustomModule,
    },
})

Or after initialization:

removeModule := f.AddModule(myCustomModule)
removeModule()

Child instance

When dealing with purely client-side applications, it is understandable that there is only one user involved, like in browser or mobile applications.

But when using Featurevisor SDK in server-side applications, where a single server instance can handle multiple user requests simultaneously, it is important to isolate the context for each request.

That's where child instances come in handy:

childF := f.Spawn(featurevisor.Context{
    // user or request specific context
    "userId": "123",
})

Now you can pass the child instance where your individual request is being handled, and you can continue to evaluate features targeting that specific user alone:

isEnabled := childF.IsEnabled("my_feature")
variation := childF.GetVariation("my_feature")
variableValue := childF.GetVariable("my_feature", "my_variable")

Similar to parent SDK, child instances also support several additional methods:

• SetContext
• SetSticky
• IsEnabled
• GetVariation
• GetVariable
• GetVariableBoolean
• GetVariableString
• GetVariableInteger
• GetVariableDouble
• GetVariableArray
• GetVariableArrayInto
• GetVariableObject
• GetVariableObjectInto
• GetVariableJSON
• GetAllEvaluations
• On
• Close

Close

Both primary and child instances support a .Close() method, that removes forgotten event listeners (via On method) and cleans up any potential memory leaks.

f.Close()

CLI usage

This package also provides a CLI tool for running your Featurevisor project's test specs and benchmarking against this Go SDK:

All three commands accept repeatable --target=<target> options. test builds only the selected Target datafiles and runs untargeted assertions plus assertions for those targets. benchmark and assess-distribution run independently against every selected Target datafile. Without --target, existing project-wide behavior is preserved. Project definitions, test specs, Target discovery, and datafile generation continue to come from the Node.js CLI.

Test

Learn more about testing here.

go run cmd/main.go test --projectDirectoryPath="/absolute/path/to/your/featurevisor/project"

Additional options that are available:

go run cmd/main.go test \
    --projectDirectoryPath="/absolute/path/to/your/featurevisor/project" \
    --quiet|verbose \
    --onlyFailures \
    --keyPattern="myFeatureKey" \
    --assertionPattern="#1"

If you want to validate parity locally against the JavaScript SDK runner, you can use the bundled example project:

cd /Users/fahad/Projects/featurevisor/featurevisor/examples/example-1
npx featurevisor test

# from this Go SDK repository root:
go run cmd/main.go test \
  --projectDirectoryPath="/Users/fahad/Projects/featurevisor/featurevisor/examples/example-1" \
  --onlyFailures

# or:
make test-example-1

Benchmark

Learn more about benchmarking here.

go run cmd/main.go benchmark \
    --projectDirectoryPath="/absolute/path/to/your/featurevisor/project" \
    --environment="production" \
    --feature="myFeatureKey" \
    --context='{"country": "nl"}' \
    --n=1000

Assess distribution

Learn more about assessing distribution here.

go run cmd/main.go assess-distribution \
    --projectDirectoryPath="/absolute/path/to/your/featurevisor/project" \
    --environment=production \
    --feature=foo \
    --variation \
    --context='{"country": "nl"}' \
    --populateUuid=userId \
    --populateUuid=deviceId \
    --n=1000

Development of this package

Setting up

Clone the repository, and install the dependencies using Go modules:

go mod download

Running tests

go test ./...

Releasing

• Manually create a new release on GitHub
• Tag it with a prefix of v, like v1.0.0

License

MIT © Fahad Heylaal