Connect to flagd

⭐ Community Toolkit

This page describes how consuming apps connect to a flagd resource that’s already modeled in your AppHost. For the AppHost API surface — adding a flagd resource, file sync, port configuration, log levels, and health checks — see flagd Hosting integration.

When you reference a flagd resource from your AppHost, Aspire injects the connection information into the consuming app as environment variables. Your app can then configure the appropriate OpenFeature flagd provider using that connection information — the pattern works the same from C#, Go, Python, or TypeScript.

Connection properties

Aspire exposes each property as an environment variable named [RESOURCE]_[PROPERTY]. For instance, the Uri property of a resource called flagd becomes FLAGD_URI.

The flagd resource exposes the following connection properties:

Property Name	Description
Host	The hostname or IP address of the flagd server
Port	The port number of the flagd HTTP/gRPC endpoint
Uri	The endpoint URI, with the format http://{Host}:{Port}

Example connection string:

Uri: http://localhost:8013

Connect from your app

Pick the language your consuming app is written in. Each example assumes your AppHost adds a flagd resource named flagd and references it from the consuming app.

C#

For C# apps, use the OpenFeature SDK with the flagd provider. Install the 📦 OpenFeature.Contrib.Providers.Flagd NuGet package in the client-consuming project:

.NET CLI

.NET CLI — Add OpenFeature.Contrib.Providers.Flagd package

dotnet add package OpenFeature.Contrib.Providers.Flagd

Program.cs (C# file-based app)

Program.cs

#:package OpenFeature.Contrib.Providers.Flagd@*

PackageReference (*.csproj)

XML — Add OpenFeature.Contrib.Providers.Flagd package reference

<PackageReference Include="OpenFeature.Contrib.Providers.Flagd" Version="*" />

In Program.cs, read the Aspire-injected connection string and register the flagd provider with the OpenFeature SDK:

C# — Program.cs

using OpenFeature;
using OpenFeature.Contrib.Providers.Flagd;

var connectionString = builder.Configuration.GetConnectionString("flagd");

await OpenFeature.Api.Instance.SetProviderAsync(
    new FlagdProvider(new Uri(connectionString!)));

Tip

The connection name passed to GetConnectionString must match the flagd resource name from the AppHost. For more information, see Add flagd resource.

Obtain a client and evaluate feature flags:

C# — ExampleService.cs

var flagClient = OpenFeature.Api.Instance.GetClient();

var welcomeBanner = await flagClient.GetBooleanValueAsync(
    "welcome-banner",
    defaultValue: false);

var backgroundColor = await flagClient.GetStringValueAsync(
    "background-color",
    defaultValue: "#000000");

Read environment variables in C#

If you prefer to read the connection URI from environment variables directly rather than through IConfiguration:

C# — Program.cs

using OpenFeature;
using OpenFeature.Contrib.Providers.Flagd;

var uri = Environment.GetEnvironmentVariable("FLAGD_URI");

await OpenFeature.Api.Instance.SetProviderAsync(
    new FlagdProvider(new Uri(uri!)));

For more information, see the OpenFeature .NET SDK documentation and the flagd .NET provider.

Go

Install the OpenFeature Go SDK and flagd provider:

Terminal

go get github.com/open-feature/go-sdk/openfeature
go get github.com/open-feature/go-sdk-contrib/providers/flagd/pkg

Read the injected environment variable, parse the URI, and configure the provider:

Go — main.go

package main

import (
    "context"
    "net/url"
    "os"
    "strconv"

    "github.com/open-feature/go-sdk/openfeature"
    flagd "github.com/open-feature/go-sdk-contrib/providers/flagd/pkg"
)

func main() {
    // Read the Aspire-injected connection URI
    uri, err := url.Parse(os.Getenv("FLAGD_URI"))
    if err != nil {
        panic(err)
    }

    port, _ := strconv.Atoi(uri.Port())

    openfeature.SetProvider(flagd.NewProvider(
        flagd.WithHost(uri.Hostname()),
        flagd.WithPort(uint16(port)),
    ))

    client := openfeature.NewClient("app")

    enabled, _ := client.BooleanValue(
        context.Background(),
        "welcome-banner",
        false,
        openfeature.NewEvaluationContext("", nil),
    )
    _ = enabled
}

For more information, see the OpenFeature Go SDK and the flagd Go provider.

Python

Install the OpenFeature SDK and flagd provider:

Terminal

pip install openfeature-sdk openfeature-provider-flagd

Read the injected environment variable, parse the URI, and configure the provider:

Python — app.py

import os
from urllib.parse import urlparse
from openfeature import api
from openfeature.contrib.providers.flagd.flagd_provider import FlagdProvider

# Read the Aspire-injected connection URI
uri = urlparse(os.getenv("FLAGD_URI"))

api.set_provider(FlagdProvider(
    host=uri.hostname,
    port=uri.port,
))

client = api.get_client()

welcome_banner = client.get_boolean_value("welcome-banner", False)
background_color = client.get_string_value("background-color", "#000000")

For more information, see the OpenFeature Python SDK and the flagd Python provider.

TypeScript

Install the OpenFeature Server SDK and flagd provider:

Terminal

npm install @openfeature/server-sdk @openfeature/flagd-provider

Read the injected environment variable, parse the URI, and configure the provider:

TypeScript — index.ts

import { OpenFeature } from '@openfeature/server-sdk';
import { FlagdProvider } from '@openfeature/flagd-provider';

// Read the Aspire-injected connection URI
const uri = new URL(process.env.FLAGD_URI!);

OpenFeature.setProvider(new FlagdProvider({
    host: uri.hostname,
    port: Number(uri.port),
}));

const client = OpenFeature.getClient();

const welcomeBanner = await client.getBooleanValue('welcome-banner', false);
const backgroundColor = await client.getStringValue('background-color', '#000000');

For more information, see the OpenFeature JavaScript SDK and the flagd Node.js provider.