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Feature Flags - Quick Start

Maturity: Beta

This module is actively being improved. Breaking changes are unlikely but possible.

Add feature toggles with percentage rollouts in under 5 minutes.

Complete Data Isolation

Primus Feature Flags runs entirely within your application. All flag evaluation happens locally using your configuration.

Providers​

The Feature Flags module supports configuration-based providers.

Provider 1: AppSettings (Standard)​

Step 1: Install Package​

dotnet add package PrimusSaaS.FeatureFlags

Step 2: Configure in Program.cs​

using PrimusSaaS.FeatureFlags;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddPrimusFeatureFlags(opts => 
    builder.Configuration.GetSection("PrimusFeatureFlags").Bind(opts));

var app = builder.Build();
app.MapControllers();
app.Run();

Step 3: Configure appsettings.json​

Add your flags configuration:

{
  "PrimusFeatureFlags": {
    "Flags": {
      "NewDashboard": {
        "Enabled": true,
        "RolloutPercentage": 50
      },
      "BetaFeature": {
        "Enabled": true,
        "EnabledForUsers": ["user-123", "user-456"]
      },
      "MaintenanceMode": {
        "Enabled": false
      }
    }
  }
}
How to Configure Values​
  • Enabled: Global switch (true/false)
  • RolloutPercentage: 0-100 integer for gradual rollout
  • EnabledForUsers: Array of specific User IDs allowed access

Step 4: Create Controller​

Create Controllers/FeatureFlagController.cs:

using Microsoft.AspNetCore.Mvc;
using PrimusSaaS.FeatureFlags;

namespace YourApp.Controllers;

[ApiController]
[Route("api/[controller]")]
public class FeatureFlagController : ControllerBase
{
    private readonly IFeatureFlagService _flags;

    public FeatureFlagController(IFeatureFlagService flags)
    {
        _flags = flags;
    }

    [HttpGet("status/{flagName}")]
    public IActionResult GetStatus(string flagName)
    {
        var isEnabled = _flags.IsEnabled(flagName);
        return Ok(new { feature = flagName, enabled = isEnabled });
    }
}

Step 5: Test Endpoints​

# Check if NewDashboard is enabled
curl http://localhost:5000/api/featureflag/status/NewDashboard

Expected Response:

{
  "feature": "NewDashboard",
  "enabled": true
}

Configuration Reference​

Feature Flag Options​

OptionTypeDefaultDescription
EnabledboolfalseMaster switch for the feature
RolloutPercentageint0Percentage of users to enable for (0-100)
EnabledForUsersstring[][]Specific user IDs to allow list

Examples​

Example 1: Conditional Logic​

if (_flags.IsEnabled("NewDashboard"))
{
    return Ok(GetNewDashboardData());
}
return Ok(GetLegacyDashboardData());

Node.js Quick Start​

Add feature flags to a Node.js service with in-memory config.

Step 1: Install Package​

npm install @primus-saas/feature-flags

Step 2: Create a FeatureFlagService​

import { FeatureFlagService } from '@primus-saas/feature-flags';

const featureFlags = new FeatureFlagService({
  flags: {
    NewDashboard: { enabled: true, rolloutPercentage: 50 },
    BetaFeature: { enabled: false, enabledForGroups: ['beta-testers'] },
  },
});

if (featureFlags.isEnabled('NewDashboard')) {
  console.log('New dashboard enabled');
}

Troubleshooting​

Issue: Flag always false​

Solution:

  1. Check if the flag name in appsettings.json matches exactly (case-sensitive).
  2. Ensure Enabled is set to true.