Flaggr vs Unleash: Which Open-Source Feature Flag Platform?

If you're evaluating open-source feature flag platforms, Flaggr and Unleash are both strong contenders. Both are open source, both support self-hosting, and both offer feature flag management with targeting rules and gradual rollouts. But their architectures, protocol support, and developer experiences differ significantly.

This comparison covers the meaningful differences to help you choose the right platform for your team.

Quick Comparison

Architecture Differences

Unleash: Traditional Server Architecture

Unleash uses a traditional architecture:

Client SDK → Unleash Server (Node.js) → PostgreSQL
                   ↓
            Unleash Edge (optional)

• PostgreSQL required — Unleash needs a running PostgreSQL instance for all deployments
• Unleash Edge — a Rust-based proxy for edge caching and reduced latency
• Client-side evaluation — SDKs periodically fetch the full feature toggle configuration and evaluate locally

Flaggr: Serverless-First Architecture

Flaggr is built on Next.js with a serverless-first approach:

Client SDK → Flaggr API (serverless functions) → Firestore / File Storage
                   ↓
            Redis (caching, pub/sub)

• No database server required — Firestore (managed) or file-based storage for development
• Serverless-native — deploys to Vercel, Cloudflare, or any serverless platform with zero infrastructure management
• Server-side evaluation — the API evaluates flags and returns only the result, keeping targeting rules private

What This Means in Practice

Operational overhead: Unleash requires you to manage PostgreSQL (backups, upgrades, monitoring, scaling). Flaggr on Firestore has zero database operations — it's fully managed. For small teams that don't want to run database infrastructure, this difference is significant.

Scaling model: Unleash scales vertically (bigger server) or horizontally with Edge proxies. Flaggr scales automatically via serverless functions — from zero to thousands of concurrent evaluations without capacity planning.

Data model: Unleash stores everything in PostgreSQL with a relational schema. Flaggr uses a document model (Firestore collections) that maps naturally to the project → service → flag hierarchy.

Protocol Support

This is one of the largest differences between the two platforms.

Unleash Protocols

Unleash primarily uses REST APIs. Client SDKs poll the server for toggle configuration and evaluate flags locally. The Unleash Proxy and Unleash Edge handle client-side SDK connections.

Flaggr Protocols

Flaggr supports five evaluation protocols:

The multi-protocol approach means teams can choose the protocol that best fits their architecture. A Go microservice might use Connect-RPC for performance, while a React app uses the REST API or SSE streaming.

OpenFeature Support

Flaggr: OpenFeature-Native

Flaggr was built around the OpenFeature specification:

• Server Provider — first-class provider for Node.js/TypeScript
• Web Provider — browser-optimized provider with SSE streaming
• OFREP v1 — any OFREP-compatible client can evaluate flags
• Standard context — targeting uses OpenFeature evaluation context natively

import { OpenFeature } from '@openfeature/server-sdk';
import { FlaggrProvider } from '@flaggr/openfeature-provider';
 
OpenFeature.setProvider(new FlaggrProvider({
  endpoint: 'https://your-flaggr-instance.com',
  serviceId: 'my-service',
}));
 
const client = OpenFeature.getClient();
const value = await client.getBooleanValue('dark-mode', false);

Unleash: OpenFeature via Community Providers

Unleash supports OpenFeature through community-maintained providers. The integration works but is a compatibility layer on top of Unleash's native SDK model rather than a core architectural choice.

Unleash does not implement OFREP, so you're limited to Unleash-specific SDK protocols even when using the OpenFeature wrapper.

Feature Comparison

Targeting Rules

Both platforms support targeting rules with user attributes, but the models differ:

Unleash uses "strategies" — named targeting configurations that can be stacked:

• Standard, GradualRollout, UserIds, IPs, Hostnames
• Custom strategies via the SDK
• Strategy constraints for additional conditions

Flaggr uses a rules-and-conditions model:

• Each rule has conditions with operators (equals, contains, in, starts_with, greater_than, etc.)
• Percentage rollout per rule with consistent hashing
• Scheduled rules for time-based activation
• Prerequisite flags for dependency chains

Flaggr's model is more flexible for complex targeting scenarios. Unleash's strategy model is simpler but requires custom strategies for non-standard conditions.

Experiments

Flaggr: Built-in A/B testing with variants, traffic allocation, and metrics tracking in the open-source version.

Unleash: Experimentation is an Enterprise-only feature. The open-source version supports variants for multivariate flags but doesn't include experiment management or statistical analysis.

Environments

Both platforms support multiple environments (development, staging, production) with independent flag configurations.

Unleash treats environments as a core concept with per-environment activation strategies.

Flaggr supports environments with per-environment flag values and targeting rules, plus environment-specific API tokens.

Audit Logging

Flaggr: Full audit logging included in the open-source version — every flag change, member update, and token operation is tracked.

Unleash: Event logging is available in the open-source version. Detailed audit logging and compliance features are in the Enterprise tier.

Alerting

Flaggr: Built-in alerting with configurable rules for error rates, latency thresholds, and evaluation anomalies. Supports webhook, Slack, and email notification channels.

Unleash: Alerting is available through integrations (Slack, Teams, Datadog) but doesn't include built-in evaluation metric alerting in the open-source version.

Developer Experience

Dashboard

Unleash has a clean, functional dashboard that's been refined over years. It's straightforward and well-documented.

Flaggr offers a modern dashboard with a command palette for keyboard navigation, real-time flag evaluation visualization, and an integrated API explorer for testing endpoints.

SDK Experience

Unleash SDKs use a polling model — they fetch the full toggle set periodically and evaluate locally. This means:

• No network call per evaluation (fast after initial fetch)
• The full toggle configuration is available in client memory (potential privacy concern)
• Polling interval creates a delay for flag updates (typically 10-15 seconds)

Flaggr SDKs support both polling and streaming:

• REST API for on-demand evaluation
• SSE streaming for real-time updates (sub-second propagation)
• Server-side evaluation keeps targeting rules private

Infrastructure as Code

Both platforms have Terraform providers. Flaggr's provider covers projects, services, flags, alert channels, alert rules, and metric sources:

resource "flaggr_project" "main" {
  name = "My Project"
  slug = "my-project"
}
 
resource "flaggr_service" "api" {
  project_id = flaggr_project.main.id
  name       = "API Service"
}
 
resource "flaggr_flag" "feature" {
  service_id    = flaggr_service.api.id
  key           = "new-feature"
  name          = "New Feature"
  type          = "boolean"
  environment   = "production"
  default_value = false
}

When to Choose Unleash

• You need mobile SDKs (iOS, Android, Flutter) — Unleash has a broader SDK ecosystem
• Your team is comfortable managing PostgreSQL infrastructure
• You want a platform with years of production hardening and a large community
• Simple strategy-based targeting is sufficient for your use cases
• You're evaluating the Enterprise tier for advanced features (change requests, SCIM, custom roles)

When to Choose Flaggr

• You want serverless-native deployment with zero database management
• Multi-protocol support matters (gRPC, Connect-RPC, OFREP)
• OpenFeature compliance is a core requirement, not a nice-to-have
• You need built-in experimentation and alerting without an enterprise license
• You're managing infrastructure with Terraform and want comprehensive IaC coverage
• Real-time flag updates (sub-second via SSE) are important
• Data privacy requires server-side evaluation (targeting rules don't leave your infrastructure)

Migration

Both platforms support the OpenFeature standard, making migration feasible:

1. Set up Flaggr alongside Unleash
2. Recreate flag configurations (or use Flaggr's import API)
3. Swap the OpenFeature provider in each service
4. Decommission Unleash once migration is complete

Flaggr's migration guide covers the process in detail, including strategies for gradual migration.

Want to try Flaggr? Get started with the quick start guide — it takes under 5 minutes with zero infrastructure setup.