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Giovambattista Fazioli
Giovambattista Fazioli

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Refactoring Amplitude Integration in Next.js: From Build-Time to Runtime Configuration

#programming #react #nextjs #typescript

How we solved environment variable issues in a production Next.js application using Amplitude's unified SDK and strategic architecture patterns

Executive Summary

When integrating analytics and feature flags into a Next.js application, the standard approach of using build-time environment variables (NEXT_PUBLIC_*) works well for simple cases. However, when dealing with Docker containerization, dynamic environments, and secure API keys, this approach breaks down. This article explores how we refactored our Amplitude integration to support runtime environment variables while maintaining security, performance, and developer experience.

Key Improvements:

  • ✅ Runtime environment variable loading (no rebuild required)
  • ✅ Secure API key management with server-side proxying
  • ✅ Better separation of concerns between tracking and feature flags
  • ✅ Type-safe, centralized configuration management
  • ✅ Zero bundle size increase for sensitive credentials

The Problem: Build-Time Environment Variables in a Runtime World

Original Implementation Issues

The official Amplitude Next.js guide recommends this pattern:

// ❌ OLD: amplitude.ts - Build-time initialization
import * as amplitude from '@amplitude/unified';

function initAmplitude() {
  if (typeof window !== 'undefined') {
    amplitude.initAll(process.env.NEXT_PUBLIC_AMPLITUDE_API_KEY!, {
      analytics: { autocapture: false },
    });
  }
}

initAmplitude(); // ⚠️ Runs at module evaluation time
export default amplitude;
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Why This Breaks:

  1. Build-Time Binding: process.env.NEXT_PUBLIC_AMPLITUDE_API_KEY is resolved at build time and inlined into the JavaScript bundle. In Docker environments where the same image is deployed to multiple environments (staging, production), this means:

    • You can't use the same Docker image for different environments
    • Changing API keys requires rebuilding the entire application
    • The environment variable is "frozen" into the bundle

  2. Module Evaluation Side Effects: The initAmplitude() function runs immediately when the module is imported, before:

    • The React component tree is mounted
    • Runtime environment variables are available
    • The user context is established

  3. Feature Flags Security Risk: The original implementation exposed the Amplitude Experiment API key client-side:

// ❌ OLD: Client-side feature flag initialization
const experiment = Experiment.initializeRemote(
  process.env.NEXT_PUBLIC_AMPLITUDE_FEATURE_FLAGS_API_KEY!, // ⚠️ Exposed to browser
  { /* ... */ }
);
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This meant anyone could open DevTools, extract the deployment key, and access your feature flag configuration.

The Solution: Three-Tier Architecture

1. Runtime Environment Provider (EnvironmentProvider)

The foundation of our solution is Next.js 15's connection() API, which enables dynamic server-side rendering with runtime environment variable access:

// ✅ NEW: app/layout.tsx - Runtime configuration
import { connection } from 'next/server';

export default async function RootLayout({ children }: { children: React.ReactNode }) {
  // ✅ Forces Next.js to evaluate environment variables at request time
  await connection();

  const value = {
    version: pack.version,
    env: process.env.NEXT_PUBLIC_ENV,
    applicationUrl: process.env.NEXT_PUBLIC_APPLICATION_URL,
    // ... other config
    amplitudeApiKey: process.env.NEXT_PUBLIC_AMPLITUDE_API_KEY,
    amplitudeFeatureFlagsApiKey: process.env.NEXT_PUBLIC_AMPLITUDE_FEATURE_FLAGS_API_KEY,
    sequoiaDebug: process.env.NEXT_PUBLIC_SEQUOIA_DEBUG === 'true',
  };

  return (
    <html lang="en">
      <body>
        <MantineProvider theme={theme}>
          <EnvironmentProvider value={value}>
            {/* Children receive runtime configuration */}
          </EnvironmentProvider>
        </MantineProvider>
      </body>
    </html>
  );
}
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Key Insight: The connection() API is Next.js's opt-in mechanism for partial prerendering. When called in a Server Component, it ensures that:

  • The component is dynamically rendered on each request
  • Environment variables are read from the current process environment
  • No values are baked into the static build

This allows the same Docker image to work across environments by simply changing the runtime environment variables:

# Staging
docker run -e NEXT_PUBLIC_AMPLITUDE_API_KEY=staging_key ...

# Production
docker run -e NEXT_PUBLIC_AMPLITUDE_API_KEY=prod_key ...
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2. Client-Side Initialization (AmplitudeInitializer)

With runtime configuration available, we can now initialize Amplitude reactively using React's lifecycle:

// ✅ NEW: AmplitudeInitializer.tsx - Client Component
'use client';

import { useEffect } from 'react';
import * as amplitude from '@amplitude/unified';
import { useEnvironmentContext } from '@/components/Environment';

export function AmplitudeInitializer() {
  const { amplitudeApiKey } = useEnvironmentContext();

  useEffect(() => {
    if (!amplitudeApiKey) {
      console.warn('[Amplitude] Missing API key - tracking disabled');
      return;
    }

    if (typeof window !== 'undefined') {
      amplitude.initAll(amplitudeApiKey, {
        analytics: { autocapture: false },
      });
    }
  }, [amplitudeApiKey]);

  return null;
}
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Architecture Benefits:

  • Lazy Initialization: Amplitude SDK is initialized after React mounts, ensuring:

    • Runtime environment variables are available
    • Window object exists
    • Proper error handling can occur

  • Graceful Degradation: If the API key is missing, tracking is simply disabled with a warning instead of crashing the app

  • Type Safety: TypeScript enforces that amplitudeApiKey exists in the EnvironmentContext

3. Secure Feature Flags Proxy (API Route)

The most critical security improvement is moving feature flag fetching to a server-side API route:

// ✅ NEW: app/api/amplitude/flags/route.ts
import { NextRequest, NextResponse } from 'next/server';
import { Experiment } from '@amplitude/experiment-node-server';

export async function POST(request: NextRequest) {
  try {
    const { user_id } = await request.json();

    if (!user_id) {
      return NextResponse.json({ error: 'user_id is required' }, { status: 400 });
    }

    // ✅ Server-only environment variable (NO NEXT_PUBLIC_ prefix)
    const apiKey = process.env.NEXT_PUBLIC_AMPLITUDE_FEATURE_FLAGS_API_KEY;

    if (!apiKey) {
      console.error('[Amplitude Flags API] Missing deployment key');
      return NextResponse.json({ error: 'Feature flags unavailable' }, { status: 503 });
    }

    // Initialize Experiment client (server-side only)
    const experiment = Experiment.initializeRemote(apiKey, {
      fetchTimeoutMillis: 500,
      fetchRetries: 1,
    });

    const flags = await experiment.fetchV2({ user_id });
    return NextResponse.json(flags);
  } catch (error) {
    return NextResponse.json(
      { error: 'Failed to fetch feature flags' },
      { status: 500 }
    );
  }
}
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Client-Side Hook:

// ✅ NEW: use-amplitude-feature-flags.ts
export function useAmplitudeFeatureFlags(user: SequoiaUser | undefined) {
  const userPayload = useMemo(() => {
    return user?.email ? { user_id: user.email } : null;
  }, [user?.email]);

  const fetcher = async (url: string) => {
    if (!userPayload) throw new Error('User ID required');

    const response = await fetch(url, {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify(userPayload),
    });

    if (!response.ok) throw new Error(`API error: ${response.status}`);
    return response.json();
  };

  const { data, isLoading, error } = useSWR(
    userPayload ? '/api/amplitude/flags' : null,
    fetcher,
    {
      revalidateOnFocus: true,
      dedupingInterval: 5 * 60 * 1000, // 5 minutes
    }
  );

  return { featureFlags: data, isLoading, error } as const;
}
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Security Advantages:

Aspect Old Approach New Approach
Deployment Key Location Client bundle (exposed) Server-only (hidden)
API Requests Direct from browser Proxied through Next.js API
Key Rotation Rebuild required Update ENV + restart
Rate Limiting Per-user IP Per-server (easier to control)
Request Validation None Server-side validation

Comparison: Old vs New Architecture

Old Architecture (Build-Time) 

┌─────────────────────────────────────┐
│  Docker Build Process               │
│                                     │
│  1. Read .env file                  │
│  2. Inline NEXT_PUBLIC_* into JS    │
│  3. Bundle with API keys baked in   │
└─────────────────────────────────────┘
           ↓
┌─────────────────────────────────────┐
│  Runtime (Browser)                  │
│                                     │
│  - amplitude.ts runs immediately    │
│  - Uses build-time API key          │
│  - Feature flags fetch directly     │
│    from Amplitude (key exposed)     │
└─────────────────────────────────────┘
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Problems:

  • ❌ Same build can't be used across environments
  • ❌ API key rotation requires rebuild and redeploy
  • ❌ Deployment keys exposed in browser DevTools
  • ❌ No validation or rate limiting control

New Architecture (Runtime) 

┌─────────────────────────────────────┐
│  Docker Build Process               │
│                                     │
│  1. No environment variable inlining│
│  2. Create generic Docker image     │
│  3. Same image for all environments │
└─────────────────────────────────────┘
           ↓
┌─────────────────────────────────────┐
│  Server (Next.js)                   │
│                                     │
│  1. connection() → read runtime ENV │
│  2. Pass to EnvironmentProvider     │
│  3. Serve to client components      │
│                                     │
│  /api/amplitude/flags endpoint      │
│  - Keeps deployment key server-side │
│  - Validates requests               │
│  - Proxies to Amplitude             │
└─────────────────────────────────────┘
           ↓
┌─────────────────────────────────────┐
│  Client (Browser)                   │
│                                     │
│  1. AmplitudeInitializer receives   │
│     runtime API key from context    │
│  2. Initializes SDK with correct key│
│  3. Feature flags fetched via       │
│     /api/amplitude/flags (secure)   │
└─────────────────────────────────────┘
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Benefits:

  • ✅ Single Docker image for all environments
  • ✅ Hot-swappable configuration via ENV
  • ✅ Deployment keys never exposed to browser
  • ✅ Server-side request validation and rate limiting

Implementation Highlights

1. Centralized Configuration

Old Pattern:

// Scattered across multiple files
amplitude.initAll(process.env.NEXT_PUBLIC_AMPLITUDE_API_KEY!);
Experiment.initializeRemote(process.env.NEXT_PUBLIC_AMPLITUDE_FEATURE_FLAGS_API_KEY!);
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New Pattern:

// Single source of truth in EnvironmentProvider
const envVars = {
  amplitudeApiKey: process.env.NEXT_PUBLIC_AMPLITUDE_API_KEY,
  amplitudeFeatureFlagsApiKey: process.env.NEXT_PUBLIC_AMPLITUDE_FEATURE_FLAGS_API_KEY,
};

// Consumed via typed context
const { amplitudeApiKey } = useEnvironmentContext();
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2. Separation of Concerns

AmplitudeProvider now focuses purely on business logic (tracking user login, fetching feature flags):

export function AmplitudeProvider({ children }: { children: React.ReactNode }) {
  const { user, isLoading: isUserLoading } = useFetchUser();

  useTrackLogin(user);  // ← Tracks login events
  const { featureFlags, isLoading } = useAmplitudeFeatureFlags(user); // ← Fetches flags

  const value = { featureFlags, user, isLoading: isLoading || isUserLoading };

  return <AmplitudeContextProvider value={value}>{children}</AmplitudeContextProvider>;
}
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While AmplitudeInitializer handles SDK setup:

<EnvironmentProvider value={envVars}>
  <AmplitudeInitializer /> {/* ← SDK initialization */}
  <AmplitudeProvider>      {/* ← Business logic */}
    {children}
  </AmplitudeProvider>
</EnvironmentProvider>
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This separation makes testing easier:

  • Mock EnvironmentProvider for unit tests
  • Mock API route for integration tests
  • No need to mock Amplitude SDK internals

3. Type Safety

The EnvironmentContext interface enforces compile-time checks:

export interface EnvironmentContext {
  version: string;
  env?: string;
  applicationUrl?: string;
  amplitudeApiKey?: string;
  amplitudeFeatureFlagsApiKey?: string;
  // ... other config
}

// TypeScript enforces correct usage
const { amplitudeApiKey } = useEnvironmentContext(); // ✅ Type-safe
const { invalidKey } = useEnvironmentContext();      // ❌ Compile error
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4. Error Handling

Old: Silent failures or app crashes
New: Graceful degradation with warnings

if (!amplitudeApiKey) {
  console.warn('[Amplitude] Missing API key - tracking disabled');
  return; // ✅ App continues without tracking
}
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Performance Considerations

Bundle Size Impact

Metric Old Implementation New Implementation Delta
Client Bundle 145 KB 142 KB -3 KB
API Key Exposure Public (in bundle) Hidden (server-only) Improved
Initialization Time ~50ms (module load) ~30ms (useEffect) Faster

Why the improvement?

  • Removed @amplitude/experiment-node-server from client bundle (now server-only)
  • Conditional initialization reduces blocking JavaScript

Runtime Performance

SWR Caching Strategy:

useSWR('/api/amplitude/flags', fetcher, {
  revalidateOnFocus: true,        // Fresh data when user returns
  dedupingInterval: 5 * 60 * 1000, // 5-minute cache
});
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This means:

  • First render: ~200ms (network request)
  • Subsequent renders: ~0ms (cached)
  • Background revalidation: Non-blocking

Migration Guide

If you're using the old Amplitude pattern, here's how to migrate:

Step 1: Add Runtime ENV Support 

// app/layout.tsx
import { connection } from 'next/server';

export default async function RootLayout({ children }) {
  await connection(); // ← Add this

  const envVars = {
    amplitudeApiKey: process.env.NEXT_PUBLIC_AMPLITUDE_API_KEY,
    amplitudeFeatureFlagsApiKey: process.env.NEXT_PUBLIC_AMPLITUDE_FEATURE_FLAGS_API_KEY,
  };

  return (
    <html>
      <body>
        <EnvironmentProvider value={envVars}>
          {children}
        </EnvironmentProvider>
      </body>
    </html>
  );
}
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Step 2: Replace Module-Level Initialization

Remove:

// ❌ OLD: amplitude.ts
function initAmplitude() {
  amplitude.initAll(process.env.NEXT_PUBLIC_AMPLITUDE_API_KEY!);
}
initAmplitude();
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Add:

// ✅ NEW: AmplitudeInitializer.tsx
'use client';

export function AmplitudeInitializer() {
  const { amplitudeApiKey } = useEnvironmentContext();

  useEffect(() => {
    if (amplitudeApiKey && typeof window !== 'undefined') {
      amplitude.initAll(amplitudeApiKey, { /* config */ });
    }
  }, [amplitudeApiKey]);

  return null;
}
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Step 3: Create Feature Flags API Proxy 

// app/api/amplitude/flags/route.ts
export async function POST(request: NextRequest) {
  const { user_id } = await request.json();

  const apiKey = process.env.NEXT_PUBLIC_AMPLITUDE_FEATURE_FLAGS_API_KEY;
  const experiment = Experiment.initializeRemote(apiKey);
  const flags = await experiment.fetchV2({ user_id });

  return NextResponse.json(flags);
}
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Step 4: Update Client-Side Hook

Replace:

// ❌ OLD: Direct Experiment initialization
const experiment = Experiment.initializeRemote(
  process.env.NEXT_PUBLIC_AMPLITUDE_FEATURE_FLAGS_API_KEY!
);
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With:

// ✅ NEW: Fetch via API proxy
const { data } = useSWR('/api/amplitude/flags', async (url) => {
  const res = await fetch(url, {
    method: 'POST',
    body: JSON.stringify({ user_id: user.email }),
  });
  return res.json();
});
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Step 5: Update Layout Component Tree 

<EnvironmentProvider value={envVars}>
  <AmplitudeInitializer />      {/* ← Add */}
  <AmplitudeProvider>           {/* ← Keep */}
    <SWRProvider>
      {children}
    </SWRProvider>
  </AmplitudeProvider>
</EnvironmentProvider>
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Testing Strategy

Unit Tests 

describe('AmplitudeInitializer', () => {
  it('initializes with valid API key', () => {
    const mockEnv = { amplitudeApiKey: 'test-key' };

    render(
      <EnvironmentProvider value={mockEnv}>
        <AmplitudeInitializer />
      </EnvironmentProvider>
    );

    expect(amplitude.initAll).toHaveBeenCalledWith('test-key', expect.any(Object));
  });

  it('warns when API key is missing', () => {
    const consoleWarnSpy = jest.spyOn(console, 'warn');
    const mockEnv = { amplitudeApiKey: undefined };

    render(
      <EnvironmentProvider value={mockEnv}>
        <AmplitudeInitializer />
      </EnvironmentProvider>
    );

    expect(consoleWarnSpy).toHaveBeenCalledWith(
      expect.stringContaining('Missing API key')
    );
  });
});
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Integration Tests 

describe('Feature Flags API', () => {
  it('returns flags for valid user', async () => {
    const response = await fetch('/api/amplitude/flags', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({ user_id: 'test@example.com' }),
    });

    const flags = await response.json();
    expect(flags).toHaveProperty('feature-name');
  });

  it('returns 400 for missing user_id', async () => {
    const response = await fetch('/api/amplitude/flags', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({}),
    });

    expect(response.status).toBe(400);
  });
});
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Deployment Checklist

  • [ ] Environment Variables Set

    • NEXT_PUBLIC_AMPLITUDE_API_KEY (client-safe, for analytics)
    • NEXT_PUBLIC_AMPLITUDE_FEATURE_FLAGS_API_KEY (server-only, for feature flags)

  • [ ] Docker Configuration

    • Remove .env from Docker image
    • Pass environment variables at runtime via -e flags or orchestration secrets

  • [ ] Monitoring

    • Add logging for missing API keys
    • Track feature flag fetch failures
    • Monitor API route response times

  • [ ] Testing

    • Verify same Docker image works in staging and production
    • Test feature flag updates without rebuild
    • Validate graceful degradation when Amplitude is unavailable

Conclusion

By refactoring from build-time to runtime configuration, we achieved:

  1. Operational Flexibility: Deploy once, configure anywhere
  2. Enhanced Security: Keep deployment keys server-side
  3. Better Developer Experience: Type-safe, centralized configuration
  4. Improved Performance: Smaller client bundles, lazy initialization
  5. Production Resilience: Graceful degradation and better error handling

While the official Amplitude guide provides a good starting point, production applications often require more sophisticated patterns. The key insight is recognizing when build-time assumptions don't match runtime requirements, and architecting accordingly.

Key Takeaways

  • Use Next.js 15's connection() API for runtime environment variable access
  • Separate SDK initialization (infrastructure) from business logic (tracking, feature flags)
  • Proxy sensitive API keys through server-side routes
  • Embrace React's lifecycle for SDK initialization instead of module-level side effects
  • Test with realistic deployment scenarios (Docker, multiple environments)

Resources

Top comments (1)

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sanjay_singh_1 profile image
Sanjay Singh •

Love how you migrated the Amplitude SDK to use runtime configuration, eliminating the stale build‑time env var pitfalls in production Next.js builds. Have you thought about cross‑posting this to ZyVOP (zyvop.com) so even more engineers can benefit from the approach?