posthog-core-workflow-b

PostHog Core Workflow B — Feature Flags & Experiments

Overview

Feature flag management, A/B experiment evaluation, and cohort analysis with PostHog. Covers boolean and multivariate flags, local evaluation for performance, experiment setup and statistical significance, and cohort creation via the API.

Prerequisites

• Completed posthog-install-auth setup
• Familiarity with posthog-core-workflow-a (event capture)
• Personal API key (phx_...) for flag management API

Instructions

Step 1: Evaluate Feature Flags (Browser)

import posthog from 'posthog-js';

// Boolean flag
if (posthog.isFeatureEnabled('new-checkout-flow')) {
  renderNewCheckout();
} else {
  renderLegacyCheckout();
}

// Multivariate flag (returns string variant name)
const variant = posthog.getFeatureFlag('pricing-page-experiment');
switch (variant) {
  case 'control':
    renderOriginalPricing();
    break;
  case 'annual-first':
    renderAnnualFirstPricing();
    break;
  case 'social-proof':
    renderSocialProofPricing();
    break;
  default:
    renderOriginalPricing(); // Fallback if flag not loaded yet
}

// Get flag payload (JSON data attached to a flag variant)
const payload = posthog.getFeatureFlagPayload('banner-config');
// payload: { text: "Spring sale!", color: "#ff6b35", discount: 20 }

// React: Wait for flags to load before rendering
posthog.onFeatureFlags(() => {
  // Flags are now loaded and ready
  const enabled = posthog.isFeatureEnabled('new-feature');
  setFeatureEnabled(enabled ?? false);
});

Step 2: Evaluate Feature Flags (Server — posthog-node)

import { PostHog } from 'posthog-node';

const posthog = new PostHog(process.env.NEXT_PUBLIC_POSTHOG_KEY!, {
  host: 'https://us.i.posthog.com',
  // Personal API key enables local evaluation (no network call per flag check)
  personalApiKey: process.env.POSTHOG_PERSONAL_API_KEY,
});

// Single flag evaluation
async function checkFlag(userId: string): Promise<boolean> {
  const enabled = await posthog.isFeatureEnabled('new-api-version', userId);
  return enabled ?? false;
}

// Multivariate flag
async function getVariant(userId: string): Promise<string> {
  const variant = await posthog.getFeatureFlag('onboarding-experiment', userId, {
    personProperties: { plan: 'pro', country: 'US' },
  });
  return (variant as string) || 'control';
}

// Get ALL flags for a user at once (one network call)
async function getUserFlags(userId: string) {
  const flags = await posthog.getAllFlags(userId, {
    personProperties: { plan: 'enterprise' },
    groupProperties: { company: { name: 'Acme' } },
  });
  // flags: { 'new-checkout': true, 'pricing-experiment': 'variant-a', ... }
  return flags;
}

// Get all flags with their payloads
async function getFlagsAndPayloads(userId: string) {
  const result = await posthog.getAllFlagsAndPayloads(userId);
  // result.featureFlags: { 'banner': true }
  // result.featureFlagPayloads: { 'banner': { text: 'Sale!' } }
  return result;
}

Step 3: Create Feature Flags via API

set -euo pipefail
# Create a boolean feature flag with percentage rollout
curl -X POST "https://app.posthog.com/api/projects/$POSTHOG_PROJECT_ID/feature_flags/" \
  -H "Authorization: Bearer $POSTHOG_PERSONAL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "key": "new-dashboard-v2",
    "name": "New Dashboard V2",
    "active": true,
    "filters": {
      "groups": [{
        "rollout_percentage": 25,
        "properties": []
      }]
    }
  }'

# Create a multivariate flag for A/B testing
curl -X POST "https://app.posthog.com/api/projects/$POSTHOG_PROJECT_ID/feature_flags/" \
  -H "Authorization: Bearer $POSTHOG_PERSONAL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "key": "checkout-experiment",
    "name": "Checkout Flow Experiment",
    "active": true,
    "filters": {
      "multivariate": {
        "variants": [
          {"key": "control", "rollout_percentage": 50},
          {"key": "streamlined", "rollout_percentage": 50}
        ]
      },
      "groups": [{"rollout_percentage": 100, "properties": []}]
    }
  }'

Step 4: Set Up an Experiment

// Track experiment exposure and goal metrics
async function runExperiment(userId: string) {
  const posthog = new PostHog(process.env.NEXT_PUBLIC_POSTHOG_KEY!);

  // PostHog automatically tracks $feature_flag_called when you evaluate
  const variant = await posthog.getFeatureFlag('checkout-experiment', userId);

  // Track the goal metric
  posthog.capture({
    distinctId: userId,
    event: 'purchase_completed',
    properties: {
      variant,
      order_value: 49.99,
      // PostHog will attribute this to the experiment automatically
    },
  });

  await posthog.flush();
  return variant;
}

Step 5: Query Experiment Results via API

set -euo pipefail
# List experiments and their status
curl "https://app.posthog.com/api/projects/$POSTHOG_PROJECT_ID/experiments/" \
  -H "Authorization: Bearer $POSTHOG_PERSONAL_API_KEY" | \
  jq '.results[] | {id, name, start_date, end_date, feature_flag_key}'

# Get experiment results
curl "https://app.posthog.com/api/projects/$POSTHOG_PROJECT_ID/experiments/EXPERIMENT_ID/results/" \
  -H "Authorization: Bearer $POSTHOG_PERSONAL_API_KEY" | \
  jq '{
    variants: [.result.variants[] | {key, count, conversion_rate: .absolute_exposure}],
    significance: .result.significance_code,
    probability: .result.probability
  }'

Step 6: Manage Cohorts via API

set -euo pipefail
# Create a behavioral cohort (users who signed up in last 30 days)
curl -X POST "https://app.posthog.com/api/projects/$POSTHOG_PROJECT_ID/cohorts/" \
  -H "Authorization: Bearer $POSTHOG_PERSONAL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "name": "Recent Signups (30d)",
    "is_calculating": true,
    "filters": {
      "properties": {
        "type": "AND",
        "values": [{
          "type": "AND",
          "values": [{
            "key": "user_signed_up",
            "type": "behavioral",
            "value": "performed_event",
            "time_value": 30,
            "time_interval": "day"
          }]
        }]
      }
    }
  }'

# List cohorts
curl "https://app.posthog.com/api/projects/$POSTHOG_PROJECT_ID/cohorts/" \
  -H "Authorization: Bearer $POSTHOG_PERSONAL_API_KEY" | \
  jq '.results[] | {id, name, count, is_calculating}'

Error Handling

Error	Cause	Solution
Flag always returns undefined	Flags not loaded yet	Use posthog.onFeatureFlags() callback
Flag returns default on server	No personalApiKey set	Add personal API key for local evaluation
Experiment not tracking	Goal event name mismatch	Verify event name matches experiment config
Cohort stuck is_calculating	Large dataset	Wait for calculation; check PostHog status
getAllFlags slow	No local evaluation	Set personalApiKey in PostHog constructor

Output

• Feature flag evaluation (boolean and multivariate)
• Server-side local evaluation for low-latency flag checks
• A/B experiment setup with goal metric tracking
• Cohort creation and management via API
• Experiment results with statistical significance

Resources

• Feature Flags Overview
• Adding Feature Flag Code
• Local Evaluation
• Experiments
• Cohorts API

Next Steps

For common errors, see posthog-common-errors.