PostHog Observability

Overview

Monitor PostHog event ingestion health, query performance, and feature flag evaluation reliability. Key signals include event ingestion rate and latency (time from posthog.capture() to event appearing in queries), feature flag evaluation latency (critical for server-side flags in hot paths), event volume by type (to detect instrumentation regressions), and API rate limit consumption.

Prerequisites

• PostHog Cloud or self-hosted instance

• Personal API key with read access

• Application instrumented with PostHog SDK

Instructions

Step 1: Monitor Event Ingestion Health

set -euo pipefail

Check recent event ingestion via the API

curl "https://app.posthog.com/api/projects/PROJECT_ID/insights/trend/?events=[{\"id\":\"$pageview\"}]&date_from=-24h"
-H "Authorization: Bearer $POSTHOG_PERSONAL_API_KEY" |
jq '.result[0].data | {total: add, last_hour: .[-1], trend: (if .[-1] > .[-2] then "up" else "down" end)}'

Step 2: Track Feature Flag Evaluation Performance

// posthog-flag-monitor.ts import PostHog from 'posthog-node'; const posthog = new PostHog(process.env.POSTHOG_API_KEY!);

async function monitorFlagLatency(flagKey: string, distinctId: string) { const start = performance.now(); const value = await posthog.getFeatureFlag(flagKey, distinctId); const duration = performance.now() - start;

emitHistogram('posthog_flag_eval_ms', duration, { flag: flagKey }); emitCounter('posthog_flag_evals_total', 1, { flag: flagKey, result: String(value) });

if (duration > 200) { # HTTP 200 OK console.warn(Slow flag evaluation: ${flagKey} took ${duration.toFixed(0)}ms); } }

Step 3: Monitor Event Volume for Billing

// Track event counts to predict billing async function checkEventVolume() { const res = await fetch(https://app.posthog.com/api/projects/${PROJECT_ID}/insights/trend/?events=[{"id":"$pageview"},{"id":"$autocapture"}]&date_from=-7d, { headers: { Authorization: Bearer ${POSTHOG_KEY} }, }); const data = await res.json(); const weeklyTotal = data.result.reduce((sum: number, series: any) => sum + series.data.reduce((a: number, b: number) => a + b, 0), 0); emitGauge('posthog_weekly_events', weeklyTotal); emitGauge('posthog_projected_monthly_events', weeklyTotal * 4.3); }

Step 4: Alert on Anomalies

groups:

• name: posthog rules:
  • alert: PostHogIngestionDrop expr: rate(posthog_events_ingested[1h]) < rate(posthog_events_ingested[1h] offset 1d) * 0.5 annotations: { summary: "PostHog event ingestion dropped >50% vs yesterday" }
  • alert: PostHogFlagSlow expr: histogram_quantile(0.95, rate(posthog_flag_eval_ms_bucket[5m])) > 500 # HTTP 500 Internal Server Error annotations: { summary: "PostHog feature flag P95 evaluation exceeds 500ms" }
  • alert: PostHogEventBudgetHigh expr: posthog_projected_monthly_events > 10000000 # 10000000 = 10M limit annotations: { summary: "Projected PostHog events exceed 10M/month (check billing tier)" }
  • alert: PostHogApiErrors expr: rate(posthog_api_errors_total[5m]) > 0.1 annotations: { summary: "PostHog API errors elevated" }

Step 5: Dashboard Panels

Track: event ingestion rate over time, event volume by type ($pageview, $autocapture, custom), feature flag evaluation latency, projected monthly events vs billing tier, API response times, and session recording count. An ingestion drop often means broken SDK instrumentation -- catch it early.

Error Handling

Issue Cause Solution

Events not appearing SDK not initialized or blocked Check browser console for PostHog errors, verify API key

Flag always returns default Flag not enabled for environment Check flag filters and rollout percentage

Event volume spike Autocapture on high-frequency element Exclude element from autocapture config

API 429 rate limited Too many insight queries Cache insight results, reduce poll frequency

Examples

Basic usage: Apply posthog observability to a standard project setup with default configuration options.

Advanced scenario: Customize posthog observability for production environments with multiple constraints and team-specific requirements.

Output

• Configuration files or code changes applied to the project

• Validation report confirming correct implementation

• Summary of changes made and their rationale

Resources

• Official monitoring documentation

• Community best practices and patterns

• Related skills in this plugin pack