# Client-side Apps

> Use OpenTelemetry on end-user controlled apps running on devices like mobile phones, desktop computers, and retail kiosks.

---

Client-side applications present unique challenges for observability compared to
server-side workloads. These apps run on devices you don't control, with varying
network conditions, hardware capabilities, and user behaviors.

## Why client-side observability matters

Traditional server-side monitoring gives you visibility into your backend
systems, but misses the complete picture of user experience. Client-side
observability helps you:

- **Understand real user experience**: See actual load times, frame rates, and
  responsiveness as users experience them.
- **Debug issues in context**: Correlate errors with device characteristics,
  network conditions, and user actions.
- **Track end-to-end transactions**: Connect client-side operations with backend
  traces for complete distributed tracing.
- **Monitor app health at scale**: Aggregate telemetry across your user base to
  identify patterns and trends.

## Key differences from server-side instrumentation

When instrumenting client apps, consider these factors:

- **Resource constraints**: Mobile devices and browsers have limited CPU,
  memory, and battery. Telemetry collection must be efficient and avoid
  impacting app performance.
- **Network variability**: Users may have slow, intermittent, or no
  connectivity. Implement offline buffering and batch exports to handle network
  unreliability.
- **Session management**: Track user sessions to group related telemetry and
  understand user journeys across multiple app launches.
- **Privacy and consent**: Client apps often collect data subject to privacy
  regulations. Plan for data minimization, consent management, and attribute
  redaction.
- **Data volume**: With potentially millions of users, sampling strategies
  become essential to manage costs while maintaining representative telemetry.

## Real User Monitoring (RUM)

OpenTelemetry supports Real User Monitoring (RUM) patterns that capture how real
users experience your application:

- **Page/screen load performance**: Time to first byte, first contentful paint,
  and full load completion.
- **User interactions**: Click events, navigation patterns, and form
  submissions.
- **Errors and crashes**: Unhandled exceptions, ANR events, and error rates.
- **Resource loading**: Network request timing, cache hit rates, and resource
  sizes.

## Connecting to your backend

Client telemetry becomes most valuable when connected to your backend traces.
Propagate trace context through your HTTP requests to maintain end-to-end
visibility:

```text
Client App → API Gateway → Backend Services → Database
    │              │              │              │
    └──────────────┴──────────────┴──────────────┘
                 Correlated Traces
```

Configure your client SDK to inject trace headers (`traceparent`, `tracestate`)
and ensure your backend services propagate this context through their
operations.

---

Section pages:

- [Android](/docs/platforms/client-apps/android/): Use OpenTelemetry in apps running on Android platforms
- [iOS](/docs/platforms/client-apps/ios/): Use OpenTelemetry in apps running on iOS and iPadOS
- [Web](/docs/platforms/client-apps/web/): Use OpenTelemetry in apps running on web browsers