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App Engine

Core Concepts

App Engine is a fully managed PaaS for building web applications and APIs without infrastructure management. Two environments: Standard (sandboxed, instant scaling) and Flexible (containers, more control).

Key Decision: Use Standard for simple web apps with variable traffic; Flexible for custom runtimes or background processing.

Standard vs Flexible

Feature Standard Flexible
Scaling 0 to N instances (instant) Min 1 instance (gradual)
Runtime Specific language versions Any Docker container
Startup Milliseconds Minutes
Pricing Instance hours ($0.05/hr) vCPU/memory hours
Free Tier 28 instance hours/day None
SSH No Yes
Timeout 60s (10m tasks) 60m

When to Use

Standard Environment

✅ Use when:

  • Simple web apps, REST APIs
  • Unpredictable traffic (scale to zero)
  • Budget-conscious (free tier)
  • Supported runtime sufficient
  • Fast startup critical

❌ Don’t use when:

  • Need custom runtime/libraries
  • Background processing required
  • Write to local disk needed
  • Need SSH access

Flexible Environment

✅ Use when:

  • Custom runtime/dependencies
  • Native code libraries
  • Background workers
  • Existing Docker containers

❌ Don’t use when:

  • Can use Standard (cheaper)
  • Need multi-region → Cloud Run
  • Need K8s control → GKE

Architecture Patterns

Services (Microservices)

app.yaml           → default service (frontend)
api.yaml           → api service (backend)
worker.yaml        → worker service (async tasks)

Benefits: Independent scaling, separate deploys, logical separation

Traffic Splitting

Use cases: Canary (5% new version), A/B testing (50/50), blue-green

Methods: IP, cookie, random

Scaling Strategies

Automatic (most common): Scale based on load Basic (Standard only): Fixed instances, idle shutdown Manual: Fixed instance count

Key Limitations

Standard:

  • 60s request timeout
  • Read-only filesystem (except /tmp)
  • Outbound HTTP/S only
  • No background threads

Flexible:

  • Always ≥1 instance (cost)
  • Slower cold start
  • No free tier

Both:

  • Single region deployment
  • No global load balancing

Integration

Native:

  • Cloud Datastore/Firestore (automatic indexing)
  • Cloud Tasks (async work)
  • Cloud Storage (static files)
  • Cloud SQL (via proxy)

Identity-Aware Proxy: Zero-trust authentication without code

Cost Optimization

Standard: Scale to zero, free tier, instance hours Flexible: Always running, higher base cost

Strategy: Use Standard unless Flexible required

Exam Focus

  • Standard vs Flexible decision criteria
  • When App Engine vs Cloud Run vs GKE
  • Scaling configurations
  • Service architecture
  • Request timeout constraints
  • Regional limitation