tips-for-scaling-web-apps.rst

Tips for Scaling Web Apps

• Magic numbers

  • 100ms ideal 50th percentile latency, 1 second 98th percentile solid
  • A medium sized physical box (8 cores, 24GB of RAM) should be able to do thousands of requests per second

• Monitoring and metrics

• Deployment strategies

  • Roll web nodes a few at a time, drain connections

  • Continuous deployment

    • Feature flags
    • Dark reads

  • Your application code should be able to handle DB in any relevant state:

    • Add new column to DB nullable
    • Deploy code to write values to it
    • Backfill values in an offline script
    • Verify integrity of all data
    • Deploy code to read from it
    • Remove other branches from code

• Static assets

  • CDN
  • Immutable asset names

• Sessions

  • Use authenticated (+ encrypted) cookies if at all possible
  • Fallback use a simple K/V store, best-effort persistence is fine (e.g. memcached)

• Load balancer

  • Put your web nodes behind one from the start
  • Min-conn is a good distribution strategy
  • Health check every few seconds

• Databases

  • For most applications a relational database is fine
  • Read replicas
  • Put analytics workloads on a different replica (WAL-E)

• Caching

  • Use memcached for 99% of workloads
  • Use it judiciously, don't depend on it