2.8 Decision cheat sheet

Which latency number do I put in the SLO? p50 for "typical user experience," p99 for the SLO, p999 only if your slowest requests correlate with your most valuable customers (the Amazon case). Skip p9999 — too expensive, too noisy, diminishing returns.

Where do I measure? Client side. Server-side timers exclude queueing delay, which is where the variance lives. Instrument both and treat the gap as a signal in itself.

Read-heavy feed: query on read, or materialize on write? Materialize when read volume × query cost ≫ write volume × fan-out. Go hybrid the moment the fan-out distribution has a long tail — write-path for the many, read-path merge for the few.

How do I stop a retry storm? Jittered exponential backoff + a retry budget (token bucket capping retries as a fraction of traffic) + circuit breakers + server-side load shedding with bounded queues. Any one alone is insufficient.

Vertical or horizontal? Vertical while it's cheap and simple — it usually is, for longer than people assume. Horizontal when you need multi-DC fault tolerance, elasticity, or you've hit the price cliff. Remember horizontal costs you explicit sharding plus all of Ch 9.

How far ahead should I design for scale? One order of magnitude. Not two. Expect to rethink the architecture at each 10×.

When is more automation wrong? When load is predictable (manual scaling has fewer surprises), and when the automation makes failures harder to troubleshoot than a manual procedure would be.

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