# Backend Performance — Derived Rules

> Derives from `domains/performance/first-principles.md` P1 (Measure First), P3 (Complexity Awareness), P4 (Resource Bounds).

## Measure First (P1)

- p50, p95, p99 latencies. The average hides the long tail.
- Throughput (req/s) under load. Saturation point (where latency rises).
- Resource utilization: CPU, memory, I/O, network. Each is a budget.

## N+1 Queries (P3 Complexity Awareness)

- A query in a loop is an N+1. It is O(N) queries instead of O(1).
- Detect with a query counter in tests. A test that issues 100 queries is failing.
- Fix with a JOIN, a batch load, or a dataloader. Never "we'll fix it later."

## Caching (P5 Caching with Intent)

- Cache what is: expensive to compute, stable, read often.
- Invalidation is designed: TTL, event-based, or version-based. Never "we'll just clear it."
- A cache without an invalidation strategy is a cache that serves stale data forever.
- Multi-level: HTTP cache → CDN → app cache → DB. Each layer has its own rules.

## Async and Concurrency (P7 Async When Independent, see `domains/concurrency/`)

- I/O-bound work is async. Don't block a thread on a network call.
- CPU-bound work is in a worker, not the request path.
- Bounded queues everywhere (P9 Bounded Queues). Unbounded = OOM.

## Database (P4 Resource Bounds, see `domains/data/indexing.md`)

- Connection pool: bounded. The DB has a connection limit; the pool respects it.
- Slow queries: logged, explained, fixed. A 10-second query is a bug.
- Pagination on large tables: cursor, not offset. Offset scans rows.

## Resource Bounds (P4)

- Memory: bounded. A request that allocates unbounded memory is a DoS vector.
- Timeouts: every external call has one. A call without a timeout is a call that can hang forever (P8 Timeout Discipline).
- File handles, DB connections, HTTP connections: all bounded, all pooled.

## What Violates Backend Performance

| Violation | Principle |
|-----------|-----------|
| N+1 query in a loop | P3 |
| No timeout on an HTTP call | P4, P8 (concurrency) |
| Unbounded in-memory sort | P4 |
| Cache with no invalidation | P5 |
| `SELECT *` | P4 (data P10) |
| Connection pool size = 1000 | P4 (DB limit) |