Jon Chery
51 lines
2.2 KiB
Markdown
51 lines
2.2 KiB
Markdown
# Backend Performance — Derived Rules
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> Derives from `domains/performance/first-principles.md` P1 (Measure First), P3 (Complexity Awareness), P4 (Resource Bounds).
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## Measure First (P1)
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- p50, p95, p99 latencies. The average hides the long tail.
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- Throughput (req/s) under load. Saturation point (where latency rises).
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- Resource utilization: CPU, memory, I/O, network. Each is a budget.
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## N+1 Queries (P3 Complexity Awareness)
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- A query in a loop is an N+1. It is O(N) queries instead of O(1).
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- Detect with a query counter in tests. A test that issues 100 queries is failing.
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- Fix with a JOIN, a batch load, or a dataloader. Never "we'll fix it later."
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## Caching (P5 Caching with Intent)
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- Cache what is: expensive to compute, stable, read often.
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- Invalidation is designed: TTL, event-based, or version-based. Never "we'll just clear it."
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- A cache without an invalidation strategy is a cache that serves stale data forever.
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- Multi-level: HTTP cache → CDN → app cache → DB. Each layer has its own rules.
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## Async and Concurrency (P7 Async When Independent, see `domains/concurrency/`)
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- I/O-bound work is async. Don't block a thread on a network call.
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- CPU-bound work is in a worker, not the request path.
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- Bounded queues everywhere (P9 Bounded Queues). Unbounded = OOM.
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## Database (P4 Resource Bounds, see `domains/data/indexing.md`)
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- Connection pool: bounded. The DB has a connection limit; the pool respects it.
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- Slow queries: logged, explained, fixed. A 10-second query is a bug.
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- Pagination on large tables: cursor, not offset. Offset scans rows.
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## Resource Bounds (P4)
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- Memory: bounded. A request that allocates unbounded memory is a DoS vector.
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- Timeouts: every external call has one. A call without a timeout is a call that can hang forever (P8 Timeout Discipline).
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- File handles, DB connections, HTTP connections: all bounded, all pooled.
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## What Violates Backend Performance
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| Violation | Principle |
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|-----------|-----------|
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| N+1 query in a loop | P3 |
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| No timeout on an HTTP call | P4, P8 (concurrency) |
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| Unbounded in-memory sort | P4 |
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| Cache with no invalidation | P5 |
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| `SELECT *` | P4 (data P10) |
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| Connection pool size = 1000 | P4 (DB limit) | |