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Prewarm

A beginner-friendly guide to understanding runtime prewarming.

On this page 2026-07-09
The Simple Idea Why Prewarm Exists Cold Path vs Warm Path What Can Be Prewarmed Prewarm vs Cache Prewarm In Multi-Tenant Systems Trade-Offs My Learning Notes Common Misunderstandings Related Concepts

A beginner-friendly guide to prewarming: preparing runtime work before the first real request needs it.

Prewarm is about avoiding the cold start.

prepare early -> serve faster later

Table of Contents

The Simple Idea

Prewarm means doing setup work before it is urgently needed.

Example:

server starts
compile tenant code
prepare caches
first request arrives
runtime is already warm

The goal is to reduce first-request cost.

Why Prewarm Exists

The first request often pays for setup:

  • loading files,
  • parsing config,
  • compiling code,
  • preparing templates,
  • opening connections,
  • building caches,
  • discovering tenants.

If that work happens during the first user request, the user waits.

Prewarm moves some of that work earlier.

Cold Path vs Warm Path

Cold path:

request arrives -> load -> parse -> compile -> run -> respond

Warm path:

request arrives -> reuse prepared work -> respond

The warm path is usually simpler and faster.

What Can Be Prewarmed

Systems may prewarm:

  • compiled bytecode,
  • templates,
  • route tables,
  • tenant metadata,
  • static caches,
  • database pools,
  • generated assets,
  • JIT outputs.

Not everything should be prewarmed.

Prewarm what is likely to be needed and expensive enough to matter.

Prewarm vs Cache

Cache stores reusable work.

Prewarm fills or prepares that work early.

cache: where prepared work lives
prewarm: when prepared work is created

They often work together.

Prewarm In Multi-Tenant Systems

Prewarm becomes interesting when many tenants exist.

The host may need to decide:

  • which tenants to warm,
  • how much memory to spend,
  • when to evict,
  • how to handle hot reload,
  • whether to warm all tenants or only active ones.

Prewarm is not just performance.

It is resource planning.

Trade-Offs

Prewarm can improve latency.

But it can also cost:

  • startup time,
  • memory,
  • CPU,
  • stale caches,
  • more complex invalidation.

Good prewarm design asks:

what should be ready before traffic arrives?

My Learning Notes

Kitwork made prewarm concrete through tenant bytecode.

If tenant code can be compiled before the first request, the runtime can avoid doing that work at the worst moment.

The lesson:

prewarm is not just speed
prewarm is choosing when the runtime pays its cost

Common Misunderstandings

"Prewarm means cache everything."

No. Prewarm should be selective.

"Prewarm is always worth it."

No. It trades startup and memory for request latency.

"Prewarm replaces caching."

No. Prewarm often fills caches. It does not replace them.

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Related: runtime, bytecode, jit

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