---
title: The Deploy Pipeline
description: What happens when you click "Deploy" — how Polymerium turns your mod list into a runnable game.
---

## What Deploy Does

Deploy turns your instance metadata — the Minecraft version, loader, and mod list — into a runnable game directory. Think of it as building your game from a recipe: the metadata is the recipe, and the deployed `build/` folder is the meal.

## What Gets Downloaded

On your first deploy, Polymerium downloads everything needed to run your game:

- **Minecraft itself** — the version JAR and assets from Mojang's servers
- **Loader libraries** — Fabric, Forge, NeoForge, or Quilt runtime files
- **Mods** — resolved from CurseForge and Modrinth, including any dependencies

All of these are stored in a **shared cache** on your disk. Once downloaded, they're never downloaded again — any instance can reuse them.

## Symlinks, Not Copies

Instead of copying mod files into each instance, Polymerium creates symbolic links from the shared cache into your `build/` directory. If you have five instances all using Sodium, there's only one copy of Sodium on your disk — every instance just links to it.

Files from your `import/`, `live/`, and `persist/` directories are also layered into `build/` in priority order. See [Four-Layer Directories →](/docs/concepts/four-layers) for how that works.

## Fast Deploys

The first deploy for a new instance downloads everything, so it takes a moment — just like any launcher. After that, things get fast:

- **Nothing changed?** Deploy is skipped entirely. Your last build is still valid.
- **Only a few mods changed?** Only those mods are updated. Everything else stays in place.

This is why launching an instance you've already deployed feels instant.

## Caching Benefits

The shared cache means your other instances benefit too. If Instance A has already downloaded a mod, and you add the same mod to Instance B, there's nothing to download — deploy just creates a new symlink.

## Learn More

- [Metadata-Driven Instances →](/docs/concepts/metadata-driven) — why instances are descriptions, not folder copies
- [Four-Layer Directories →](/docs/concepts/four-layers) — how files are layered into the game directory
