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Metro has a multi-layered cache: you can set up multiple caches to be used by Metro instead of one. This has several advantages, on this page we will explain how the caches work.

Why Cache?​

Caches give big performance benefits, they can increase the speed of a bundler with more than tenfold. However, many systems use a non-persistent cache. With Metro we have a more sophisticated way of caching with a layer system. For example, we can store the cache on a server. Because of this all bundlers connected to the same server can use the shared cache. As a result the initial build time for CI servers and local development become significantly lower.

We want to store caches in multiple places as to always have a cache to fallback to. That's why there is a multi-layered cache system.

Cache Fetching & Saving​

There is an ordering mechanism to determine which cache to use. For retrieving a cache we go through the caches from top to bottom until we find a result, for saving a cache we do the same until we find a store that has the cache.

Let's say you have two cache stores: one on a server and one on your local file system. You would specify that in this way:

const config = {
cacheStores: [
new FileStore({/*opts*/}),
new HttpStore({/*opts*/})

Metro will first look into the FileStore when we retrieve a cache. If it can't find the cache there it will check HttpStore, and so on. Finally if there's no cache there it will generate a new cache itself. As soon as the cache has been generated, Metro will go again from top to bottom to store the cache in all stores. This also happens if a cache is found. For example, if Metro finds a cache in the HttpStore it will store it in FileStore as well.