New Loaders API: Enhancing Stale Caching and Tracing

Leverage Stale Caching and Tracing on all loaders at
05/12/2023Tiago Gimenes


Stale caching has long been the secret behind the speed and reliability of websites. With ecommerce APIs often experiencing latency between 1 to 7 seconds, crafting a rendering engine that responds in a sub-second timescale requires a meticulously designed caching layer. Today, we're excited to announce a pivotal shift in how stale caching operates on, introducing a simpler API for developers and significantly improving observability.

In the past, utilizing's stale caching layer meant developers had to employ a custom fetch implementation provided by the framework. This custom fetch modified the fetched URL, routing the request through deco's CDN and incorporating stale caching. For example, when fetching, the framework would actually retrieve"". However, with the growing number of apps on decohub, some services already provided libraries for interacting with their APIs, rendering our custom fetch implementation impractical. Moreover, integrating tracing was challenging due to the framework's lack of control over HTTP connections. Consequently, we're transitioning away from the custom fetch implementation, relocating the caching layer to our loaders. This new API offers seamless support for both stale caching and tracing.

Opting into the New Features: A Simplified Transition Process

To start using the new loaders API, set the environment variable:


Enable Stale Caching

By default, stale caching remains inactive in loaders. Activating this feature requires a simple modification in your loader's code:

// ... loader code

export const cache = 'stale-while-revalidate'

You have the option to set the cache variable to either stale-while-revalidate or no-store, with the default being no-store.

Now, your loader is stale cached! To confirm, open a page where your loader is used, add a ?<em></em>d query string to the URL, and observe tracing information in the terminal, including cache states such as HIT, MISS, STALE, and BYPASS.

[200] 1312ms /camisa-masc-classic-linen-ml-verde-forest-46661-206/p
[====================] 1312ms 200 /camisa-masc-classic-linen-ml-verde-forest-46661-206/p
[ ] 0ms load-page
[ ] 23ms router
[ ===================] 1234ms load-data
[ ===================] 1233ms Product Page@sections
[ ] 0ms PDP [email protected]
[ ===================] 1231ms STALE PDP [email protected]
[ ] 47ms render-to-string

HIT: Cache considered fresh (standard max-age: 60 seconds). STALE: Cache not fresh, but stale content served (background validation triggered). MISS: Loader is cacheable, but data not in the cache. BYPASS: Loader not cacheable, possibly due to specific configurations.

Note that the cache key varies with the props content passed to the loader. To vary the loader's response with the Request object, read the next section.

Varying the Cache

Some loaders may need to vary based on properties available only in the Request object, such as query strings, cookies, etc. To achieve this, open your loader's file and export the cacheKey function:

export const cacheKey = (req: Request, ctx: AppContext): string => '';

This function allows the developer to precisely describe how the cache should vary.


To illustrate, let's consider a practical scenario. Suppose you're building a loader for fetching a paginated response, with the pagination information stored in a ?page= parameter. Here's how you can correctly implement this loader:

interface Props {
// ... loader props

const loader = (props: Props, req: Request, ctx: AppContext) {
// Implementation details for fetching paginated response

export const cache = 'stale-while-revalidate'

// Vary the cache with the page parameter
export const cacheKey = (req: Request, ctx: AppContext) => {
const url = new URL(req.url)
return url.searchParams.get('page')

In this example, the loader function encapsulates the logic for fetching a paginated response, while the cache setting ensures stale-while-revalidate caching. The cacheKey function intelligently varies the cache based on the page parameter, allowing for a dynamic and efficient caching strategy tailored to your specific use case.


These changes mark the beginning of forthcoming improvements in both performance and observability for websites built on We'll commence testing this new feature and gradually roll it out for our customers by default.

For more info, check out the original PR on GitHub.