Preloaded queries

Preloaded queries allow your Hydrogen app to discover and preload nested queries within a page.

In the following diagram, none of the queries (shopInfo, CollectionDetails, layoutContent, or Localization) have been preloaded:

The shopInfo and CollectionDetails queries must finish loading before layoutContent can start. Subsequently, layoutContent must finish loading before localization can start. This behavior is called a request waterfall, and it can severely reduce your app performance.

Because none of these queries have dependencies, it would be ideal if they all started to load in parallel up front. This process is called preloading queries, and Hydrogen does it by default:

Preloaded queries work by storing the query fetcher for the next time the same URL is requested. The first time a route is loaded, no queries are preloaded. The second time a route is loaded, Hydrogen remembers which queries were necessary to render the same route.

By default, all cached queries are preloaded. However, not all queries should be preloaded. For example, any query that returns personalized results shouldn't be preloaded. Just like caching personalized queries, preloading personalized queries can disperse data across requests. You can disable preloaded queries by setting cache to CacheNone or explicitly opting out with preload: false:

In development mode, Hydrogen detects request waterfalls, and warns you that the query isn't preloaded:

Preloaded queries work similarly for routes with parameters, like Product and Collection, except that each route with a different parameter has its own preloaded queries.

For example, the first time a user visits /collections/freestyle-collection, no queries will be preloaded. Similarly, the first time a user visits /collections/backcountry-collection, no queries will be preloaded. However, the second time that a user visits either route, queries will be preloaded. It's important that each route has separate preloaded queries, otherwise the results of one might show up for the results of the other.

You can tell Hydrogen to preload a query everywhere. The preload property takes a Boolean value or a string. When the value is *, Hydrogen will preload the query for every request. This option can be helpful for menu links in your navigation, allowing you to preload a query on button click or while you animate a transition to another page:

If you have a suspense waterfall detected, then you can log request timing information by enabling the logger.showQueryTiming property in your Hydrogen configuration file.

The showQueryTiming property logs the timeline of when queries are being requested, resolved, and rendered. If a query is preloaded, but isn't being used, then a warning displays in the server log:

If a query is being double loaded, then a warning displays in the server log. This error typically happens when the preloaded query isn't the same query as the one requested:

• ShopifyProvider
• fetchSync
• useShopQuery
• useQuery

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