caching.md

id	title
caching	Caching Examples

Please thoroughly read the Important Defaults before reading this guide

Basic Example

This caching example illustrates the story and lifecycle of:

• Query Instances with and without cache data
• Background Refetching
• Inactive Queries
• Garbage Collection

Let's assume we are using the default cacheTime of 5 minutes and the default staleTime of 0.

• A new instance of useQuery(['todos'], fetchTodos) mounts.
  • Since no other queries have been made with this query + variable combination, this query will show a hard loading state and make a network request to fetch the data.
  • It will then cache the data using ['todos'] as the unique identifiers for that cache.
  • The hook will mark itself as stale after the configured staleTime (defaults to 0, or immediately).
• A second instance of useQuery(['todos'], fetchTodos) mounts elsewhere.
  • Because this exact data exists in the cache from the first instance of this query, that data is immediately returned from the cache.
• A background refetch is triggered for both queries (but only one request), since a new instance appeared on screen.
  • Both instances are updated with the new data if the fetch is successful
• Both instances of the useQuery(['todos'], fetchTodos) query are unmounted and no longer in use.
  • Since there are no more active instances of this query, a cache timeout is set using cacheTime to delete and garbage collect the query (defaults to 5 minutes).
• Before the cache timeout has completed another instance of useQuery(['todos'], fetchTodos) mounts. The query immediately returns the available cached value while the fetchTodos function is being run in the background to populate the query with a fresh value.
• The final instance of useQuery(['todos'], fetchTodos) unmounts.
• No more instances of useQuery(['todos'], fetchTodos) appear within 5 minutes.
  • This query and its data are deleted and garbage collected.