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 gcTime of 5 minutes and the default staleTime of 0.
A new instance of useQuery({ queryKey: ['todos'], queryFn: fetchTodos }) mounts.
Since no other queries have been made with the ['todos'] query key, this query will show a hard loading state and make a network request to fetch the data.
When the network request has completed, the returned data will be cached under the ['todos'] key.
The hook will mark the data as stale after the configured staleTime (defaults to 0, or immediately).
A second instance of useQuery({ queryKey: ['todos'], queryFn: fetchTodos }) mounts elsewhere.
Since the cache already has data for the ['todos'] key from the first query, that data is immediately returned from the cache.
The new instance triggers a new network request using its query function.
Note that regardless of whether both fetchTodos query functions are identical or not, both queries' status are updated (including isFetching, isPending, and other related values) because they have the same query key.
When the request completes successfully, the cache's data under the ['todos'] key is updated with the new data, and both instances are updated with the new data.
Both instances of the useQuery({ queryKey: ['todos'], queryFn: fetchTodos }) query are unmounted and no longer in use.
Since there are no more active instances of this query, a garbage collection timeout is set using gcTime to delete and garbage collect the query (defaults to 5 minutes).
Before the cache timeout has completed, another instance of useQuery({ queryKey: ['todos'], queryFn: fetchTodos }) mounts. The query immediately returns the available cached data while the fetchTodos function is being run in the background. When it completes successfully, it will populate the cache with fresh data.
The final instance of useQuery({ queryKey: ['todos'], queryFn: fetchTodos }) unmounts.
No more instances of useQuery({ queryKey: ['todos'], queryFn: fetchTodos }) appear within 5 minutes.
The cached data under the ['todos'] key is deleted and garbage collected.