Caching

React Query caching is automatic out of the box. It uses a stale-while-revalidate in-memory caching strategy (popularized by HTTP RFC 5861) and a very robust query deduping strategy to always ensure a query's data is always readily available, only cached when it's needed, even if that query is used multiple times across your application and updated in the background when possible.

At a glance:

• The cache is keyed on a deterministic hash of your query key.
• By default, query results become stale immediately after a successful fetch. This can be configured using the staleTime option at both the global and query-level.
• Stale queries are automatically refetched whenever their query keys change (this includes variables used in query key tuples), when they are freshly mounted from not having any instances on the page, or when they are refetched via the query cache manually.
• Though a query result may be stale, query results are by default always cached when in use.
• If and when a query is no longer being used, it becomes inactive and by default is cached in the background for 5 minutes. This time can be configured using the cacheTime option at both the global and query-level.
• After a query is inactive for the cacheTime specified (defaults to 5 minutes), the query is deleted and garbage collected.

A Detailed Caching Example

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' and fetchTodos as the unique identifiers for that cache.
  • A stale state is scheduled using the staleTime option as a delay (defaults to 0, or immediately).
• A second instance of useQuery('todos', fetchTodos) mounts elsewhere.
  • Because this exact data exist in the cache from the first instance of this query, that data is immediately returned from the cache.
• Both instances of the useQuery('todos', fetchTodos) query are unmounted and no longer in use.
  • Since there are no more active instances to 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.