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RxDB uses a QueryCache which optimizes the reuse of queries at runtime. This makes sense especially when RxDB is used in UI-applications where people move for- and backwards on different routes or pages and the same queries are used many times. Because of the event-reduce algorithm cached queries are even valuable for optimization, when changes to the database occur between now and the last execution.

Cache Replacement Policy

To not let RxDB fill up all the memory, a cache replacement policy is defined that clears up the cached queries. This is implemented as a function which runs regularly, depending on when queries are created and the database is idle. The default policy should be good enough for most use cases but defining custom ones can also make sense.

The default policy

The default policy starts cleaning up queries depending on how much queries are in the cache and how much document data they contain.

  • It will never uncache queries that have subscribers to their results
  • It tries to always have less than 100 queries without subscriptions in the cache.
  • It prefers to uncache queries that have never executed and are older then 30 seconds
  • It prefers to uncache queries that have not been used for longer time

Other references to queries

With JavaScript, it is not possible to count references to variables. Therefore it might happen that an uncached RxQuery is still referenced by the users code and used to get results. This should never be a problem, uncached queries must still work. Creating the same query again however, will result in having two RxQuery instances instead of one.

Using a custom policy

A cache replacement policy is a normal JavaScript function according to the type RxCacheReplacementPolicy. It gets the RxCollection as first parameter and the QueryCache as second. Then it iterates over the cached RxQuery instances and uncaches the desired ones with uncacheRxQuery(rxQuery). When you create your custom policy, you should have a look at the default.

To apply a custom policy to a RxCollection, add the function as attribute cacheReplacementPolicy.

const collection = await myDatabase.addCollections({
humans: {
schema: mySchema,
cacheReplacementPolicy: function(){ /* ... */ }