Most API functions are intelligently cached on the server side. For example, topic entities such as concepts and chemical compounds are versioned by the server. Documents are also versioned, so a document cached may be robustly and reliably kept by the server.
In some use cases, it is also important to keep a client-side cache. For example, a high-traffic web page that always or frequently shows the same documents should keep those documents in a client-side cache. Concept names that are frequently displayed should also be kept in a client-side cache.