Layer 2 cache and request cache vs collection cache?

Question

As I understand it, the second level cache will be used when objects are loaded using their primary key. This includes a selection of associations. I can only think of the methods session.get (), session.load, where the second level cache will be displayed.

If an association is a collection or some other entity, how can it be cached? For example: -

@Cacheable public class Department{ private List Employees; private DepatmentDetail detail ; } 

How can I link employees and drill down on caching? I think I need to mention @cache above employee associations and details. But that didn't work?

when the developer does department.getEmployees (), hibernate will internally run the ie request

  select * from employees where deptId =1; 

Now, if I use a cache request, in which I explicitly make the request above and get the results, the request will be run again on db. why the request is run again. I think this is due to the way hibernate internally saves the result of the second level cache and request cache (they can be stored in separate regions). If someone can shed light on this aspect, it will be great.

Answer 1

See the links below for details.

• Hibernate caching

• Hibernate caching community edition

• Hibernate: Truly Understanding L2 Caches and Queries

Request Level Cache:

Hibernate also implements a cache for requests that integrate tightly with the second-level cache.

This is an optional feature and requires two additional areas of the physical cache that store the results of the cached query and timestamps when the table was last updated. This is only useful for queries that often run with the same parameters.

Second level cache

Hibernate is compatible with multiple second-tier cache providers. Any implementation can be used for a second level cache.

Difference:

The sole purpose of the query cache is to cache requests, while the second cache can also be used to cache other caches.

The request cache is provided internally by Hibernate, while for the second level cache you must select some external second level cache, such as Infinispan, EHCache, etc.

Answer 2

The second level cache has a hash table, such as a structure inside, to store data. The key here will be the identifier of the object, and the value will be the dehydrated value of the object. To get data from this L2 cache, you must have the key identifier ie. Thus, you can use it with methods in which you select an object by id.

This scenario changes if the query_cache option is included in the L2 cache. The query cache stores the query and matches the identifiers of the corresponding result objects. Now, even if you do not select id (using JPQL or HQL OR queryDsl), hibernate checks if the same query is launched earlier, and if so, get the list of identifiers from the query cache. After that, objects from the L2 cache corresponding to the same identifiers are returned.

Answer 3

We need to explicitly put @Cache(usage=CacheConcurrencyStrategy.<VALUE>) in the collection and @Cacheable in the corresponding collection class. There is a very good explanation for the L2 cache here .