in my project i have enabled hibernate query caching (using ehcache) and 2nd level entity caching for some of the entities . also in the documentation it is mentioned that query caching does not store the complete entity object and only stores the entity identity. So we should enable 2nd level entity caching as well in order to achieve better performance.
I was wondering if query caching region and entity caching region should be same ? or is it ok for them to be different and still entity will be returned from 2nd level cache if its a result of query cached in query cache?
Another question I have is, is it ok to configure different 2nd level cache regions for different entities based on business categorization ?
The query cache uses its own region, just like entities should use individual regions.
This allows you to configure each region based on the cache access patterns.
It does not matter whether the query cache is in one region while the entities being cached are in different regions. If will work just fine.
I have heard that after session.close() , the objects are in this session will be removed from cache.
If 'yes'
then why 'session2' object retrieving object from cache?
I am closing 'session1' when I'm fetching data first time(query executing) and data will be stored in cache(default first level cache), RIGHT !
But, after creating 'session2' object, I'm still able to retrieve particular object(no query is executing), means it is taking object from cache !
....Why ?
In image, I have paste my code
Image : my java files, table, persistent class, output
There are cache leves in Hibernate that you need to understand.
First-level cache:
The first-level cache is the Session cache and is a mandatory cache
through which all requests must pass. The Session object keeps an
object under its own power before committing it to the database.
Second-level cache:
Second level cache is an optional cache and first-level cache will
always be consulted before any attempt is made to locate an object in
the second-level cache. The second-level cache can be configured on a
per-class and per-collection basis and mainly responsible for caching
objects across sessions.
Query-level cache:
Hibernate also implements a cache for query resultsets that integrates
closely with the second-level cache.
All is quoted from here Cache Hibernate Tutorial. So in short the First Level of cache of that Session is removed. But that doesn't mean that the other Session Cache or the Level 2 cache need to be removed, because has his own Cache. Hope it helped you.
We need to cache the results of a query; the query returns the whole table of a database. The problem is that the database is changed externally by other application; the Java application making the query is notified with the exact primary key of the row changed.
Is it possible to replace only the changed element from the query cache, not the whole list?
Is this the 1st level cache (from EntityManager) 2nd level Cache (from EntityManagerFactory) or a different cache?
If it is possible can this be done from JPA?
entityManager.refersh(entity);
or is this query cache the 2nd level JPA cache:
entityManagerFactory.getCache().evict(cls, primaryKey);
or only possible through Hibernate/EclipseLink API?
If is not possible, in order to achieve this, would calling entityManager.find() on all elements do it?
I haven't find anything useful neither in Hibernate documentation nor in EclipseLink documentation. Hibernate supports regions and refreshing only regions, but we need entity-level refresh granularity.
Later edit to clarify my findings.
Following the link posted by #Chris in the comment I have found out that what I wanted is actually supported by EclipseLink but only for Oracle Database (there is possible to implement own handler for other vendors; but the call from database is not standardized and differs from vendor to vendor). I have not found if Hibernate supports this or not.
Anyhow the query cache from EclipseLink had some very poor performance compared with Spring Cache (based on CocurrentMap) or with custom based cache so will remain with Spring Cache or Custom Cache over Spring Jdbc.
EntityManager.refresh() is what you want - it refreshes the entity from what is in the database. This should also update the entity in the shared cache if you are not in a transaction, otherwise you may need to use the entityManagerFactory.getCache().evict(cls, primaryKey); as well to clear the second level shared cache so it can be read into it as well later on.
I just want some general info about standard purpose of using L1 cache and L2 cache.
I'm curious because I'm investigating the system with terracotta as 2nd level cache and I've found that it also has 1st-level cache.
L1 Cache is the cache that exists per Hibernate session, and this cache is not shared among threads. This cache makes use of Hibernate's own caching.
L2 Cache is a cache that survives beyond a Hibernate session, and can be shared among threads. For this cache you can use either a caching implementation that comes with Hibernate like EHCache or something else like JBossCache2
In JPA/Hibernate (and other similar ORM tools), the L1 cache is the transactional cache i.e. the entities stored from when you open a transaction to when you close it. This is almost never a shared cache (other threads can't make use of it). In JPA, this would usually be held by the EntityManager.
The L2 cache is a full (typically) shared cache. If you have multiple threads/queries pulling in data, then they can make use of entities that have already been retrieved by other threads that are still live in the cache. In JPA, this would usually be held by the EntityManagerFactory.
GaryF is not wrong, but is not technically right :-) Anton is more correct on this, but to complement his answer:
First Level Cache: this is a "cache" which stores all the entities known by a specific session. So, if you have 3 transactions inside this session, it'll hold all entities touched by all three transactions. It gets cleared when you close the session or when you perform the "clear" method.
Second Level Cache: this is a "real" cache and is delegated to an external provider, such as Infinispan. In this cache, you have full control over the contents of the cache, meaning that you are able to specify which entries should be evicted, which ones should be retained longer and so on.
If Hibernate is anything similar to NHibernate (which it is, except the other way round), the Session is the first-level cache. Except that it is not cache in a general sense, but rather an identity map.
L1 By default enabled, you have to add some third party library like EH cache, Redis for L2.
You can't disable L1 in hibernate.
L1:The first-level cache is the cache per Hibernate Session cache and is a mandatory cache through which all requests must pass and this cache is not shared among threads.
L2:The second-level cache can be configured on a per-class and per-collection basis and mainly responsible for caching objects across sessions.L2 Cache is a cache that survives beyond a Hibernate session and can be shared among threads.
As mentioned in this article these are important differences :
First level Cache vs. Second-level Cache in Hibernate Now that we have
got some basic understanding of the first level and second level
cache, here are some differences between them:
The primary difference is that the first level cache is maintained
at the Session level while the second level cache is maintained at the
SessionFactory level.
The data stored in the first level cache is accessible to the only
Session that maintains it, while the second level cache is accessible
to all.
The first level cache is by default enabled while the second level
cache is by default disabled.
A couple of things to know about Hibernate's first level cache:
You can use the Session.evict() to remove the loaded entity from
the first level cache, can use the refresh() method to refresh the
cache, and can use the clear() method to remove all entities in cache.
You cannot disable the first level cache, it is always enabled.
Hibernate entities or database rows remain in cache only until
Session is open, once Session is closed, all associated cached data is
lost.
Read more: https://www.java67.com/2017/10/difference-between-first-level-and-second-level-cache-in-Hibernate.html#ixzz7B8pzzLzL
Does Hibernate use cache (second level or otherwise) if all I am doing is batch inserts?
No entities are being requested from the database, and no generators are used.
Also, would StatelessSession vs Session change the answer? What if I was using a Session with a JDBC batch size of 50? The cache I will be using is Ehcache
Doe Hibernate use cache (second level or otherwise) if all I am doing is batch inserts?
Newly inserted Entities instances are cached in the L1 cache (the session-level cache) before they are flushed to the database (see the section 13. Batch processing), hence the need to flush and clear your session regularly to prevent OOM.
Also, would StatelessSession vs Session change the answer?
Yes. As written in section 13.3. The StatelessSession interface : A StatelessSession has no persistence context associated with it and does not provide many of the higher-level life cycle semantics. In particular, a stateless session does not implement a first-level cache nor interact with any second-level or query cache.
What if I was using a Session with a JDBC batch size of 50?
This just means that you should flush/clear the session every 50 insert.