What are the First and Second Level caches in N Hibernate

Question

Can anyone explain in simple words what First and Second Level caching in Hibernate NHibernate are

User · Answer

First Level Cache

Session object holds the first level cache data. It is enabled by default. The first level cache data will not be available to entire application. An application can use many session object.

Second Level Cache

SessionFactory object holds the second level cache data. The data stored in the second level cache will be available to entire application. But we need to enable it explicitly.

User · Answer

by default  NHibernate uses first level caching which is Session Object based  but if you are running in a multi-server environment  then the first level cache may not very scalable along with some performance issues  it is happens because of the fact that it has to make very frequent trips to the database as the data is distributed over multiple servers   in other words NHibernate provides a basic  not-so-sophisticated in-process L1 cache out of box  However  it doesn   t provide features that a caching solution must have to have a notable impact on the application performance    so the questions of all these problem is the use of a L2 cache which is  associated with the session factory objects  it reduces the time consuming trips to the database so ultimately increases the app response time

User · Answer

There s a pretty good explanation of first level caching on the Streamline Logic blog   Basically  first level caching happens on a per session basis where as second level caching can be shared across multiple sessions

User · Answer

First Level Cache

Session object holds the first level cache data. It is enabled by default. The first level cache data will not be available to entire application. An application can use many session object.

Second Level Cache

SessionFactory object holds the second level cache data. The data stored in the second level cache will be available to entire application. But we need to enable it explicitly.

User · Answer

Here some basic explanation of hibernate cache     First level cache is associated with    session    object  The scope of cache objects is of session  Once session is closed  cached objects are gone forever  First level cache is enabled by default and you can not disable it  When we query an entity first time  it is retrieved from database and stored in first level cache associated with hibernate session  If we query same object again with same session object  it will be loaded from cache and no sql query will be executed  The loaded entity can be removed from session using evict   method  The next loading of this entity will again make a database call if it has been removed using evict   method  The whole session cache can be removed using clear   method  It will remove all the entities stored in cache   Second level cache is apart from first level cache which is available to be used globally in session factory scope  second level cache is created in session factory scope and is available to be used in all sessions which are created using that particular session factory  It also means that once session factory is closed  all cache associated with it die and cache manager also closed down  Whenever hibernate session try to load an entity  the very first place it look for cached copy of entity in first level cache  associated with particular hibernate session   If cached copy of entity is present in first level cache  it is returned as result of load method  If there is no cached entity in first level cache  then second level cache is looked up for cached entity  If second level cache has cached entity  it is returned as result of load method  But  before returning the entity  it is stored in first level cache also so that next invocation to load method for entity will return the entity from first level cache itself  and there will not be need to go to second level cache again  If entity is not found in first level cache and second level cache also  then database query is executed and entity is stored in both cache levels  before returning as response of load   method

User · Answer

In a second level cache  domain hbm files can be of key mutable and value false    For example     In this domain class some of the duration in a day remains constant as the universal truth  So  it can be marked as immutable across application

User · Answer

There s a pretty good explanation of first level caching on the Streamline Logic blog   Basically  first level caching happens on a per session basis where as second level caching can be shared across multiple sessions

User · Answer

1 1  First-level cache     First-level cache always Associates with the Session object  Hibernate uses this cache by default  Here  it processes one   transaction after another one  means wont process one transaction many   times  Mainly it reduces the number of SQL queries it needs to   generate within a given transaction  That is instead of updating after   every modification done in the transaction  it updates the transaction   only at the end of the transaction    1 2  Second-level cache     Second-level cache always associates with the Session Factory object  While running the transactions  in between it loads the   objects at the Session Factory level  so that those objects will be   available to the entire application  not bound to single user  Since   the objects are already loaded in the cache  whenever an object is   returned by the query  at that time no need to go for a database   transaction  In this way the second level cache works  Here we can use   query level cache also    Quoted from  http   javabeat net introduction-to-hibernate-caching

User · Answer

1 1  First-level cache     First-level cache always Associates with the Session object  Hibernate uses this cache by default  Here  it processes one   transaction after another one  means wont process one transaction many   times  Mainly it reduces the number of SQL queries it needs to   generate within a given transaction  That is instead of updating after   every modification done in the transaction  it updates the transaction   only at the end of the transaction    1 2  Second-level cache     Second-level cache always associates with the Session Factory object  While running the transactions  in between it loads the   objects at the Session Factory level  so that those objects will be   available to the entire application  not bound to single user  Since   the objects are already loaded in the cache  whenever an object is   returned by the query  at that time no need to go for a database   transaction  In this way the second level cache works  Here we can use   query level cache also    Quoted from  http   javabeat net introduction-to-hibernate-caching

User · Answer

First-level cache Hibernate tries to defer the Persistence Context flushing up until the last possible moment  This strategy has been traditionally known as transactional write-behind  The write-behind is more related to Hibernate flushing rather than any logical or physical transaction  During a transaction  the flush may occur multiple times   The flushed changes are visible only for the current database transaction  Until the current transaction is committed  no change is visible by other concurrent transactions  Due to the first-level cache  Hibernate can do several optimizations   JDBC statement batching prevent lost update anomalies  Second-level cache A proper caching solution would have to span across multiple Hibernate Sessions and that   s the reason Hibernate supports an additional second-level cache as well  The second-level cache is bound to the SessionFactory life-cycle  so it   s destroyed only when the SessionFactory is closed  typically when the application is shutting down   The second-level cache is primarily entity-based oriented  although it supports an optional query-caching solution as well  When loading an entity  Hibernate will execute the following actions    If the entity is stored in the first-level cache  then the cached object reference is returned  This ensures application-level repeatable reads  If the entity is not stored in the first-level cache and the second-level cache is activated  then Hibernate checks if the entity has been cached in the second-level cache  and if it were  it returns it to the caller  Otherwise  if the entity is not stored in the first or second-level cache  it will be loaded from the DB

User · Answer

There s a pretty good explanation of first level caching on the Streamline Logic blog   Basically  first level caching happens on a per session basis where as second level caching can be shared across multiple sessions

User · Answer

by default  NHibernate uses first level caching which is Session Object based  but if you are running in a multi-server environment  then the first level cache may not very scalable along with some performance issues  it is happens because of the fact that it has to make very frequent trips to the database as the data is distributed over multiple servers   in other words NHibernate provides a basic  not-so-sophisticated in-process L1 cache out of box  However  it doesn   t provide features that a caching solution must have to have a notable impact on the application performance    so the questions of all these problem is the use of a L2 cache which is  associated with the session factory objects  it reduces the time consuming trips to the database so ultimately increases the app response time

User · Answer

1 1  First-level cache     First-level cache always Associates with the Session object  Hibernate uses this cache by default  Here  it processes one   transaction after another one  means wont process one transaction many   times  Mainly it reduces the number of SQL queries it needs to   generate within a given transaction  That is instead of updating after   every modification done in the transaction  it updates the transaction   only at the end of the transaction    1 2  Second-level cache     Second-level cache always associates with the Session Factory object  While running the transactions  in between it loads the   objects at the Session Factory level  so that those objects will be   available to the entire application  not bound to single user  Since   the objects are already loaded in the cache  whenever an object is   returned by the query  at that time no need to go for a database   transaction  In this way the second level cache works  Here we can use   query level cache also    Quoted from  http   javabeat net introduction-to-hibernate-caching

User · Answer

Here some basic explanation of hibernate cache     First level cache is associated with    session    object  The scope of cache objects is of session  Once session is closed  cached objects are gone forever  First level cache is enabled by default and you can not disable it  When we query an entity first time  it is retrieved from database and stored in first level cache associated with hibernate session  If we query same object again with same session object  it will be loaded from cache and no sql query will be executed  The loaded entity can be removed from session using evict   method  The next loading of this entity will again make a database call if it has been removed using evict   method  The whole session cache can be removed using clear   method  It will remove all the entities stored in cache   Second level cache is apart from first level cache which is available to be used globally in session factory scope  second level cache is created in session factory scope and is available to be used in all sessions which are created using that particular session factory  It also means that once session factory is closed  all cache associated with it die and cache manager also closed down  Whenever hibernate session try to load an entity  the very first place it look for cached copy of entity in first level cache  associated with particular hibernate session   If cached copy of entity is present in first level cache  it is returned as result of load method  If there is no cached entity in first level cache  then second level cache is looked up for cached entity  If second level cache has cached entity  it is returned as result of load method  But  before returning the entity  it is stored in first level cache also so that next invocation to load method for entity will return the entity from first level cache itself  and there will not be need to go to second level cache again  If entity is not found in first level cache and second level cache also  then database query is executed and entity is stored in both cache levels  before returning as response of load   method

User · Answer

In a second level cache  domain hbm files can be of key mutable and value false    For example     In this domain class some of the duration in a day remains constant as the universal truth  So  it can be marked as immutable across application

User · Answer

First-level cache Hibernate tries to defer the Persistence Context flushing up until the last possible moment  This strategy has been traditionally known as transactional write-behind  The write-behind is more related to Hibernate flushing rather than any logical or physical transaction  During a transaction  the flush may occur multiple times   The flushed changes are visible only for the current database transaction  Until the current transaction is committed  no change is visible by other concurrent transactions  Due to the first-level cache  Hibernate can do several optimizations   JDBC statement batching prevent lost update anomalies  Second-level cache A proper caching solution would have to span across multiple Hibernate Sessions and that   s the reason Hibernate supports an additional second-level cache as well  The second-level cache is bound to the SessionFactory life-cycle  so it   s destroyed only when the SessionFactory is closed  typically when the application is shutting down   The second-level cache is primarily entity-based oriented  although it supports an optional query-caching solution as well  When loading an entity  Hibernate will execute the following actions    If the entity is stored in the first-level cache  then the cached object reference is returned  This ensures application-level repeatable reads  If the entity is not stored in the first-level cache and the second-level cache is activated  then Hibernate checks if the entity has been cached in the second-level cache  and if it were  it returns it to the caller  Otherwise  if the entity is not stored in the first or second-level cache  it will be loaded from the DB

User · Answer

There s a pretty good explanation of first level caching on the Streamline Logic blog   Basically  first level caching happens on a per session basis where as second level caching can be shared across multiple sessions

User · Answer

1 1  First-level cache     First-level cache always Associates with the Session object  Hibernate uses this cache by default  Here  it processes one   transaction after another one  means wont process one transaction many   times  Mainly it reduces the number of SQL queries it needs to   generate within a given transaction  That is instead of updating after   every modification done in the transaction  it updates the transaction   only at the end of the transaction    1 2  Second-level cache     Second-level cache always associates with the Session Factory object  While running the transactions  in between it loads the   objects at the Session Factory level  so that those objects will be   available to the entire application  not bound to single user  Since   the objects are already loaded in the cache  whenever an object is   returned by the query  at that time no need to go for a database   transaction  In this way the second level cache works  Here we can use   query level cache also    Quoted from  http   javabeat net introduction-to-hibernate-caching

[nhibernate] What are the First and Second Level caches in (N)Hibernate?

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