Can anyone explain in simple words what First and Second Level caching in Hibernate/NHibernate are?
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/
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.
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.
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.
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.
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.
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.
Related
I have an Apache Geode setup, connected with external Postgres datasource. I have a scenario where I define an expiration time for a key. Let's say after T time the key is going to expire. Is there a way so that the keys which are going to expire can make a call to an external datasource and update the value incase the value has been changed? I want a kind of automatic syncing for my keys which are there in Apache Geode. Is there any interface which i can implement and get the desired behavior?
I am not sure I fully understand your question. Are you saying that the values in the cache may possibly be more recent than what is currently stored in the database?
Whether you are using Look-Aside Caching, Inline Caching, or even Near Caching, Apache Geode combined with Spring would take care of ensuring the cache and database are kept in sync, to some extent depending on the caching pattern.
With Look-Aside Caching, if used properly, the database (i.e. primary System of Record (SOR), e.g. Postgres in your case) should always be the most current. (Look-Aside) Caching is secondary.
With Synchronous Inline Caching (using a CacheLoader/CacheWriter combination for Read/Write-Through) and in particular, with emphasis on CacheWriter, during updates (e.g. Region.put(key, value) cache operations), the DB is written to first, before the entry is stored (or overwritten) in the cache. If the DB write fails, then the cache entry is not written or updated. This is true each time a value for a key is updated. If the key has not be updated recently, then the database should reflect the most recent value. Once again, the database should always be the most current.
With Asynchronous Inline Caching (using AEQ + Listener, for Write-Behind), the updates for a cache entry are queued and asynchronously written to the DB. If an entry is updated, then Geode can guarantee that the value is eventually written to the underlying DB regardless of whether the key expires at some time later or not. You can persist and replay the queue in case of system failures, conflate events, and so on. In this case, the cache and DB are eventually consistent and it is assumed that you are aware of this, and this is acceptable for your application use case.
Of course, all of these caching patterns and scenarios I described above assume nothing else is modifying the SOR/database. If another external application or process is also modifying the database, separate from your Geode-based application, then it would be possible for Geode to become out-of-sync with the database and you would need to take steps to identify this situation. This is rather an issue for reads, not writes. Of course, you further need to make sure that stale cache entries does not subsequently overwrite the database on an update. This is easy enough to handle with optimistic locking. You could even trigger a cache entry remove on an DB update failure to have the cache refreshed on read.
Anyway, all of this is to say, if you applied 1 of the caching patterns above correctly, the value in the cache should already be reflected in the DB (or will be in the Async, Write-Behind Caching UC), even if the entry eventually expires.
Make sense?
I understand that NHibernate's 2nd level cache works at the SessionFactory level. I'm using a connection provider in my NHibernate configuration to work with my multi-tenant environment. I'm wondering how the 2nd level cache interacts with the connection provider. That is, does it intelligently maintain a cache for each separate connection string, or do all connections share the same cache?
Obviously sharing the same cache is a huge no-no since the same query will return different results based on the database it is hitting.
The life time of the second level cache is tied to the session factory and not to an individual session. Once an entity is loaded by its unique id and the second level cache is active the entity is available for all other sessions (of the same session factory).
So 2nd level works only thru sessions of the same session factory.
Please look First and Second Level caching in NHibernate for more details
We have a brownfield multi-user application (99% Delphi, 1% .net) which uses NHibernate for the persistency of the .net modules. In my application I can add categories to some entity. If I select one and decide to not use it (thus removing the category again) I has been loaded by NHibernate and will stay in the session's first level cache. Now, if some other user deletes this category and I try to save my entity my application throws an exception because the object loaded doesn't exist anymore.
my question: is there a way to check if my cache has items loaded which don't exist anymore? and if so, is there a way to remove non-exist entities from my cache?
So what happens:
I load an entity (added to session cache)
I add a category (added to session cache)
Someone else deletes the category from the database.
I save my entity and the exception occurs because the category doesn't exist anymore.
It's still in the session cache. It would be nice if I could (automatically) remove it from my session's cache? is there a way to clean up the cache and remove objects that don't exist anymore?
Regards, Ted
There's no option in NHibernate to do it automatically, at least not with ISession. You could use IStatelessSession for loading, since it doesn't have first-level cache, but you'll lose many other features that ISession provides.
You could also call ISession.Clear() to clear the session (first-level) cache, or ISession.Evict() to evict certain entities from session, but that's not automatic.
How long do you keep your session object? Maybe you need a different session management context.
If the lifespan of your session is shorter, you can still achieve entity caching, but with second-level cache. SysCache2 is one of second-level cache providers that has a support for SqlCacheDependency. This means that you could set cache expiration when some objects in database change.
What's the proper way to manage persisted entities in the 1st level cache during thread/session lifetime? Actually, there is only one or two persisted instances per thread (session) that should be present at all times, for referencing them from other (transient) entities before saving.
Since I need to clear the session cache every once in a while (because otherwise it gets filled and painfully slow), what I am not sure is - should I:
Evict all entities from the session, except the ones I need?
Clear the entire session, and reload necessary entities?
Create a new session and reload necessary entities?
I think you should close previous Session and open new one. If objects that you want to cache are heavy for loading per each session you sould use Second level caching.
I've got a web application that is 99% read-only, with a separate service that updates the database at specific intervals (like every 10 minutes). How can this service tell the application to invalidate it's second-level cache? Is it actually important? (I don't actually care if I have too much stale data) If I don't invalidate the cache how much time is needed to the records to get updated (if using SysCache)
You can manually dispose your 2nd level cache for a specific entity, entity type or a collection.
From http://knol.google.com/k/fabio-maulo/nhibernate-chapter-16-improving/1nr4enxv3dpeq/19#
For the second-level cache, there are methods defined on ISessionFactory for evicting the cached state of an instance, entire class, collection instance or entire collection role.
sessionFactory.Evict(typeof(Cat), catId); //evict a particular Cat
sessionFactory.Evict(typeof(Cat)); //evict all Cats
sessionFactory.EvictCollection("Eg.Cat.Kittens", catId); //evict a particular collection of kittens
sessionFactory.EvictCollection("Eg.Cat.Kittens"); //evict all kitten collections
If you are OK with the possibility of having some stale data, just set the default expiration to something you are comfortable with, and you'll be set.
Example:
<property name="cache.default_expiration">120</property>
This sets the default expiration to two minutes, so you'll never see stale data older than that.