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A DSO is a DSpace Object (org.dspace.content.DSpaceObject). Most everything in DSpace is a DSO (e.g. Site, Community, Collection, Item, Bitstream, EPerson, Group). A DSO is saved to the database. Bitstreams are a special type of DSO that have binary storage (of a file) in addition to data in the database.
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The context object can be set to a privileged mode that can bypass all authorization checks.
The context object contains a cache 
of objects.manages/maintains a list of "events" to dispatch after a commit() (or when dispatchEvents() is called). These events represent changes to objects in the system, and are responded to by Event Consumers.
The context object interacts with the DBConnection class to manage database commits, connection pooling, transactions, etc. The default DBConnection used is the HibernateDBConnection, which manages the Hibernate Session, Transaction, etc (more on that below).
Differences between DSpace 5 and DSpace 6 Context
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In DSpace 6, Hibernate manages the DB connection pool. Each thread grabs is associated with a unique Hibernate Session (which corresponds to a DB connection ) from in the pool). This means two Context objects could may use the same DB connection (if they are in the same thread). In other words, code can no longer assume each new Context() is treated as a new/separate database transaction.
- This is controlled by the configuration "
hibernate.current_session_context_class", which in DSpace is configured to use a separate Session per Thread: https://github.com/DSpace/DSpace/blob/dspace-6_x/dspace/config/hibernate.cfg.xml#L16
| Warning |
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Please don't change this the property "hibernate.current_session_context_class" unless you really know what you are doing. It has huge impact on the software architecture. Changing the configuration without changing parts of DSpace's source code will probably result in a malfunctioning installation (and could result in data loss). |
Curation Context (Curator.curationContext())
A context object built to be shared between Curation tasks.
Context Configurations
The DSpace Context Object can be constructed as a read-only context or a batch context. This mode determines how hibernate will flush changes to the database. It is good behavior to store the old context mode before changing it and to set it back to the old mode when you're done with your work. This reduces problems when code parts that needs to be able to update stored content calls parts of DSpace's code that uses a read only or a batch mode.
Read Only Context in DSpace 6
The Context object can be set to a read only mode for enhanced performance when processing many database objects in a read-only fashion.
Curation Context (Curator.curationContext())
Objects read by a read only context are not intended to be modified. The context object should not be committed.
The read only context is intended for data security. Implementations using this context should not be able to accidentally save changes.
Batch Context in DSpace 6
Hibernate provides a mechanism to submit a large batch of changes to a database in a memory-efficient manner.
See https://docs.jboss.org/hibernate/orm/3.3/reference/en-US/html/batch.htmlA context object built to be shared between Curation tasks.
Database Interaction (before DSpace 6.x)
All Database actions requires require the presence of a Context object.
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The DSpace Context object has methods (like uncacheEntity() and reloadEntity()) which can manage objects cached within this Hibernate Session (via HibernateDBConnection).
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A few tips on working with Hibernate Sessions (all gleaned from https://developer.atlassian.com/confdev/development-resources/confluence-architecture/hibernate-sessions-and-transaction-management-guidelines)
- Hibernate sessions are not thread-safe
- Therefore, any new DSpace code must ensure it is not attempting to share objects or Sessions between threads. Instead, pass around UUIDs, so the new thread can load the referenced object in a new Session.
- The more objects you load during the lifetime of a Session, the less efficient each query will be
- So, be sure to use
Context.commit()orContext.uncacheEntity()when you are done with an object - (recommendation: offer very specific/limited guidance on when to call uncacheEntity())
- So, be sure to use
- Because Hibernate has built-in Session caching, it is not recommended to cache objects elsewhere in your code. If you must perform other caching, store UUIDs instead
- Caching objects elsewhere is likely to result in a
LazyInitializationExceptionif the object (cached elsewhere) outlives its Session. See "Common Hibernate Error Messages" below.
- Caching objects elsewhere is likely to result in a
Context Configurations
The DSpace Context Object can be constructed as a read-only context or a batch context. This mode determines how hibernate will flush changes to the database.
Read Only Context in DSpace 6
Objects read by a read only context are not intended to be modified. The context object should not be committed.
The read only context is intended for data security. Implementations using this context should not be able to accidentally save chages.
Do read only connections also have optimized performance?
Batch Context in DSpace 6
Hibernate provides a mechanism to submit a large batch of changes to a database in a memory-efficient manner.
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The Life-cycle of a DSO with Hiberate
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Overview
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