# OmeroContext

The entire OMERO application (on a single JVM) resides in a single ome.system.OmeroContext. Each call belongs additionally to a single org.hibernate.Session (which can span over multiple calls) and to a single ome.model.meta.Event (which is restricted to a single task).

The container for all OMERO applications is the OmeroContext (https://github.com/ome/omero-common/blob/v5.7.3/src/main/java/ome/system/OmeroContext.java). Based on the Spring configuration backing the context, it can be one of `client`, `internal`, or `managed`. The use of a ServiceFactory simplifies this usage for the client.

## Hibernate sessions

A `Hibernate Session` comprises a ` Unit-of-Work <https://www.martinfowler.com/eaaCatalog/unitOfWork.html>`_ which translates for OMERO’s OME-Remote Objects model to a relational database. It keeps references to all Database-backed objects so that within a single session, object-identity stays constant and object changes can be persisted.

A session can span multiple calls by being disconnected from the underlying database transaction, and then reconnected to a new transaction on the next call (see https://github.com/ome/omero-server/blob/v5.7.3/src/main/java/ome/tools/hibernate/SessionHandler.java for the implementation).

For information about Events see OMERO events and provenance.