The backbone of LibreHealth is the core API. This API has methods for all of the basic functions like adding/updating a patient, adding/updating a concept, etc. These methods are provided in services. There are classes named PatientService, ConceptService, EncounterService, ObsService, etc. The Data Model groups the database tables into "domains." Each domain is a separate colored box. The breakdown of domains/tables is essentially a visual representation of the service separation.

The Context is a static class to allow the application to save on memory. Only one PatientService object, one ConceptService object, etc (and of course the associated DAO's) are instantiated. The Context's services are split into two categories: methods for the Services and for Users. The services are kept in the aptly named ServiceContext class. This is instantiated only once and is stored statically in the Context. The getter methods for the services simply pass through Context to the ServiceContext. The StaticContext properties are set via our Spring application Context and Dependency Injection. The UserContext contains methods for acting on users: authentication and authorization, logging in, logging out, etc. A different UserContext is instantiated for every user accessing the system. The "current" UserContext is stored on the current thread. When that user is done, the UserContext is taken off of the thread and put into the user's session variable (in the case of the webapp). When the user accesses the system again, the UserContext is taken off of the user's session and placed onto the thread again. In the webapp, this manipulation is done by the OpenmrsFilter class that wraps around every call to the server. Similar to the services, the methods on the Context class pass through to the current UserContext on the current thread.

Every access to the system must be defined within a "unit of work". This unit is bordered by calls to Context.openSession() and Context.closeSession(). In the webapp, these calls are done in OpenmrsFilter and most developers don't have to worry about making those calls. However, any developer of an external application or a thread spinoff (like Hl7InQueueProcessor and FormEntryProcessor) will need to be sure to include open/closeSession calls or risk leaking database connections. Read more about the API

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