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What is a persistence layer?
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The availability of such paths means that every node in openEHR data is addressable using a meaningful path, opening the way for some novel possibilites in data storage, particularly relational storage.
Is openEHR a proprietary data format?
No, it's the opposite of proprietary. openEHR data are defined by the Reference Model specifications published by openEHR since 2001. These specifications define every detail of openEHR data, and are available in UML (XMI) form and XML Schema form.
But what if openEHR data are stored on a proprietary (i.e. commercial) database?
A great deal of production data (probably the majority) in the world in all industries, including health, are stored on proprietary databases such as Oracle, IBM DB2, Microsoft SQL Server. Indeed some of the openEHR vendor solutions are deployed on these databases. However, the data always follow the openEHR Reference Model, and can always be retrieved in the standard open XML Schema form, as well as the object form defined by the RM, via the EHR Service interface, whose calls return openEHR RM structures as Java, C# or other programming language objects.
This is in contrast with EMR solutions that define a proprietary schema for the database and no logical Reference Model.
What about Performance?
Regardless of what kind of persistence mechanism is chosen, performance of storage and retrieval is important, if the system is to be scalable to large numbers of users and databases accesses. Object-oriented data generally takes the form of fine-grained hierarchical structures, and openEHR data is no exception. Storing data at its finest granularity is almost guaranteed to be infeasible for scalable systems. Retrieval tests on typical object data stored in fine-grained form almost always reveal extreme inefficiency. Addressing this problem usually means storing the data at a coarser granularity, i.e. converting the fine-grained in-memory data into "blobs" and storing them instead. The questions raised by doing that include:
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