SMILA/Specifications/RecordStorage

!!! UNDER CONSTRUCTION !!!

Description

As Berkeley XML DB will not be available in eclipse in the near future, we need an open source alternative to store record metadata. There is no requirement to use an XML database onyl, any storage that allows us to persist record metadata will suffice.

RecordStorage Interface

Here is a proposal for a RecordStorage interface. It contains only the basic functionality without any query support.

interface RecordStorage
{
    Record loadRecord(Id id);
    void storedRecord(Record record);
    void removeRecord(Id id);
    boolean existsRecord(Id id)
}

RecordStorage based on JDBC database

At the moment all access to records is based on the record ID. The record ID is the primary key when reading/writing records. It would be easily possible to store the records in a relational database, using just one table with columns for the record ID (the primary key) and a second one to store the record itself. The record could be stored as a BLOB or CLOB:

• BLOB: the record is just serialize into a byte[] and stored as a BLOB
• CLOB: the record's XML representation could be stored in a CLOB. Extra method calls to parse/convert the record from/to XML needs to bee applied wheenreading/writing the records (performance impact in comparison to using a BLOB). But this would offer some options to include WHERE clauses accessing the CLOB in SQL queries

Because the String representation of IDs can be really long, an alternative could be to store a hash of the String. (This hash has to be computed whenever accessing the database) In addition one could also add another column to store the source attribute of the record ID. This would allow easy access on all records of a datasource to handle the use-case "reindexing without crawling"

For advanced use-cases (e.g. Mashup) query support is needed (compare XQJ), e.g. to select all records of a certain mime type. It would be possible to add more columns or join tables for selected record attributes. Another option is to do postprocessing of selected records, filtering those records that do not match the query filter. This is functional equal to a SQL select but of course performance is very slow.

When implementing a JDBC RecordStorage one should take care to use database neutral SQL statements, or make the statements to use configurable. A good practice could be to implement the reading/writing in DAO objects, so that database specific implementations of the DAOs could be provided to make use of special features. Most databases offer imporved support for BLOBs and CLOBs.

A good choice for an open source database is Apache Derby. The Apache License 2.0 is compatible to EPL, the database has a low footprint (2MB) and can be used in process as well as in client/server network mode. It is also already commited to Orbit. For a productive environment it would be easily possible to swicth to any other JDBC database, like Oracle.

RecordStorage based on relational database using eclipseLink

EclipseLink offers various options to persists Java objects. Below we go into detail about using eclipseLink with JPA (Java Persistence Api):

Overview on JPA

A mapping of Java classes to a relational database schema is created by using annotations in java code or providing an XML configuration. The classes to be persisted (called Entities) are in general represented ny database tables, member variables as columns in those tables. There are some requirements to be met:

• an entity class must provide a non argument constructor (either public or protected)
• antity classes must be top level classes, no enums or interfaces

There exists two kinds of access types, where onyl one kind is usablee per entity:

• field based: direct access on member variables
• property based: JavaBean like access via getter- and setter-methods

An Entity must have a unique Id, this can be either

• a simple key (just one member variable) (@Id)
• a composite key using multiple member varibales. This implies the usage of an additional primary key class that contains the same member varibales (same name and type) as the entity class (@Id + @idClass)
• an embedded key (@EmbeddedId)

Entities can have relations to other entities or contain embedded classes. Embedded classes aree not entities themselfes (but must meet the same requirements) and do not have a unique Id. They "belong" to the entity object embedding them. Version 1.0 of the JPA specification demands only support of one level of embedded objects. If more levels are supported depends on the implementation. Collections are also not allowed as embedded classes.

For more information see ejb-3_0-fr-spec-persistence.pdf.

PoC eclipseLink JPA RecordStorage using Oracle DB and Smila datamodel

• a JPA RecordStorage is always based on a concrete implementation of entities to persist. So it is not possible to implement a generic RecordStorage for any Record implementation but only for a specific implementation, in this case our default implementation RecordImpl, IdImpl , ...
• the classes of the Smila datamodell cannot be used as is, but have to meet the requirements of JPA (e.g. no argument constructor)
• the default implementation uses interfaces of other entity classes as types for memeber variables. These have to be replaced by concrete implementatuion types, for example

@Entity
public class RecordImpl implements Record, Serializable {
  @EmbeddedId
  private IdImpl _id;
  ...
}

instead of

@Entity
public class RecordImpl implements Record, Serializable {
  @EmbeddedId
  private Id _id;
  ...
}

• the Smila coding conventions implicate some issues: the names for tables/columns are automatically generated by JPA using class and member varibale names. The leading _ used in Smila for member variables leads to invalid SQL statements (at leats with Oracle). Therefore it is necessary to define the names for every tables, join tables and columns manually by using annotations or xml configuration
• the Smila objects (Records, Id, Attribute, Annotation) are all structured recursively and most make also use of Collections or Maps as members
  • eclipseLink supports N levels of embedded objects
  • recursive embedding of objects is NOT supported
  • Collections/Maps of embedded classes are not supported in JPA 1.0 (will be supported in JPA 2.0). eclipseLink offers a so called DescriptorCustomizer which could be used to implement such a support (not considered in die PoC, needs further analysis)
  • so it is not possible modeling the Smila classes as ambedded objects, which would have been the most natural approach as all data belongs to a single record object
• as an alternative one could try to model the relations between the various datamodell classes. This means all classes have to be annotated as separate Entity objects and each needs to have an own unique Id. Besaides the Record object, no other object has a single member that could be used as a primary key. This is a major problem as for example a object of type Attribute is only unique by creating a primary key over all member varibales, which in addition are Lists. Class LiteralImpl has another problem, as the meemnber _value is of type Object, which is represented as a Blob in the database. But Blobs cannot be used as part of a primary key
  • Daniel Stucky : we could check if automatically generated Ids are useful in this scenario. This means that all Smila Entity objects have to add a new meember varibale (e.g. int _persistentId ). Most likely this approach still fails because of the recursive structure

Links

• [http://db.apache.org/derby/] (Derby homepage)
• [http://wiki.eclipse.org/EclipseLink/] (EclipseLink homepage)
• [http://jcp.org/en/jsr/detail?id=220] (JPA Specification)