SMILA/Specifications/RecordStorage

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 databse

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.