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4 Storage Policies

Sometimes STIL needs to store the data from a table for later use. This is necessary for instance when it creates a StarTable object by reading a VOTable document: it parses the XML by reading through from start to finish, but must be able to supply the cell data through the StarTable's data access methods without doing another parse later. Another example is when converting a sequential-only access table to a random-access one (see the example below, and Section 2.3.3) - the data must be stored somewhere they can be accessed in a non-sequential way at a later date.

The obvious thing to do is to store such data in object arrays or lists in memory. However, if the tables get very large this is no longer appropriate because memory will fill up, and the application will fail with an OutOfMemoryError (Java's garbage collection based memory management means it is not much good at using virtual memory). So sometimes it would be better to store the data in a temporary disk file. There may be other decisions to make as well, for instance the location or format (perhaps row- or column-oriented) to use for a temporary disk file.

Since on the whole you don't want to worry about these choices when writing an application, STIL provides a way of dealing with them which is highly configurable, but behaves in a 'sensible' way if you don't take any special steps. This is based around the StoragePolicy class.

A StoragePolicy is a factory for RowStore objects, and a RowStore is an object to which you can write the metadata and data of a table once, and perform random access reads on it at a later date. Any of the STIL classes which need to do this sort of table data caching use a StoragePolicy object; they have policy get/set methods and usually constructors which take a StoragePolicy too. Application code which needs to stash table data away should follow the same procedure.

By way of example: the randomTable method takes a (possibly non-random-access) table and returns a random-access one containing the same data. Here is roughly how it does it:

    static StarTable randomTable( StarTable seqTable, StoragePolicy policy )
            throws IOException {

        // Get a new row store object from the policy.
        RowStore rowStore = policy.makeRowStore();

        // Inform the row store about the table metadata - we do this by
        // passing the table itself, but this could be a data-less StarTable
        // object if the data were not available yet.
        rowStore.acceptMetadata( seqTable );

        // Loop over the rows in the input table, passing each one in turn
        // to the row store.
        RowSequence rowSeq = seqTable.getRowSequence();
        while ( rowSeq.next() ) {
            rowStore.acceptRow( rowSeq.getRow() );
        }

        // Inform the row store that there are no more rows to come.
        rowStore.endRows();

        // Extract and return a table from the row store.  This consists of
        // the metadata and data we've written in there, but is guaranteed
        // random access.
        return rowStore.getStarTable();
    }
Most times you won't have to write this kind of code since the STIL classes will be doing it behind the scenes for you.


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STIL - Starlink Tables Infrastructure Library
Starlink User Note252
STIL web page: http://www.starlink.ac.uk/stil/
Author email: m.b.taylor@bristol.ac.uk