11.6.1. Writing JavaScript Filters

The JavaScript interface to the replicator enables filters to be written using standard JavaScript with a complete object-based interface to the internal Java objects and classes that make up the THL data.

For more information on the Rhino JavaScript implementation, see Rhino.

The basic structure of a JavaScript filter is as follows:

// Prepare the filter and setup structures



// Perform the filter process; function is called for each event in the THL


// Get the array of DBMSData objects
    data = event.getData();

    // Iterate over the individual DBMSData objects
      // Get a single DBMSData object
      d = data.get(i);

      // Process a Statement Event; event type is identified by comparing the object class type

      if (d = instanceof com.continuent.tungsten.replicator.dbms.StatementData)
        // Do statement processing
      else if (d = instanceof com.continuent.tungsten.replicator.dbms.RowChangeData)
        // Get an array of all the row changes
        rows = data.get(i).getRowChanges();

        // Iterate over row changes
          // Get the single row change
          rowchange = rows.get(j);

          // Identify the row change type
          if (rowchange.getAction() == "INSERT")


The following sections will examine the different data structures, functions, and information available when processing these individual events. Logging Information and Exceptions

Information about the filtering process can be reported into the standard trepsvc.log file by using the logger object. This supports different methods according to the configured logging level:

  • logger.info() — information level entry, used to indicate configuration, loading or progress.

  • logger.debug() — information will be logged when debugging is enabled, used when showing progress during development.

  • logger.error() — used to log an error that would cause a problem or replication to stop.

For example, to log an informational entry that includes data from the filter process:

logger.info("regexp: Translating string " + valueString.valueOf());

To raise an exception that causes replication to stop, a new ReplicatorException object must be created that contains the error message:

if(col == null)
  throw new com.continuent.tungsten.replicator.ReplicatorException(
    "dropcolumn.js: column name in " + schema + "." + table +
    " is undefined - is colnames filter enabled and is it before the dropcolumn filter?"

The error string provided will be used as the error provided through trepctl, in addition to raising and exception and backtrace within the log.