Read Tutorial

Like in the Write Example some parameters must be defined. For simplicity, this time only the folder of the DRUMS-table must be given.

GlobalParameters gp = new GlobalParameters(folderOfDRUMS);

Now, DRUMS can be opend for reading.

DRUMS drums = DRUMSInstantiator.openTable(gp);

To perform efficient selects, it is recommended to use the DRUMSReader. Use the factory method #getReader() in the DRUMS-object to get an instance.

DRUMSReader reader = drums.getReader();

Single selects

Single selects can be performed using #select(keys[]...) from the DRUMS-instance or #get(keys[]...) from the DRUMSReader.

 List result = drums.select(key1, key2, ..., keyN);

 List result = reader.get(key1, key2, ..., keyN); 

Range selects

Range selects can only be performed using the DRUMSReader. The method #getRange(byte[], byte[]) must be used carefully. The returned list might be very large and might need a lot of memory.

List result = reader.getRange(lowerKey, upperKey); 

Iterator

DRUMS provides also the option to use an iterator to scan a large range of the table.

DRUMSIterator iterator = drums.getIterator();
while (iterator.hasNext() ) { // do something };
iterator.close();

Filtering data

DRUMS doesn't provide the possibility to formulate detailed select conditions like MySQL (... WHERE ...). Therefore the returned lists of records must be filtered.

Iterator iterator = result.iterator();
while (iterator.hasNext()) {
    Data record = iterator.next();
    if (condition is fullfilled) {
        // add element to a final result list
    }
}

drums.close();

Example

For a concrete example please have a look at BioDRUMS. This file provides a full functional example for performing the above mentioned operations on a filled DRUMS table.