Data is stored in Accumulo in a distributed sorted map. The Keys of the map are broken up logically into a few different parts, as seen in the image below.
Row ID - Unique identifier for the row.
Column Family - Logical grouping of the key. This field can be used to partition data within a node.
Column Qualifier - More specific attribute of the key.
Column Visibility - Security label controlling access to the key/value pair.
Timestamp - Generated automatically and used for versioning.
The value is where the actual data is stored. For brevity, we often refer to the 3 parts of the column as the family, qualifier and visibility.
Take a closer look at the Mutation object created in the first exercise:
Mutation mutation = new Mutation("id0001"); mutation.put("hero","alias", "Batman");
It can be broken down as follows:
Row ID: id0001 Column Family: hero Column Qualifier: alias Value: Batman
For this exercise add a few more rows to the GothamDB table. Create a row for Robin (id0002), who is a hero that also wears a cape and his name is “Dick Grayson”. Create a row for Joker (id0003), who is a villain with an “Unknown” name and doesn't wear a cape. Build and run.
Notice how the data is printed in sorted order. Accumulo sorts by Row ID then family and then qualifier.