Data is stored in Accumulo in a distributed sorted map. The Key's 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 mutation1 = new Mutation("id0001"); mutation1.put("hero","alias", "Batman");
It can be broken down as follows:
| Row ID: | id0001 | | Column Family: | hero | | Column Qualifier: | alias | | Value: | Batman |
As an exercise, add a few more rows to the GothamPD 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. Compare your output to the output displayed on the next page.
Notice how the data is printed in sorted order. Accumulo sorts by Row ID, then Family, and then Qualifier.