FoundationDB’s lexicographically ordered data model permits finding keys based on their order (for example, finding the first key in the database greater than a given key). Key selectors represent a description of a key in the database that could be resolved to an actual key by FDBC.Transaction.get_key/3 or used directly as the beginning or end of a range in FDBC.Transaction.get_range/4.

For example:

# Selects the first key following apple.
FDBC.KeySelector.greater_than('apple')

# Selects the second key following apple.
selector = FDBC.KeySelector.greater_than('apple')
%{selector | selector.offset + 1}

All possible key selectors can be constructed using one of the four 'common form; constructors and a positive or negative offset. Alternatively, general key selectors can be manually constructed by specifying:

• A reference key (binary)
• An equality flag (boolean)
• An offset (integer)

To 'resolve' these key selectors FoundationDB first finds the last key less than the reference key (or equal to the reference key, if the equality flag is true), then moves forward a number of keys equal to the offset (or backwards, if the offset is negative). If a key selector would otherwise describe a key off the beginning of the database (before the first key), it instead resolves to the empty key <<>>. If it would otherwise describe a key off the end of the database (after the last key), it instead resolves to the key '\xFF' (or '\xFF\xFF' if the transaction has been granted access to the system keys).