Key

A document key: an opaque byte string ordered as unsigned bytes.

Bytes rather than String because the ordering is the load-bearing part. An LSM-tree merges sorted runs, so the comparator is not a detail of the API — it is the one thing every segment ever written has to agree with, forever. Unsigned lexicographic order over bytes is the only choice that is stable under UTF-8: it makes key order agree with code-point order for text keys, while still accepting keys that are not text at all.

String.compareTo would have been the wrong comparator even for string keys — it compares UTF-16 units, so U+FF21 sorts before U+10000 in UTF-8 and after it in UTF-16. The Variant codec has the same rule for field names, for the same reason.

Keys are immutable; the bytes handed to of are copied, and toByteArray copies back out. An empty key is legal and sorts first.