To find keys without scanning, an SSTable needs a table of contents - but first the reader has to know where the data ends and that table begins. Today you append an index and a fixed-size footer, and teach the reader to stop at the data boundary.
Append an index and a fixed-size footer to an SSTable, and bound the reader to the data section.
To look a key up without scanning, the file needs a table of contents, and the groundwork for it is knowing where the records stop. Today you append two things after the data: an index (built from the byte offset of each record, which you record as you write them) and a small, fixed-size footer holding the offset where the index begins - which is exactly where the data section ends.
The footer is the bootstrap. A reader opening the file cold seeks to
fileSize - footerSize, reads the footer, and learns the data boundary without
scanning anything. The critical continuity point: your existing iterator and Get
walked records to the end of the file - now they must stop at that boundary, or
they would try to decode index and footer bytes as if they were records. Bound them
to [0, dataEnd) and the trailer stays invisible to normal reads. The index is now
on disk, ready for the next lesson to actually use.
// file layout:// [data: records...][index: (key, recordOffset) pairs][footer]// footer is a FIXED size at EOF holding the index's start offset// (which is also where the data section ends). record each// record's offset as you write the data, so the index can be built.// OpenSSTable: read footer at (fileSize - footerSize) -> dataEnd,// and bound Iterator/Get to [0, dataEnd) so the trailer is ignored.