Memtable
BTreeMap with tombstones under a single writer and a reader-writer lock, held only for one insert. Deletes stay until compaction so older snapshots remain correct.
Design
Defend every choice.
Spindle’s interview value is being able to walk the write path, the crash story, and the compaction amp without flipping open a wiki. This page is the short map — the long form lives in the repo.
WAL + fsync
Append-before-memtable. EveryWrite by default; group-commit optional. If we die after fsync but before memtable insert, replay restores the ACK’d write.
SSTable layout
Data blocks → bloom meta → index → 48-byte footer with magic SPNDLSST. Sparse index, restart-interval prefix compression, trailing CRC.
Compaction
Leveled ×10. L0 file-count trigger and per-level byte thresholds. Tombstones drop only at the bottom level.
Snapshots pin old versions
Compaction keeps every version a live snapshot can still see, so a long-lived snapshot holds old data on disk until it is dropped.