Two engines, one contract

Two engines, one contract:

public interface Engine {
    void put(String key, String value);   // upsert
    java.util.Optional<String> get(String key);
    java.util.List<String> keys();        // sorted, for determinism
    int count();
}

InMemory: a ConcurrentHashMap behind the interface. Determinism comes from keys() sorting — iteration order of a hash map is otherwise an implementation detail that would leak into reports.

JsonlFile: the source of truth is an append-only file, one JSON-ish record per line: {"op":"put","k":"a","v":"1"}. put appends then updates an in-memory index (write-through); keys() sorts the index the same way. Replay — reading the file and re-applying operations — rebuilds the state after any "restart"; the checkpoint simulates one by constructing a second engine over the same path and comparing reports.

This is the Shape of every real persistence layer: an ordered event log you can replay, an index you can rebuild, and a read path that never lies about what has been acknowledged.