toydb_15.py

    def set(self, key: str, value: Any):
        """Set a key-value pair"""
        with self.lock:
            if not isinstance(key, str):
                raise ValueError("Key must be a string")

            # 1. Safety first: Write to WAL
            self.wal.set(key, value)

            # 2. Write to memory table (fast!)
            self.memtable.add(key, value)

            # 3. If memory table is full, save to disk
            if self.memtable.is_full():
                self._flush_memtable()

    def _flush_memtable(self):
        """Flush memtable to disk as new SSTable"""
        if not self.memtable.entries:
            return  # Skip if empty

        # Create new SSTable with a unique name
        sstable = SSTable(str(self.base_path / f"sstable_{len(self.sstables)}.db"))
        sstable.write_memtable(self.memtable)
        # Add to our list of SSTables
        self.sstables.append(sstable)

        # Create fresh memory table
        self.memtable = MemTable()

        # Create a checkpoint in WAL
        self.wal.checkpoint()

        # Compact if we have too many SSTables
        if len(self.sstables) > self.max_sstables:
            self._compact()