What is a write-ahead log (WAL)?

class WAL:
    def __init__(self):
        self.log = []      # append-only (a file on disk in reality)
        self.state = {}    # the "data pages"
    def set(self, key, value):
        self.log.append(("set", key, value))  # 1. write-ahead (fsync) FIRST
        self.state[key] = value               # 2. then apply
    def recover(self):
        self.state = {}
        for op, key, value in self.log:       # replay on restart
            if op == "set":
                self.state[key] = value
        return self.state

wal = WAL()
wal.set("balance", 100)
wal.set("balance", 80)
# simulate crash + restart: rebuild state purely from the log
print("recovered:", wal.recover())   # {'balance': 80}