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hardSystem Design

What is a write-ahead log (WAL)?

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01

Understand the problem

Explain WAL.

wal
02

Attempt it yourself

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03

Study the solution

Step 1: Outline use cases and constraints

Gather requirements and scope the problem. Ask questions to clarify use cases and constraints. Discuss assumptions.

Use cases

We'll scope the problem to handle only the following use cases

  • User needs to understand wal to make architecture decisions

Solution ready — 2 min read

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04

Read the code

WAL: log then apply, recover by replay
Run Playground
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}
05

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