I started with the log format and that went okay, length-prefixed records with a checksum at the end, pretty standard stuff.
Start by clarifying requirements (durability, performance, recovery time) and then walk through the design of an append-only write-ahead log with checksums, covering put/delete semantics, replay, and optional snapshotting. Emphasize how you handle partial writes via checksums and truncation, and discuss trade-offs between fsync frequency and durability.
Pro tip: Demonstrate awareness of real-world constraints: fsync is expensive, so batching or group commit can improve throughput while maintaining durability; also mention that torn writes are detected via checksums and that the log is truncated at the last valid record.
Ask about durability guarantees (e.g., must every acknowledged write survive a crash?), performance targets, and whether snapshots are needed. State assumptions about single-node, in-memory store with disk-based WAL.
Define a record structure with a header (magic, length, checksum, sequence number, operation type) and payload (key, value). Use append-only writes and include a checksum (e.g., CRC32) to detect corruption.
For put and delete, write the record to the log and fsync before acknowledging to the client. Discuss trade-offs: fsync per operation vs. batching (group commit) to amortize cost.
On startup, read the log sequentially, validate each record's checksum, and apply valid operations to rebuild the in-memory state. Stop at the first invalid or partial record and truncate the log there.
Periodically write a snapshot of the in-memory state to disk, then truncate the log up to the snapshot point. On recovery, load the latest snapshot and replay only subsequent log entries.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.