A record is evidence, not durability

• Encode the page change before reasoning about recovery.
• Separate cached pages, buffered WAL, durable WAL and durable pages.
• LSN orders history; PrevLSN follows one transaction.

Derive the byte budget

Fixed header = 34 bytes; CRC = 4 bytes.

Before=ABC (3), After=xy (2): total = 43.

Payloads: [34,37), [37,39); CRC: [39,43).

Decode one complete record

Bounds → magic/type/reserved → exact length → non-update fields → CRC → copy.

A matching CRC does not establish valid semantics or a durable commit.

Crash trace

Durable WAL=8; pageLSN=8. Update 10 and commit 11 are appended.

• Before WAL flush: both can disappear.
• Flush through 10: updated page may be written.
• Flush through 11: durable commit may be acknowledged.
• Crash before page write: recovery must redo committed history.

Your turn

Before length 2; After length 4. Derive ranges and total size.

Durable WAL=25; pageLSN=27; buffered commit=28. Which flush permits the page write? Which permits acknowledgment?

Check the lesson reveals, then implement WAL Record Encoding.