Author
Pat Helland
1 reading card from 1 book · 2015.
1 card
Immutability Changes Everything · 2015
A correction is a new row, not an eraser.
An accountant does not erase a wrong entry; they add a correcting one. The ledger stays complete, anyone can redo the arithmetic, and "the balance" is a derived value, not a fact. Helland shows that modern computing is moving the same way: transaction logs, event logs, Parquet files written once, even SSDs that do not overwrite in place. When data is immutable, whole classes of problems vanish — lost updates, inconsistent reads, "who changed this?" — and two precious things appear: audit is free and any state can be recomputed. In a data warehouse, facts are naturally immutable: a sale happened. Corrections become new rows with the opposite sign or with a validity period, and late-arriving facts are appended, not "patched" into the past. Dimensions keep their history through versions — the next lesson. The only real conflict is with the GDPR right to erasure, for which there is an answer compatible with immutability, in lesson 19.
“There is an inexorable trend towards storing and sending immutable data. We need immutability to coordinate at a distance and we can afford immutability, as storage gets cheaper.”