Delta Lake & Lakehouse Deep Dive

SECTION 1: DELTA LAKE INTERNALS (1.5 hours)

Q1: What is Delta Lake? And what is the transaction log?

Simple Explanation: Think of a normal data lake — you store files (like Parquet) in cloud storage (Azure ADLS). But there's a big problem: if two people write to the same folder at the same time, data can get corrupted. There's no "undo" button. There's no way to know what changed.

Delta Lake solves this. It adds a "smart layer" on top of your Parquet files. This smart layer is called the transaction log (stored in a folder called _delta_log/). It's like a diary that records every change — "file X was added", "file Y was removed", "schema changed", etc.

Real-world analogy: Imagine a hotel booking register. Every time a booking is made or cancelled, the receptionist writes it in a numbered logbook (commit 1, commit 2, commit 3...). If someone asks "what did our bookings look like yesterday?", you can replay the logbook up to yesterday. That logbook = Delta transaction log.

Why do we need it?

• Without Delta: Two booking agents update same file → data gets corrupted
• With Delta: Transaction log ensures only one change goes through at a time (like a queue)

Technical details: