Data lake
A data lake is a place where a company stores its data as files, as it arrives and untransformed, and decides later how to read it.
Select any words here to ask your Maestro about them.
Each month's sales file (Parquet) lands in your data lake as is. In March, the export renamed the amount column to total. You run SUM(amount) by month. What does March show?
How it works: everything lands in the lake as it arrives, untransformed: CSV and JSON files, Parquet, images or logs, in cheap storage like Amazon S3. The structure is decided when you read (schema on read), not when you save.
Lake or warehouse: a data warehouse stores clean tables with a fixed structure, ready for reports. A lake stores everything, more cheaply and without deciding up front what it's for. Many companies use both: the lake takes everything in, and an ETL process moves the clean parts into the warehouse.
What it's for: keeping data you don't yet know how you'll use, or that doesn't fit in a table, like text, images or sensor data, and training machine learning models.
How it's used
Lake, warehouse or lakehouse
Where things live
- Data lake: files as they arrive (CSV, JSON, Parquet, images, logs) in cheap storage like S3
- Data warehouse: clean tables with a fixed structure, ready to query with SQL
- marked: Lakehouse: the lake's files with a table format on top (Delta Lake, Iceberg) so you can query them like a warehouseDatabricks made the idea popular: the lake's low cost with the warehouse's order.
When to look closer
Four files for October
sales/2026/10/
- $ ls sales/2026/10/
- sales_oct.csv sales_oct_final.csv sales_oct_final_v2.csv sales_oct_USE_THIS.csv
Your lake's October folder holds four sales files. Which one has the month's real sales?
Ask your Maestro about Data lake
It answers from the free course page closest to this word, the way it answers while you study.
You: What does "Data lake" mean, in plain words?
Your Maestro: I'll explain it with a case of yours: it sticks better than a definition. Where do the files your systems export end up today?
Asking about: Answers from the page "How do I read a file from another country correctly in pandas?"
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Related terms
- Data warehouse
A data warehouse is a database built for analysis: it brings data from many systems into one place, keeps its history and organizes it for queries.
- ETL (extract, transform, load)
ETL (extract, transform, load) copies data out of the systems where it's created, cleans and reshapes it, and loads it into a data warehouse so it can be analyzed together.
- CSV
A CSV (comma-separated values) is a plain-text file that holds a table: one row per line, with commas between the columns.
- JSON
JSON is a text format for structured data: labels and values inside curly braces, lists inside square brackets. Programs use it to pass data to each other.