Cloud data lake architecture for organisations with high-volume event data, unstructured data (documents, images, logs), or compliance requirements mandating raw record retention that make a warehouse-only approach insufficient. Storage layer: AWS S3, Google Cloud Storage, or Azure Data Lake Storage Gen2 as the raw landing zone, with S3 Intelligent-Tiering or GCS lifecycle policies automatically moving cold data to cheaper storage tiers after 90 days. Table format: Apache Iceberg (preferred for new builds) providing ACID transactions, schema evolution (adding and renaming columns without rewriting the full dataset), time-travel queries (SELECT * FROM table AS OF TIMESTAMP '2024-01-01'), and row-level deletes for GDPR right-to-erasure compliance on the lake; Delta Lake for organisations already in the Databricks ecosystem. Medallion architecture with enforced data contracts: Bronze layer (raw, source-faithful data in Parquet format, schema-on-read, retention-for-compliance); Silver layer (cleaned, standardised, deduplicated, with schema enforced and data quality tests passing, the layer analysts and ML engineers use for exploration); Gold layer (business-logic transformed, metric-ready tables optimised for query performance, the layer BI tools and production dashboards connect to). Separation of compute from storage enables cost efficiency: AWS Athena, BigQuery Omni, or Trino for ad-hoc SQL queries on S3/GCS data; Apache Spark on AWS EMR for heavy transformation jobs; dbt with Spark adapter for dbt-managed transformations on the lake. Data governance: Apache Atlas or AWS Glue Data Catalog for metadata management, table lineage, and PII classification tagging; column-level masking of PII fields for non-privileged users.