Druid

1. Design and Architecture

   Process types, deep storage, metadata store and ZooKeeper.

2. Segments

   Segment file format, time chunking and columnar storage.

3. Ingestion

   Batch and streaming ingestion overview.

4. Ingestion Spec

   Timestamp, dimensions, metrics, rollup and granularity.

5. Data Rollup

   Pre-aggregating data at ingestion time.

6. Streaming Ingestion

   Kafka and Kinesis supervisors with exactly-once ingestion.

7. SQL-Based Ingestion

   Multi-stage query engine for ingestion and queries.

8. Querying

   Native and SQL query interfaces.

9. Druid SQL

   SQL syntax and translation to native queries.

10. Data Management

    Updates, deletion, compaction and schema changes.

11. Retention and Tiering

    Load and drop rules and historical tiers.

12. Security Overview

    Authentication, authorization and TLS.

13. Configuration Reference

    Service configuration and tuning.

14. Operations and Tuning

    Cluster tuning, metrics and monitoring.

Each section links to the official page. Titles and summaries by Extra Credit; the docs belong to their authors.