Time Travel in LakeHouses

Time Travel in the context of a Data Lakehouse refers to the ability to query or access data as it existed at a specific point in the past, or to query data based on snapshot IDs. This feature is primarily enabled by open table formats like Apache Iceberg, Apache Hudi, and Delta Lake, which manage versions of table metadata and data files.

Instead of only seeing the current state of a table, time travel allows users to "go back in time" to examine historical states, reproduce experiments, audit changes, or recover from errors.

How it Works

Open table formats achieve time travel by:

1. Versioning Table Metadata: Each operation that modifies a table (e.g., an INSERT, UPDATE, DELETE, or schema change) creates a new version or "snapshot" of the table's metadata. This metadata tracks the data files that constitute the table at that specific point in time.
2. Immutable Data Files: Data files in the lake (e.g., Parquet, ORC files on S3) are typically immutable. When data is updated or deleted, new files are written with the changes, and old files are marked as no longer part of the current snapshot but are retained for a configurable period.
3. Snapshot Isolation: Queries can be directed to read a specific snapshot of the table based on either:
   • A timestamp (e.g., "SELECT * FROM my_table TIMESTAMP AS OF '2023-10-26 10:00:00Z'").
   • A snapshot ID (e.g., "SELECT * FROM my_table VERSION AS OF 1234567890123456789").

The table format's metadata tells the query engine which data files to read for that particular snapshot.

Key Use Cases and Benefits

• Reproducibility:
  • Re-run queries or machine learning models on the exact same data they were originally trained or run on.
  • Debug issues by examining the state of data at the time an error occurred.
• Auditing and Governance:
  • Track changes to data over time for compliance or regulatory purposes.
  • Understand how data has evolved.
• Error Recovery and Rollbacks:
  • Easily revert a table to a previous known-good state in case of accidental data corruption, bad writes, or application errors. This is much simpler than traditional backup/restore operations.
• Incremental Processing:
  • Process only the data that has changed since a previous snapshot, enabling more efficient incremental ETL pipelines.
• Historical Analysis:
  • Analyze trends or compare data across different time periods directly.
• Testing and Development:
  • Test pipeline changes on consistent historical snapshots of production data.

Implementation in Open Table Formats

• Apache Iceberg: Maintains a manifest list for each snapshot, which points to manifest files, which in turn list the actual data files. Timestamps and snapshot IDs are stored in table metadata.
• Delta Lake: Uses a transaction log (the "_delta_log") that records all changes as ordered, atomic commits. Each commit creates a new version of the table.
• Apache Hudi: Supports different table types (Copy on Write, Merge on Read) that provide varying trade-offs for write/read performance and offer snapshot isolation for queries.

Considerations

• Storage Overhead: Retaining old data files and metadata versions consumes storage. Table formats provide mechanisms to "expire" old snapshots and their associated data files (vacuuming/compaction) to manage storage costs.
• Retention Policies: Organizations need to define how long historical versions should be retained based on their specific needs (auditing, recovery, cost).
• Query Engine Support: The query engine (e.g., Spark, Trino, Presto, Flink) must be compatible with the chosen table format and its time travel syntax.

Time travel is a powerful feature that significantly enhances the reliability, manageability, and analytical capabilities of data lakehouses, bringing them closer to the functionalities offered by traditional data warehouses. While RisingWave itself focuses on processing current and incoming streaming data to produce fresh results, its output can be sunk into lakehouse tables (e.g., Iceberg tables) that offer time travel capabilities for the data at rest.