Multi-version Concurrency Control

How MVCC works

MVCC maintains multiple versions of each data record, each tagged with transaction timestamps or version numbers. When a transaction modifies data:

1. A new version is created rather than overwriting existing data
2. The old version is retained for ongoing transactions that might need it
3. Each transaction sees a consistent snapshot based on its start time

Benefits in time-series systems

In time-series databases, MVCC is particularly valuable because:

1. Time-series data is often append-heavy with concurrent reads
2. Historical queries need consistent point-in-time views
3. Real-time analytics require isolation from ongoing writes

For example, a financial system can simultaneously:

• Ingest live market data
• Run historical analysis
• Serve real-time dashboards

Each operation sees a consistent version of the data without blocking others.

Version cleanup and storage considerations

MVCC systems must manage version cleanup (vacuum) to prevent unbounded storage growth:

1. Track the oldest active transaction
2. Identify versions no longer visible to any transaction
3. Remove obsolete versions while maintaining referential integrity

The cleanup process balances:

• Storage efficiency
• Query performance
• Transaction isolation guarantees

Implementation approaches

Modern databases implement MVCC through various strategies:

1. Append-only storage: New versions are appended to tables or write-ahead logs
2. Time-travel storage: Versions are organized by timestamp for efficient historical access
3. Delta storage: Only changes are stored between versions

These approaches often combine with other features like snapshot isolation for additional consistency guarantees.