How does Liquibase track which changes have been applied to a database?

Liquibase uses two system tables to track applied changes:

1. DATABASECHANGELOG: Records each changeset that has been executed

   • ID: Changeset identifier
   • AUTHOR: Changeset author
   • FILENAME: Source changelog file
   • DATEEXECUTED: When the changeset was run
   • MD5SUM: Checksum of the changeset content
   • EXECTYPE: How it was executed (EXECUTED, FAILED, SKIPPED, etc.)

2. DATABASECHANGELOGLOCK: Prevents multiple instances from modifying the database simultaneously

   • ID: Lock identifier
   • LOCKED: Boolean indicating if database is locked
   • LOCKGRANTED: Timestamp when lock was acquired
   • LOCKEDBY: Information about who holds the lock

When Liquibase runs, it:

1. Checks the lock table to ensure exclusive access
2. Compares changelog contents with DATABASECHANGELOG records
3. Executes only new/unrun changesets
4. Updates tracking tables with execution results