Java.DBMigrationTools.What are preconditions in Liquibase?

✅ Short Answer

Preconditions in Liquibase are conditions that must be true before a changeset or changelog is applied.
If the precondition fails, Liquibase can be told to halt, warn, or continue, depending on your configuration.

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🔎 Detailed Explanation

A precondition acts like a safeguard — for example:

• “Only run this change if a table exists”
• “Skip this if a column is already present”
• “Apply this only on PostgreSQL”

You can define preconditions:

• At the changelog level (applies to all changesets)
• At the changeset level (applies to one changeset only)

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✅ Common Use Cases for Preconditions

Use Case	Precondition Type
Ensure a table exists	tableExists
Ensure a column doesn’t already exist	not, columnExists
Restrict to a DBMS (e.g., PostgreSQL)	dbms
Block updates on production	runningAs (based on user)

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🧪 Example – XML format

<changeSet id="1" author="alice">
  <preConditions onFail="HALT">
    <not>
      <tableExists tableName="users"/>
    </not>
  </preConditions>

  <createTable tableName="users">
    <column name="id" type="int" autoIncrement="true"/>
    <column name="username" type="varchar(255)"/>
  </createTable>
</changeSet>

✅ This change will only run if the users table does not exist.
If the table does exist, Liquibase will halt the migration (because onFail="HALT").

🛠️ Other Precondition Types

• sqlCheck – Run custom SQL that returns true/false
• changeLogPropertyDefined
• columnExists
• sequenceExists
• foreignKeyConstraintExists
• customPrecondition – write your own in Java

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📌 Key Takeaways

✅ Preconditions let you add logic before a changeset runs, preventing errors or unintended changes.
✅ You can halt, warn, or skip based on the result.
✅ They are useful for idempotency, environment targeting, and fail-safety.