What is a Liquibase precondition and why would you use one?

A precondition is a condition that Liquibase checks before executing a changeSet or the entire changeLog. If the condition is not met, Liquibase can either halt execution, skip the changeSet, mark it as run, or issue a warning — depending on the onFail and onError settings. Preconditions let you write defensive migrations that check the actual database state before blindly executing DDL.

Common use cases:

• Verify a table or column already exists before trying to modify it (useful for changeSets that might have been applied manually in some environments).
• Check that a specific database type or version is in use before executing vendor-specific SQL.
• Confirm a required extension or configuration exists before proceeding.

<changeSet id="add-index-if-table-exists" author="eve">
    <preConditions onFail="MARK_RAN">
        <tableExists tableName="order_item"/>
        <not>
            <indexExists indexName="idx_order_item_product"/>
        </not>
    </preConditions>
    <createIndex tableName="order_item" indexName="idx_order_item_product">
        <column name="product_id"/>
    </createIndex>
</changeSet>

The onFail attribute controls what happens if the precondition is not met: HALT (stop everything), CONTINUE (skip this changeSet), MARK_RAN (record it as executed without running it), or WARN (log a warning and continue). Choosing the right onFail behavior is important — MARK_RAN is useful when a change may have been applied manually and you want Liquibase to catch up without re-applying, while HALT is appropriate for critical prerequisites.