What is Liquibase ?

1. Liquibase is an open source database-independent library for tracking, managing and applying database changes.
2. All changes to the database are stored in XML files and identified by a combination of an "id" and "author" tag as well as the name of the file itself.
3. A list of all applied changes is stored in each database which is consulted on all database updates to determine what new changes need to be applied.
4. Liquibase executes changes based on this XML file to handle different revisions of database structures and data.
5. When you first run a changelog, LiquiBase manages those changelogs by adding two tables into your database.
databasechangelog: maintains the database changes that were run.
databasechangeloglock: ensures that two machines don't attempt to modify the database at one time.
6. Limitations do exist such that it will not export triggers, stored procedures, functions and packages.

Sample changeLog file:

 The above is an example of creating table EMPLOYEE and adding columns into it.


<?xml version="1.0" encoding="UTF-8"?>
<databaseChangeLog xmlns="http://www.liquibase.org/xml/ns/dbchangelog"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.liquibase.org/xml/ns/dbchangelog http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-2.0.xsd">
      
    <changeSet author="waheed" id="123456789-1">
        <createTable tableName="EMPLOYEE">
            <column autoIncrement="true" name="EMPLOYEE_ID" type="BIGINT">
                <constraints nullable="false" primaryKey="true" />
            </column>
            <column name="NAME" type="VARCHAR(255)" />
            <column name="GENDER" type="VARCHAR(2)" />
            <column name="COUNTRY" type="VARCHAR(255)" />
            <column name="ABOUT_YOU" type="VARCHAR(255)" />
        </createTable>
    </changeSet>
</databaseChangeLog>