How to work with liquibase, a concrete example - database

Question

Following the quickstart on liquibase i've created a changeset (very dumb :) )
Code:
<?xml version="1.0" encoding="UTF-8"?>
<databaseChangeLog
xmlns="http://www.liquibase.org/xml/ns/dbchangelog/1.6"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.liquibase.org/xml/ns/dbchangelog/1.6
http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-1.6.xsd">
<changeSet id="1" author="me">
<createTable tableName="first_table">
<column name="id" type="int">
<constraints primaryKey="true" nullable="false"/>
</column>
<column name="name" type="varchar(50)">
<constraints nullable="false"/>
</column>
</createTable>
<createTable tableName="new_table">
<column name="id" type="int">
<constraints primaryKey="true" nullable="false"/>
</column>
</createTable>
</changeSet>
</databaseChangeLog>
I've created a clean schema and i've launched the migrate command.
Liquibase created the database, with the support tables databasechangelog and ..lock.
Now how i can track the changes?? i've modified the changeset adding a new createTable element but when i try the command "update" liquibase tells me this
Migration Failed: Validation Failed:
1 change sets check sum
so i don't think to have understood the way to work with liquibase.
Someone may point me to the right direction??
Thanks

Answer 1

You should never modify a <changeSet> that was already executed. Liquibase calculates checksums for all executed changeSets and stores them in the log. It will then recalculate that checksum, compare it to the stored ones and fail the next time you run it if the checksums differ.
What you need to do instead is to add another <changeSet> and put your new createTable element in it.
QuickStart is good readin' but it is indeed quick :-) Check out the full manual, particularly its ChangeSet section.

Answer 2

This currently accepted answer is slightly out of date based on changes in Liquibase 2.x. In the 2.x version, Liquibase will still fail if the md5 checksum has changed for a changeset, but you can specify the runOnChange attribute if you want to be able modify it.
From the documentation:
runOnChange - Executes the change the first time it is seen and each time the change set has been changed

Answer 3

If it's a change to a changeset that basically has already been done, you can manually modify the database so that its md5 for that changeset matches the new one. Good for minor textual changes. Or you can delete that changeset row from your table.