Cassandra CQL migration tool

cqlmigrate is a library for performing schema migrations on a cassandra cluster.

It is best used as an application dependency, but can also be used standalone.

Adding as a Gradle dependency

repositories {
    jcenter()
}

compile 'uk.sky:cqlmigrate:0.9.9'

Adding as a Maven dependency

<repositories>
    <repository>
        <id>jcenter</id>
        <name>jcenter</name>
        <url>http://jcenter.bintray.com</url>
    </repository>
</repositories>

<dependency>
  <groupId>uk.sky</groupId>
  <artifactId>cqlmigrate</artifactId>
  <version>0.9.8</version>
</dependency>

Cassandra Prerequisites

The locks keyspace and table needs to be created before running any migrations.

CREATE KEYSPACE IF NOT EXISTS cqlmigrate WITH replication = {'class': 'REPLICATION_CLASS', 'replication_factor': REPLICATION_FACTOR };
CREATE TABLE IF NOT EXISTS cqlmigrate.locks (name text PRIMARY KEY, client text);

Library usage

To apply all .cql files located in /cql in the classpath:

import com.datastax.driver.core.*;

// Configure locking for coordination of multiple nodes
CassandraLockConfig lockConfig = CassandraLockConfig.builder()
        .withTimeout(Duration.ofSeconds(3))
        .withPollingInterval(Duration.ofMillis(500))
        .withConsistencyLevel(ConsistencyLevel.ALL)
        .withLockKeyspace("cqlmigrate")
        .build();

// Create a Cassandra session
Cluster cluster = Cluster.builder.addContactPoint("localhost").build();
Session session = cluster.connect();

// Create a migrator and run it
CqlMigrator migrator = CqlMigratorFactory.create(lockConfig);
Path schemas = Paths.get(ClassLoader.getSystemResource("/cql").toURI());
migrator.migrate(session, "my_keyspace", asList(schemas));

The migrator will look for a bootstrap.cql file for setting up the keyspace.

Standalone usage

$ java -Dhosts=localhost,192.168.1.1 -Dkeyspace=my_keyspace -Ddirectories=cql-common,cql-local -jar cqlmigrate.jar

Specify credentials, if required, using -Dusername=<username> and -Dpassword=<password>.

What it does

1. Checks all nodes are up and their schemas are in agreement.
2. Tries to acquire a lock for the keyspace. If it can't initially be acquired it will continue to retry at a set polling time until the timeout is reached.

3. Looks for a bootstrap.cql file and runs it first. This file should contain the keyspace definition:

   CREATE KEYSPACE my_keyspace
    WITH replication = {'class': 'SimpleStrategy', 'replication_factor': 1 };

4. Applies .cql files one by one, sorted by filename in ascending order. It is suggested to prefix the files with a datetime to order them:

   /cql/2015-05-02-19:20-create-airplanes-table.cql
   /cql/2015-05-03-14:19-add-manufacturer-column.cql

   Any previously applied files will be skipped.

5. Releases the lock.

schema_updates table

This table is used to determine what has been previously applied.

SELECT * FROM schema_updates;

 filename                         | applied_on               | checksum
----------------------------------+--------------------------+------------------------------------------
           0001-create-tables.cql | 2015-04-08 12:10:04+0100 | ec19dfac7ede62b2a40c0f39706b237cd5c30da6
                 0002-dataset.cql | 2015-04-08 12:10:04+0100 | 4fa2d6c4fae9950f0c9140ae2eb57fe689192b4a
            0003-initial-date.cql | 2015-04-08 12:10:04+0100 | 19d0c9522b6464a06b18192c6e04233f83e78a84

(3 rows)

It also maintains a checksum to ensure the script hasn't changed since it was last applied.

locks keyspace and table

The locks keyspace replication class and factor can be configured using the LocksConfig. This table is used to keep track of what locks are currently in place, and relies on Cassandra's lightweight transactions.

SELECT * FROM locks;

 name                                | client
-------------------------------------+--------------------------------------
 airplanes_keyspace.schema_migration | 2a4ec2ae-d3d1-4b33-86a9-eb844e35eeeb

(1 rows)

Each lock will be deleted by cqlmigrate once the migration is complete.

Supported Cassandra versions

This project has been tested against the following versions:

• DSE 5.0.2 (3.0.8)
• 2.1.7
• DSE 4.7.3 (2.1.8)
• 2.2.2
• 2.2.3

Caveats

Cassandra is an eventually consistent, AP database, and so applying schema updates are not as simple as a traditional relational database.

• Certain schema changes can cause data corruption on cassandra. Be very careful changing a schema for a table that is being actively written to. Generally, adding columns is safe (but be careful with collection types). Test the migration before rolling out to production.
• AP properties of Cassandra also apply to schema updates - so it is possible for a cluster to have an inconsistent schema across nodes in case of split brain or other situation. cqlmigrate tries to alleviate this with appropriate consistency levels.

Cql File Comments

There are a number of ways to add comments to your cql files.

For inline comments prepend -- to your comment, e.g:

-- Select Queries
SELECT * FROM schema_updates;

For multiline comments wrap them with /* and */, e.g:

/*
    Added by John Smith
    19th September 2017
*/
SELECT * FROM schema_updates;