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Managing (Schema) Migrations in Cassandra

Mitch Gitmansenior software engineerGridPoint, Inc.

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05/03/2023

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migrationA word with many meanings.

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disclaimer…

image © Ana Camamiel

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What I mean by migrations• Live-data migrations

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One-off as opposed to ETL

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What I mean by migrations• Source-driven migrations

− Schema migrations

− Reference data migrations

− Test/sample data migrations

• CQL commands as opposed to real data (sstables), generally

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source control versioning

artifact versioningpublish

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Database refactoring

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• Integration test & functional test automation (bootstrap-ability)• CI server pipelines

• Containerization??

• Consistency & repeatability across environments− Local developer box

− Dev environments

− Integration & QA environments

− Staging

− Production

Source-driven DB refactoring—the benefits

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We need tools!• Built into web application frameworks

• Standalone

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What do (perhaps) all these tools have in common?

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They’re relational. They’re for SQL.

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NoSQL Distilled

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Chapter 12. Schema Migrations

"We have seen that developing and maintaining an application in the brave new world of schemaless databases requires careful attention to be given to schema migration."

either/or:• RDBMS = strong schema• NoSQL = no schema

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CREATE TABLE entities ( doc_id int, attribute_name String, attribute_value String, ... PRIMARY KEY(doc_id, attribute_name));

• partition keys & clustering keys• table-per-query denormalization• shift from Thrift to CQL

• Thrift: super columns & super column families • CQL: collection types

“metadata-driven documents in columnar storage:”

THE EXCEPTION

Does Cassandra like weak schemas?

So how have teams been managing their keyspace & table definitions?

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The Cassandra migration tools landscape

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• Flyway: First-class Cassandra support.− Requires JDBC.− https://github.com/flyway/flyway/issues/823

• Pillar: Scala tool.• mutagen-cassandra: Java tool, Astyanax driver.• Trireme: Python tool.• cql-migrate: Python tool.• mschematool: Python tool.

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What’s the secret behind DB migration tools?

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The migrations version tracking table

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Migration tool philosophies

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>

SQL

SQL

>

© Martha Stewart Living Omnimedia Inc. © Harpo Print, LLC

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Flyway for Cassandra

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• First-class Flyway• Faked-out Flyway

migrations

(in SQL)CQL

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The tradeoff

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• Store the migrations tracking table in an RDBMS

+

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Programmatically invoke Flyway

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CassandraFlywayCallback

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implements FlywayCallback

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Two-step process

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source control

artifact repository

build time

deploy time

MigrationsBuilder

FlywayMigrator

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The migrations source

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The input to MigrationsBuilder

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build time

Run MigrationsBuilder for CQL:

Run MigrationsBuilder for SQL:

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The generated migrations

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The output from MigrationsBuilder

build time

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The generated SQL script

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Faking out Flyway

build time

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deploy time

Run FlywayMigrator for CQL:

Run FlywayMigrator for SQL:java -classpath /…/flyway-migrator-postgresql.jar \

com.gridpoint.tools.migrator.flyway.FlywayMigrator postgresql

java -classpath /…/flyway-migrator-cassandra.jar \

com.gridpoint.tools.migrator.flyway.FlywayMigrator cassandra

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deploy time

flyway-migrator-postgresql.jarflyway-migrator-cassandra.jar

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The migrations version tracking table

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The Cassandra incarnation

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Best practices

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• Variations on versions− Version control: f94c7d7f8b130df360a4e9e4f586eafc618ddc50− Artifact repository: 3.5.1− Migration tool: 201505270800 or 10 or whatever you want− Effective contract versions—multiple versions can coexist at runtime

• Consistent deployment across environments• Failure handling• Baselining• Rollbacks?• Check schema agreement

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Schema agreement

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https://datastax.github.io/java-driver/2.1.8/features/metadata/

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Cassandra… migrations… limitations

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• Limitations of our Flyway-based solution− You need a relational database− Not open-sourced

• Limitations of source-driven migrations, in general

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Static vs. dynamic tables

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Deploy time vs. runtime

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Dedicated migration application vs. part of main application

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Source-driven, but…

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• The orchestration is in source control

• Actual data rather than CQL commands− Not necessarily live data− Maybe doesn’t need to be in source control

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Embracing polyglot persistence

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A unified migrations solution

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Takeaways

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•challenging•exciting

•routine•boring+

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Thank you!

Mitch Gitmanmgitman@gridpoint.commgitman@nilistics.netmgitman@gmail.comskeletal presence @ LinkedIn