Community database support

Flyway is an open source project and so database compatibility can be contributed by the open source community. Redgate will review submissions but the capabilities of the driver are dependent on the requirements of the originator.

Contributing Database Compatibility to Flyway

Flyway follows an Open Source model for the Community edition. We welcome code contributions through Pull Requests on the Flyway Community DB Support GitHub page. This article will provide help with contributing code to make Flyway compatible with a new database platform.

Flyway supports migrations for a large number of database platforms in a unified and consistent way. It does this by abstracting away the details of each database into a set of classes for each platform, plus factory classes that construct the appropriate objects for the database at hand; all communication with the database is done through a JDBC connection. The advantage of this approach is that JDBC is a widely adopted standard; with little more than a JDBC driver and knowledge of the SQL dialect used by a database it is possible to make Flyway compatible with your database of choice.

You will need…

• A JDBC driver for your database.
• A Java IDE that builds with Java 17 or higher.

Note for contributors: Flyway will not typically package database drivers for community databases in the CLI bundle so we will need to know where end-users can download the preferred JDBC driver for your database.

Getting started

Build your database support plugin

Fork the Flyway Community DB Support repository. You should be able to open the project and see a number of modules.

1. Create a module for your database and add it to the list of modules in the flyway-community-db-support pom.xml
2. Create a new folder foo in org.flywaydb.community.database to contain your new classes.
3. In the folder create classes FooDatabase (subclassed from Database), FooSchema (subclassed from Schema), and FooTable (subclassed from Table). These classes make up Flyway’s internal representation of the parts of your database that it works on.
4. Create class FooParser (subclassed from Parser). This represents a simplified version of a parser for your database’s dialect of SQL. When finished it will be able to decompose a migration script into separate statements and report on serious errors, but it does not need to fully understand them.
5. Create a class FooDatabaseType subclassed from BaseDatabaseType and implement the CommunityDatabaseType interface in the folder your created. This class acts as the collation class that brings together all the classes you created before. Implement the required methods. There are also some optional methods you can override to customize the behavior.
   • createSqlScriptFactory - To use a custom SqlScriptFactory
   • createSqlScriptExecutorFactory - To use a custom SqlScriptExecutorFactory
   • createExecutionStrategy - To use a custom DatabaseExecutionStrategy
   • createTransactionalExecutionTemplate - To use a custom ExecutionTemplate
   • setDefaultConnectionProps - To set custom default connection properties
   • shutdownDatabase - To run any necessary code to cleanup the database on shutdown
   • detectUserRequiredByUrl - To skip prompting for user if the URL contains user information (e.g. user property, login file)
   • detectPasswordRequiredByUrl - To skip prompting for password if the URL contains password information (e.g. key file, or password property)
6. Create class FooConnection subclassed from Connection<FooDatabase>This represents a JDBC connection to your database. You probably won’t use it in isolation but it is an important component of a JdbcTemplate, which provides numerous convenience methods for running queries on your database.
   In the constructor of FooConnection, you can use the jdbcTemplate field of Connection to query for any database properties that you need to acquire immediately and maintain as part of the state of the connection. You will need to override the following methods as a minimum:
   • doRestoreOriginalState() - to reset anything that a migration may have changed
   • getCurrentSchemaNameOrSearchPath() - to return the current database schema for the connection, if this is a concept in your database, or the default schema name if not.
   • doChangeCurrentSchemaOrSearchPath() - to change the current database schema, if this is a concept in your database. If not, use the default which is a no-op.
   • getSchema() - to return a constructed FooSchema object.
7. Implement methods for FooDatabase to customize it to fit the SQL conventions of your database:
   • doGetConnection() - to return a new FooConnection
   • ensureSupported() - to determine which versions of your database will be supported by Flyway. During development, you can leave this as a no-op.
   • getRawCreateScript() - to return SQL appropriate for your database to create the schema history table. Refer to an existing database type to see the column types needed. The table name will be provided by the table argument. If the baseline argument is true, this method should also insert a row for the baseline migration.
   • supportsDdlTransactions() - to return whether the database can support executing DDL statements inside a transaction or not.
   • getBooleanTrue() and getBooleanFalse() - to return string representations of the Boolean values as used in your database’s dialect of SQL. Typically these are “true” and “false”, but could be, for example, “1” and “0”
   • catalogIsSchema() - to return true if the database uses a catalog to represent a single schema (eg. MySQL, SQLite); false if a catalog is a collection of schemas.
   • You may need to provide your own implementations of these methods:
     • getSelectStatement() - to return SQL appropriate for your database to select all rows from the history table with installed_rank greater than a parameter value.
     • getInsertStatement() - to return SQL appropriate to insert a row into the history table with nine parameter values (corresponding to the table columns in order).
     • supportsEmptyMigrationDescription() - if your database can’t support an empty string in the description column of the history table verbatim (eg. Oracle implicitly converts it to NULL), override this to return false.
     • doQuote() - to return an escaped version of an identifier for use in SQL. Typically this is the provided value with a double-quote added either side, but could be, for example, square brackets either side as in SQL Server.
8. You may want to add overrides for FooParser to customize it to fit the SQL dialect your database uses:
   • The constructor should call the superclass constructor with a peek depth. This determines how far in advance the parser looks to determine the nature of various symbols. 2 is a reasonable start, unless you know your database has two-character entities (like SnowflakeDB’s $$ for javascript delimiters) in which case start at 3.
   • getDefaultDelimiter() if your database uses something other than a semicolon to delimit separate statements
   • getIdentifierQuote() if your database uses something other than a double-quote to escape identifiers (eg. MySQL uses backticks)
   • getAlternativeIdentifierQuote() if your database has a second way to escape identifiers in addition to double-quotes.
   • getAlternativeStringLiteralQuote() if your database has a second way to mark string literals in addition to single-quotes (eg. MySql allows double-quotes)
   • getValidKeywords() if your database has a different set of valid keywords to the standard ones. It’s not strictly necessary to include keywords that cannot be found in migration scripts.
   • There are other overrides available for handling more complex SQL.
9. Add overrides for FooSchema to customize it to fit the SQL dialect your database uses:
   • doExists() - to query whether the schema described exists in the database
   • doEmpty() - to query whether the schema contains any sub-objects eg. tables, views, procedures.
   • getObjectCount() - to query the number of objects of a given type that exist in the schema
   • doCreate() - to create the schema in the database
   • doDrop() - to drop the schema in the database
   • doClean() - to drop all the objects that exist in the schema
   • doAllTables() - to query for all the tables in the schema and return a populated array of FooTable objects
   • getTable() - to return a FooTable object for the given name
10. Add overrides for FooTable to customize it to fit the SQL dialect your database uses:
    • doDrop() - to drop the table
    • doExists() - to query whether the table described exists in the database
    • doLock() - to lock the table with a read/write pessimistic lock until the end of the current transaction. This is used to prevent concurrent reads and writes to the schema history while a migration is underway. If your database doesn’t support table-level locks, do nothing.
11. Finally, expose your database support to the Flyway engine by adding the full qualified name of your BaseDatabaseType class to <your module>/src/main/resources/META-INF/services/org.flywaydb.core.extensibility.Plugin

How to test your plugin

Testing with a prebuilt Flyway CLI package

This is a quick and easy way to verify that things are working as you expect

1. Download and install the latest Flyway CLI package
2. Put your freshly built flyway-database-foo.jar and JDBC driver in the /jars folder of your Flyway install directory.
3. Configure Flyway to talk to your database
4. Run your tests

Testing Flyway with source code

This is more complex but allows you to set breakpoints and debug more easily.

Get your environment setup for developing flyway

Copy the file /flyway-commandline/src/main/assembly/flyway.toml.example to an accessible location on your machine and rename it to flyway.toml. This location will be a temporary ‘scratch’ area for testing. Use this copy to set up the following properties:

[environments.sample]
   url = # - the JDBC URL of your development database
   user = # - the user account
   password = # - the password to the database
[flyway]
   locations = # - to point to an accessible folder where you can put test migrations.
   environment = "sample"

You can now set up a run configuration in your IDE that will compile Flyway and run using your newly created configuration:

• Main class: org.flywaydb.commandline.Main
• Program arguments: info -X -configFiles=<scratch location>\flyway.toml
• Classpath of module: flyway-commandline

Flyway itself should start. Since Flyway doesn’t yet support your database you should see a message like:

org.flywaydb.core.api.FlywayException: ERROR: No database found to handle jdbc:FooDb://<host>:<port>/<databasename>

You’re now ready to start adding that database support. We’re going to assume your database platform is called FooDb. Change the obvious naming conventions to suit your database.

When you’re ready, add your freshly built flyway-database-foo.jar and JDBC driver to the classpath using your IDE.

Run your tests

• Run the flyway info build configuration and see an empty version history. Congratulations! You have got a basic implementation up and running. You can now start creating migration scripts and running flyway migrate on them.
• Basic SQL scripts should run with few problems, but you may find more edge cases, particularly in Parser.
  • Look at the existing overrides for existing platforms for examples of how to deal with them.
• If you find you need to make more invasive changes in the core of Flyway, please raise an issue on the appropriate repository.
  • We will need to test bigger changes ourselves against all our test instances before we can accept them.

Submit your PR for approval

You will need to have:

• Have completed every section of this tutorial
• Submitted your code as a Pull Request for our review, remembering to include supporting material (e.g. test code, results, screenshots etc.) to prove compatibility
• Completed any requested code changes
• Signed the Flyway CLA for your PR

Once your PR is accepted

• We will add it to the flyway command line module and assemblies.
• We’ll include in the next release of FLyway