Add multi-datasource module: two databases with per-database DataSource, Flyway, EntityManagerFactory and transaction manager on Boot 4.1

Working setup plus one small application per way of getting it wrong (no @Primary, @Primary only, url vs jdbc-url, wrong repository package, Flyway auto-configuration), plain vs named @Transactional, and ChainedTransactionManager with a failing commit. Transcripts are written by the tests.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Uu7q8vPeREyT4218EJPzz1

multi-datasource/output/01-two-databases.txt

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+# Two databases, each with its own DataSource, Flyway, EntityManagerFactory and transaction manager
+
+
+--- the two DataSources (one pool each, configured separately) ---
+customersDataSource  pool=customers-pool  max=4  url=jdbc:h2:mem:customers  primary=true
+ordersDataSource     pool=orders-pool     max=8  url=jdbc:h2:mem:orders  primary=false
+
+--- Flyway ran once per database, each with its own history ---
+customers flyway_schema_history: [1 customer]
+orders    flyway_schema_history: [1 purchase order, 2 index customer]
+
+--- what each EntityManagerFactory manages ---
+EntityManagerFactory beans: [customersEntityManagerFactory, ordersEntityManagerFactory]
+customersEntityManagerFactory    entities=[Customer]
+ordersEntityManagerFactory       entities=[PurchaseOrder]
+
+--- transaction managers ---
+beans: [customersTransactionManager, ordersTransactionManager]
+customersTransactionManager    primary=true
+ordersTransactionManager       primary=false
+
+--- save one customer and one order through the two repositories ---
+customers database tables: [CUSTOMER, flyway_schema_history]
+orders    database tables: [PURCHASE_ORDER, flyway_schema_history]
+rows: customers.customer=1  orders.purchase_order=1

multi-datasource/output/02-no-primary.txt

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+# Two DataSource beans, none marked @Primary
+
+startup failed: BeanCreationException -> NoSuchBeanDefinitionException
+root cause: org.springframework.beans.factory.NoSuchBeanDefinitionException: No bean named 'entityManagerFactory' available

multi-datasource/output/03-primary-only-wrong-database.txt

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+# One @Primary DataSource and nothing else: it starts, and it writes to the wrong database
+
+application started: yes
+EntityManagerFactory beans: [entityManagerFactory]
+customers database tables: [CUSTOMER, PURCHASE_ORDER]
+orders    database tables: []
+rows in the customers database: customer=1  purchase_order=1
+hibernate.hbm2ddl.auto on that factory: create-drop

multi-datasource/output/04-url-vs-jdbc-url.txt

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+# DataSourceBuilder.create().build() bound to app.datasource.customers.url
+
+startup failed: BeanCreationException -> BeanInstantiationException -> IllegalArgumentException
+root cause: java.lang.IllegalArgumentException: dataSource or dataSourceClassName or jdbcUrl is required.

multi-datasource/output/05-builder-defaults.txt

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+# A hand-built EntityManagerFactory that sets no Hibernate properties of its own
+
+hibernate.physical_naming_strategy = org.hibernate.boot.model.naming.PhysicalNamingStrategySnakeCaseImpl
+hibernate.implicit_naming_strategy = org.springframework.boot.hibernate.SpringImplicitNamingStrategy
+hibernate.hbm2ddl.auto             = null
+saved a Customer whose field is createdAt into a column created_at: id=1

multi-datasource/output/06-flyway-auto-configuration.txt

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+# Spring Boot's Flyway auto-configuration when there are two databases
+
+
+--- A. nothing configured: Flyway targets the @Primary DataSource and scans classpath:db/migration ---
+startup failed: BeanCreationException -> BeanCreationException -> FlywayException
+root cause: org.flywaydb.core.api.FlywayException: Found more than one migration with version 1
+Offenders:
+-> <multi-datasource>/target/classes/db/migration/customers/V1__customer.sql (SQL)
+-> <multi-datasource>/target/classes/db/migration/orders/V1__purchase_order.sql (SQL)
+
+--- B. spring.flyway.locations=classpath:db/migration/customers ---
+startup failed: BeanCreationException -> PersistenceException -> SchemaManagementException
+root cause: org.hibernate.tool.schema.spi.SchemaManagementException: Schema validation: missing table [purchase_order]
+
+application started (Hibernate validation switched off so the databases can be inspected)
+customers database tables: [CUSTOMER, flyway_schema_history]
+orders    database tables: []
+
+--- C. as B, plus one hand-made Flyway bean for the orders database ---
+startup failed: BeanCreationException -> PersistenceException -> SchemaManagementException
+root cause: org.hibernate.tool.schema.spi.SchemaManagementException: Schema validation: missing table [customer]
+
+application started (Hibernate validation switched off so the databases can be inspected)
+customers database tables: []
+orders    database tables: [PURCHASE_ORDER, flyway_schema_history]

multi-datasource/output/07-wrong-package.txt

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+# A repository for Customer scanned against the orders EntityManagerFactory
+
+startup failed: BeanCreationException -> IllegalArgumentException
+root cause: java.lang.IllegalArgumentException: Not a managed type: class com.ankurm.multids.customers.Customer

multi-datasource/output/08-which-transaction-manager.txt

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+# A service method that writes to both databases, then throws
+
+
+--- @Transactional (no name): Spring picks the @Primary manager, customersTransactionManager ---
+thrown: java.lang.IllegalStateException: boom after both writes
+rows after the rollback: customers.customer=0  orders.purchase_order=1
+
+--- @Transactional("ordersTransactionManager"): the other database is now the one that rolls back ---
+thrown: java.lang.IllegalStateException: boom after both writes
+rows after the rollback: customers.customer=1  orders.purchase_order=0

multi-datasource/output/09-chained-transaction-manager.txt

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+# ChainedTransactionManager over the customers and orders managers
+
+
+--- 1. business exception after both writes: both roll back ---
+rows: customers.customer=0  orders.purchase_order=0
+
+--- 2. the orders database refuses to commit; orders is LAST in the list ---
+thrown: org.springframework.transaction.HeuristicCompletionException (outcome: rolled back)
+rows: customers.customer=0  orders.purchase_order=0
+
+--- 3. the orders database refuses to commit; orders is FIRST in the list ---
+thrown: org.springframework.transaction.HeuristicCompletionException (outcome: mixed)
+rows: customers.customer=1  orders.purchase_order=0

multi-datasource/output/10-chained-transaction-manager-jar.txt

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+# Is org.springframework.data.transaction.ChainedTransactionManager still shipped, and is it deprecated?
+
+jar:      spring-data-commons-4.1.1.jar
+class:    org/springframework/data/transaction/ChainedTransactionManager.class
+
+--- javap -v: class-level annotations ---
+RuntimeVisibleAnnotations:
+  0: #245()
+    java.lang.Deprecated
+--- javap -v: Deprecated attribute ---
+Deprecated: true

multi-datasource/output/11-enable-jpa-repositories-not-repeatable.txt

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+# javac on a class carrying two @EnableJpaRepositories (scripts/facts/RepeatedEnable.java)
+
+RepeatedEnable.java:7: error: EnableJpaRepositories is not a repeatable annotation interface
+@EnableJpaRepositories(basePackages = "a.orders", entityManagerFactoryRef = "ordersEntityManagerFactory")
+^
+1 error

multi-datasource/output/12-builder-packages.txt

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+# What does EntityManagerFactoryBuilder.Builder.packages(Class...) do with the classes it is given?
+
+jar: spring-boot-jpa-4.1.1.jar
+
+--- javap -c: the calls made inside packages(java.lang.Class<?>...) ---
+invokespecial #34                 // Method java/util/HashSet."<init>":()V
+invokestatic  #35                 // Method org/springframework/util/ClassUtils.getPackageName:(Ljava/lang/Class;)Ljava/lang/String;
+invokeinterface #41,  2           // InterfaceMethod java/util/Set.add:(Ljava/lang/Object;)Z
+invokestatic  #47                 // Method org/springframework/util/StringUtils.toStringArray:(Ljava/util/Collection;)[Ljava/lang/String;