Modulithic Applications
 with Spring Boot Building better monoliths Oliver Drotbohm ƀ odrotbohm@pivotal.io / odrotbohm

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Monolith VS. Microservice 3 Module
 A Module
 B Module
 C Module
 B Module
 C Module
 A Bounded Context Deployment Unit Internal invocation External invocation

Monolith 4 The application packages the implementation of multiple Bounded Contexts as a single deployment unit. Easy to refactor Easy to test the overall system Likely to degrade unless explicitly managed Harder to test individual Bounded Contexts

Microservices 5 A bounded context defines the boundaries of the deployment artifact. Often parts of the context are even deployed as separate processes. ○ Bounded Context interaction is remote ○ Testing individual modules Hard to refactor context boundaries Hard to test the overall system

How to maintain structure in a monolithic codebase? 6 !

Design goals 7 1. Minimally invasive As little additional metadata as possible required. Advanced customizations might need additional information. 2. Test support Individual modules should be testable standalone. 3. Minimize distance to development workflow Feedback about architecture violations as fast as possible.

Development workflow 8 Compiler Tests Build Runtime Distance to edit / run cycle Where do I want my checks to live?

The Sample Codebase https://github.com/st-tu-dresden/salespoint 9

10 Orders Business
 Time Catalog Inventory Component Dependency Type dependency

Moduliths https://github.com/odrotbohm/moduliths Binaries available from https://repo.spring.io 11

Moduliths 12 1. A convention to map contexts to packages Packages directly nested underneath the main application package are considered context ones.

Package Conventions 13 ….modulith ….modulith.moduleA ….modulith.moduleB Visibility modifiers and the compiler enough to hide internals.

Moduliths 14 1. A convention to map contexts to packages Packages directly nested underneath the main application package are considered context ones. 2. Simple set of access rules and API to verify Only access components in module API packages.

Access Rules 15 Moduliths verifies a couple of rules imposed on your modules. No cyclic dependencies Only allows explicitly allowed dependencies Prevents field injection "

Module dependency verification DEMO 16

Access Rules 17 Moduliths verifies a couple of rules imposed on your modules. No cyclic dependencies Only allows explicitly allowed dependencies Prevents field injection " Use module base package as API package

API Package Convention 18 ….modulith ….modulith.moduleA ….modulith.moduleB Visibility modifiers and the compiler enough to hide internals.

API Package Convention 19 ….modulith ….modulith.moduleA ….modulith.moduleA.internal ….modulith.moduleB ….modulith.moduleB.internal Access to components residing in internal packages
 forbidden and checked during tests.

Moduliths 20 1. A convention to map contexts to packages Packages directly nested underneath the main application package are considered context ones. 2. Simple set of access rules and API to verify Only access components in module API packages. 3. Test support to bootstrap modules Integration with Spring test context framework and Spring Boot integration test support to limit bootstrap (component & entity scanning) to only the involved packages.

Web Business logic Data access @Data…Test Module A Module B Module C @WebMvcTest @…Test ?

22 package com.acme @SpringBootApplication class MyApplication { … } package com.acme.moduleA @ModuleTest // Instead of @SpringBootTest class MyModuleTests { … } Applies package conventions Bootstraps single module for test

24 Orders Business
 Time Catalog Inventory Component Dependency Type dependency

Single module bootstrap DEMO 25

Orders 28 Business
 Time Catalog Inventory Component Dependency Type dependency

Detect and remove violation DEMO 29

30 Orders Business
 Time Catalog Inventory Component Dependency Type dependency 
 Event Event publication

Dependency types 32 1. Simple type dependency References to aggregates and entities of other modules. 2. Component dependency References to other Spring beans, required to bootstrap the module 3. Events & event listeners Dependency on some other module’s event type. Interesting as they’re great hooks for testing. #☝Avoid!

33 Orders Business
 Time Catalog Inventory Component Dependency Type dependency 
 Event Event publication

Multi module bootstrap DEMO 34

Bootstrap modes 35 1. Standalone Bootstraps the module the test is contained in. Components in other modules the bootstrapped module depends on need to be provided through @MockBean. 2. Direct dependencies Bootstraps the module including the modules it directly depends on. Transitive dependencies have to be mocked just like in standalone mode. 3. All dependencies Bootstraps the entire sub-tree of modules starting from the current one.

Moduliths 36 1. A convention to map contexts to packages Packages directly nested underneath the main application package are considered context ones. 2. Simple set of access rules and API to verify Only access components in module API packages. 3. Test support to bootstrap modules Integration with Spring test context framework and Spring Boot integration test support to limit bootstrap (component & entity scanning) to only the involved packages. 4. Documentation support PlantUML integration via Simon Brown’s Structurizr to document module structure and dependency types.

Documentation DEMO 37

Tool ecosystem and alternative approaches 38

Multiple Artifacts 39 Every module port becomes a dedicated build artifact. Dedicated control over dependencies Test scope is defined by the artifact Potential explosion of number of artifacts Internal structure visible to the outside

Java Module System 40 Every module becomes a Java 9+ module. ○ Strong coupling between module and artifact ○ Semantics of the „opens“ keyword ○ No test support

External Tools 41 External tools (e.g. jQAssistant, Sonargraph, jDepend) integrated into the build and executed in the CI environment are used to signal structural violations. Most extensive option Distance to development workflow

Development workflow 42 Compiler Tests Build Runtime Moduliths Maven Modules Jigsaw / OSGi jQA etc. Applies rules Invasiveness (bigger is stronger)

Moduliths 43 1. A convention to map contexts to packages Packages directly nested underneath the main application package are considered context ones. 2. Simple set of access rules and API to verify Only access components in module API packages. 3. Test support to bootstrap modules Integration with Spring test context framework and Spring Boot integration test support to limit bootstrap (component & entity scanning) to only the involved packages. 4. Documentation support PlantUML integration via Simon Brown’s Structurizr to document module structure and dependency types.

Resources 44 Moduliths Project website on GitHub Majestic Modular Monoliths Video on Youtube Modular Monoliths Simon Brown – Video on YouTube Refactoring to a System of Systems Video on YouTube

Thank you! 45 Oliver Drotbohm ƀ odrotbohm@pivotal.io / odrotbohm