Flowing retail sample application

This sample application showcases concepts and alternatives to implement

• a simple order application

in the context of

• Domain Driven Design (DDD)
• Event Driven Architecture (EDA)
• Microservices (µS)

Key facts:

• Written in Java
• As simple as possible to show concepts, not build for production usage. Hint: we know some parts in the code skip well known best practices and patterns, but we focussed on making the code easy to understand. For example we prefer to duplicate code, if this means you have to read one class less to understand what a component is doing.

Links

• Introduction blog post by Bernd Rücker: https://blog.bernd-ruecker.com/flowing-retail-demonstrating-aspects-of-microservices-events-and-their-flow-with-concrete-source-7f3abdd40e53

Overview and architecture

Flowing retail simulates a very easy order processing system. The business logic is separated into the following microservices:

• The core domains communicate via messages with each other.
• Messages might contain events or commands.

This is the stable nucleus for flowing retail.

Alternatives

Now there are a couple of options you can choose of when running / inspecting the example.

Channel technology

You can choose between:

• Apache Kafka as event bus (option kafka, default).
• RabbitMQ as AMQP messaging system (option rabbit).

Long running processes

In order to support long running processes there are multiple options, which are very interessting to compare:

• Domain entities store state (option entity)
• Camunda workflow engine orchestrates using BPMN models (option camunda, default)
• Camunda workflow engine orchestrates using a technical DSL (option camunda-dsl)

Note that every component does its own parts of the overall order process. As an example this is illustrated using BPMN and showing the Order and Payment Service with their processes:

Run the application

• Download or clone the source code
• Run a full maven build

mvn install

• Start all components by in one Java process
  • Channel (e.g. Kafka which also requires Zookeeper)
  • All microservices

mvn -f starter exec:java

If you want to select options you can also do so:

mvn -f starter exec:java -Dexec.args="rabbit camunda-dsl"

You can also import the projects into your favorite IDE and start the following class yourself:

starter/io.flowing.retail.command.SimpleStarter

• Now you can place an order via http://localhost:8085
• You can inspect all events going on via http://localhost:8086

TODO ZONE

Using Kafka

• Can be started built in, but you can also install and run yourself
• Port = default = ##

When installed yourself, create topic "flowing-retail"

kafka-topics.sh --create --zookeeper localhost:2181 --replication-factor 1 --partitions 1 --topic flowing-retail

You can query all topics by:

kafka-topics.sh --list --zookeeper localhost:2181

Using RabbitMQ

• Must be installed and started yourself
• Port = default = ##

Using Camunda

You can inspect what's going on using Cockpit:

• Download Camunda Distribution of your choice
• Configure Datasource to connect to: jdbc:h2:tcp://localhost:8092/mem:camunda
  • In Tomcat distribution this is configured in server/apache-tomcat-8.0.24/conf/server.xml
  • In Wildfly distribution this is configred in server/wildfly-10.1.0.Final/standalone/configuration/standalone.xml
• Best: Do not start job executor
• Run it and you can use cockpit normally

If you want to restart microservices and keep cockpit running, you have to make sure your JDBC connection pool destroys stale connections. In Wildfly you can add a valdidation to your datasource, so the config will look like this:

<datasource jndi-name="java:jboss/datasources/ProcessEngine" pool-name="ProcessEngine" ...>
    <connection-url>jdbc:h2:tcp://localhost:8092/mem:camunda</connection-url>
    <driver>h2</driver>
    <transaction-isolation>TRANSACTION_READ_COMMITTED</transaction-isolation>
    <security>
        <user-name>sa</user-name>
        <password>sa</password>
    </security>
    <validation>
        <check-valid-connection-sql>select 1</check-valid-connection-sql>
        <validate-on-match>false</validate-on-match>
    </validation>
</datasource>