Day 6: Command Handlers

Typically we try to keep the efforts at 5-10 Minutes.

You’ll find complete implementation for day 6 in the git-repo on the branch “day-6”

https://github.com/dilgerma/eventsourced-quiz

Welcome

Yesterday we generated our Commands and Events from the config.json.

Let’s generate some Command Handlers today!

Step 1: Setup the Application

In the terminal, switch to your checked out repository

Make sure to run “npm install”

Start the application using “npm run dev”

The application is accessible in the browser on “localhost:3000”

Step 2: Examine the Template

In the “templates” folder, you will find a new simple template in the file “QuizApi.ts.tpl”

Step 3: Generate Command Handlers

Today we will configure a code generator that can generates command handlers.

In the index.js, you will find the new function “generateCommandHandlers”

generateCommandHandlers(){
   // provide the implementation here
}

Goal is to generate a Command Handler for each command.

They should look like this:

function handleCreateQuiz(command: CreateQuizCommand) {
  return []
}

Don’t worry, we will generate the applied Events directly tomorrow.

Follow these instructions:

1) Find all commands

let commands = config.slices.flatMap(slice => slice.commands)

2) Iterate over all commands

let commandHandlers = commands.map(command => {
 ` // logic to create command handlers
})

3) Implement the generator for command handlers and imports

let commandImports = commands.map(command => {
      let commandTitle = slugify(command.title, "")
      return `import {${commandTitle}} from "@/app/api/commands/${commandTitle}";`
}).join("\n")

let commandHandlers = commands.map(command => {
     let commandTitle = slugify(command.title, "")
     return `
         let handle${commandTitle} = (command: ${commandTitle}): Event[] => {
              return []
         }
}).join("\n");

The logic here is very simple. Iterate over all commands, generate the command title using “slugify”.

This will change something like “Create Quiz” to “CreateQuiz”

This generates a prerendered String, that we apply to a template. I typically do not use the template functionalities, but I generate them in Code.

this.fs.copyTpl(
     this.templatePath(`src/components/QuizApi.ts.tpl`),
     this.destinationPath(`./app/api/QuizApi.ts`),
     {
          commandImports: commandImports,
          createQuizCommandHandler: commandHandlers
      }
)

We now have generated all Command Handlers.

Step 4: Run the Code Generator

Run the Code Generator

docker run -ti -p 3001:3000 -v $PWD:/workspace --name codegen --rm nebulit/codegen

Within the container, run:

gen .generator/app

Tomorrow we will generate and apply the correct result events.

See you tomorrow!

Tools you need

• Git: https://github.com/dilgerma/eventsourced-quiz
• Accelerate Miro Toolkit: https://miro.com/marketplace/eventmodeling/
• Miro Board: https://miro.com/app/board/uXjVLDkT_FA=/

Things you should consider

If you already want to dive deeper into Eventmodeling and Eventsourcing - here are some links for you.

• Book “Understanding Eventsourcing”
• Companion Online Course - “Implementing Eventsourcing” (includes a full license of the Miro Toolkit)
• Podcast “Eventmodeling & Eventsourcing”
• Eventmodelers.de
• Eventmodeling Tutorial