Middleware

Heimdall offers a special type of Interceptor called Middleware. In this document we will explain what is a Middleware and which situations it should or should not be used.

What is a Middleware

A Middleware is a maven java project that can be added as a Interceptor for a specific endpoint. There is a maven archetype to provide an easy way to create a middleware.

When should you use a Middleware

A Middleware interceptor should be used when the simple act of redirect a request is not enough or when none of the Heimdall provided interceptors have the effect you want.

That way the middleware is capable of modify a request, call multiple Api's, and modify the response in order to achieve the desired result.

If some sort of data persistence is needed inside the Middleware, Heimdall offers a NoSQL database (MongoDB) for that purpose.

When should you NOT use a Middleware

A Middleware is not an Api, it is a request interceptor. In cases where you have some indication that your middleware can evolve to something bigger, that is a warning that you might need to think about a new Api.

A few more situations where a Middleware is not recommended:

• File generation
• Relational database connections

Creating a Middleware

This tutorial shows how to create a Heimdall middleware. The main goals are:

• Create a Middleware from the archetype
• Make a request that is intercepted by the middleware
• Debug the Middleware

Create a Middleware from the archetype

To create a Middleware we recommend that you use the archetype. To do that follow the Archetype tutorial.

After following the tutorial you should have a project with a target/ folder in it. The file with jar-with-dependencies in its name is the one we want.

Testing your Middleware

To test your Middleware you are going to need Heimdall running locally. To achive that follow this tutorial. The only diference from that is that we will use a differente type of Interceptor.

Adding a Middleware to Heimdall

To add your Middleware to Heimdall you will need access to the front-end. Try the following link http://localhost:3000

Adding a Interceptor

Choose your Api, then the Interceptor tab. In this page choose the Resource and Operation you the Middleware to intercept and add the Middleware interceptor.

In the pop-up window fill the fields as shown below.

• Name: MiddlewareExampleGET
• Life Cycle: OPERATION
• Content: <your.middleware.groupId>.MiddlewareExampleGET

Hit save.

Adding the Middleware

Go to the Middleware tab.

To upload the Middleware file you need to add a middleware version.

• Version: 1.0.0 (this is a unique number)
• Choose the *jar-with-dependencies.jar file previously created

Testing your work

Using some sort of REST tool or the browser make a GET request to: http://localhost:800/<api_basepath>/<operation> . In this example you should get as a return the Conductor front page.

Debugging a Middleware

In order to debug your middleware there are some steps you need to do first that are directly related to the IDE that you are using.

1. Eclipse IDE

   1. In the project execution options, add the following under VM options:

      • -Dspring.profiles.active=dev

   2. Run heimdall-gateway in debug mode

2. Intellij IDEA

   1. In the run options for heimdall-gateway add the following to Environment -> VM options:

      • -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=5005

   2. In your middleware project select Run -> Edit Configurations

   3. Click + and choose the Remote option.

      • Add a name
      • Transport: Socket
      • Debug Mode: Attach
      • Host: localhost
      • Port: 5005

   4. Run heimdall-gateway normally

After this configurations all you are ready to add breakpoints on your middleware code and start debugging.