How PCF works internally when deploying Spring boot project - spring-boot

Question

Will PCF run the Tomcat embedded in Spring boot jar or it runs its own tomcat. In PCF we never mention the port number. In the classic approach, we start 3 Tomcat instances in different port number and have apache server before that. Does PCF work the same way.

Answer 1

This is a pretty broad question, so I'm going to answer with a pretty broad response but link you to where you can find more details, if you are inclined to dig in more.
How PCF works internally when deploying Spring boot project
Will PCF run the Tomcat embedded in Spring boot jar or it runs its own tomcat.
When you run cf push -p my/cool/file.jar (or even file.war), the cf cli extracts everything from that archive and pushes it up to CF. CF stores your app files & then your app is staged.
During staging, the Java build pack runs. It looks at all the files that were pushed & tries to determine what to do with them. It knows how to handle several different types of apps [1], including both standard WAR files & Spring Boot apps. The build pack will check your app to see if its one of the supported types in order [2] and will select the first match.
After selecting the type of app, it runs through and installs what is necessary to run your app. For a Spring Boot app, that's basically just the JVM. For a WAR file, it installs Tomcat & a JVM. In addition, it writes out the configuration & start up commands necessary for CF to launch your app.
At this point staging is complete and you have what is called a "droplet". If you have any additional questions on the staging workflow, read here for more details [3].
At this point, the app would be started. The platform takes the droplet that was created and executes the command specified by the build pack to start the app [4]. If all goes well, your app will then be up and running on CF.
In PCF we never mention the port number.
Correct. The platform will tell you the port on which your application should listen. For Java apps, you don't have to do anything. The Java buildpack will handle configuring Spring Boot or Tomcat to start and listen on the correct port.
For Spring Boot apps, you can look at the start up command to see how it's doing this. For Tomcat, it happens in the server.xml that's generated by the Java buildpack [5].
In the classic approach, we start 3 Tomcat instances in different port number and have apache server before that. Does PCF work the same way.
Yes and no. Each application you run can have multiple instances. If you scaled your app to have three instances, that would be roughly like having 3 Tomcat instances in your classic approach.
The main difference is that there would be no Apache Web Server in front. On Cloud Foundry, this is not necessary because it has it's own load balancer called Gorouter [6] which handles proxying traffic to your app and load balancing across available app instances.
This is a classic mistake that people new to CF make. They try to replicate classic architectures and shoehorn a reverse proxy into their app when it's not necessary. This is one of the benefits of CF. It handles routing traffic to your app in a scalable way, freeing you up to focus more on your app.
[1] See "Standard Containers -> https://github.com/cloudfoundry/java-buildpack#additional-documentation
[2] https://github.com/cloudfoundry/java-buildpack/blob/master/config/components.yml#L18
[3] https://docs.cloudfoundry.org/concepts/how-applications-are-staged.html
[4] https://docs.cloudfoundry.org/concepts/overview.html#apps-anywhere
[5] https://github.com/cloudfoundry/java-buildpack/blob/master/resources/tomcat/conf/server.xml#L21
[6] https://docs.cloudfoundry.org/concepts/architecture/#routing

Answer 2

No. The simple answer is each instance of ur application jar is running in its own container within PCF. So there is no clash of using the same port numbers.