ApplicationMaster in YARN Architecture

Question

I have recently started learning Hadoop and I am finding it difficult the overall architecture or job execution flow w.r.t. YARN. I was following the official documentation on YARN where I found that:

ApplicationMaster has the responsibility of negotiating appropriate resource containers from the Scheduler (ResourceManager)

and ApplicationsManager is responsible for negotiating the first container for executing the ApplicationMaster.

Link : http://hadoop.apache.org/docs/current/hadoop-yarn/hadoop-yarn-site/YARN.html

Following are the things that I am confused with:

1. The documentation says that ApplicationMaster has the responsilibility to request ResourceManager for Container. Now, who is responsible for providing the container for ApplicationMaster in the first place?
2. How does YARN knows about creating a container for ApplicationMaster?

Answer 1

Let me help you with your confusion:

1. YARN client submits the application to ResourceManager, providing it all of the information that is required in order to launch the ApplicationMaster for the application.

2. Now, the resource manager has a component called ApplicationManager who is responsible for for accepting job-submissions and negotiating the first container for running ApplicationMaster. So, first container is launched for Application Master who is responsible for requesting the resources/containers from ResourceManager for running the very application. 

Hope your doubt is clear.