Understanding Jenkins plugins and agent

Question

I was trying to understand jenkins agents. This page asks to first create jenkins docker agent. But it doesnt say where to execute these steps?

Q1. Should we be executing these steps on node or a machine which we want to designate as agent?

The next step asks to setup an agent through Jenkins UI:

Q2. Above is nothing but the Jenkins controller UI right?

But above UI does not seem to accept IP address of the agent node on which we staarted docker agent.

Q3. Does Jenkins controller automatically discovers running agents reachable on the network?

Q4. What are exactly Jenkins plugins in relation with agents? Jenkin glossary defines plugin as "an extension to Jenkins functionality provided separately from Jenkins Core." But that does not explain much of its nature or functionality. This page also explain plugin installation and management on the controller, but doesnt explain exact nature of their functionality.

Q4.1. Do plugins run jobs of agent nodes? For example, does Android Emulator plugin installed on controller installs and runs android emulator on available agent?

Q4.2. If yes is the answer to Q4.1, does every plugin need corresponding process to be installed on the agent so that agent can carry out functionality specified in the pluin on the controller?

PS: Am a noob in Jenkins and overall DevOps stuff and just trying to wrap my head around Jenkins

Answer 1

Q1. Should we be executing these steps on node or a machine which we want to designate as agent?

You can execute them where ever you want. If you want the agent to run on your local machine, then run it locally. If you want the agent to run on the same hardware as the Jenkins Controller then run the docker command on the Jenkins machine. If you want to run it on a cloud or in a Kubernetes pod, then run the command their.

Q2. Above is nothing but the Jenkins controller UI right?

Yes

But above UI does not seem to accept IP address of the agent node on which we staarted docker agent.

It has a Host field. Put the IP address or hostname into it.

Q3. Does Jenkins controller automatically discovers running agents reachable on the network?

No, that is what the above controller UI page is for. You are telling Jenkins, this is how to connect to an agent.

Q4, Q4.1, Q4.2

These should really be a new question but I will give you a brief explanation.

Agents and plugins are not really related. Plugins are installed on the Jenkins controller and will run where you run your job/pipeline. So this could be on the controller or on an Agent. Also plugins can help you setup agents, like the Amazon Ec2 plugin.

Jenkins can be tough to learn but it is the most powerful CICD engine that exists as long as you use scripted pipelines. Another place that might be helpful is https://community.jenkins.io/