Using Jenkins agents

The Jenkins architecture is designed for distributed build environments. It allows us to use different environments for each build project balancing the workload among multiple agents running jobs in parallel.

The Jenkins controller is the original node in the Jenkins installation. The Jenkins controller administers the Jenkins agents and orchestrates their work, including scheduling jobs on agents and monitoring agents. Agents may be connected to the Jenkins controller using either local or cloud computers.

The agents require a Java installation and a network connection to the Jenkins controller. View the 3 minute video below for a brief explanation of Jenkins agents.

What is a Jenkins Agent

Configuring agents with Docker

Jenkins agents can be launched in physical machines, virtual machines, Kubernetes clusters, and with Docker images. This section connects Docker agents to Jenkins with SSH.

Environment

To run this guide you will need a machine with:

Java installation
Jenkins installation
Docker installation
SSH key pair

If you need help to install Java, Jenkins and Docker please visit the section Installing Jenkins.

Generating an SSH key pair

In a terminal window run the command: ssh-keygen -f ~/.ssh/jenkins_agent_key
Provide a passphrase to use with the key

Confirm the output looks something like this:

ubuntu@desktop:~$ ssh-keygen -f ~/.ssh/jenkins_agent_key
Generating public/private rsa key pair.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/ubuntu/.ssh/jenkins_agent_key
Your public key has been saved in /home/ubuntu/.ssh/jenkins_agent_key.pub
The key fingerprint is:
SHA256:XqxxjqsLlvDD0ZHm9Y2iR7zC6IbsUlMEHo3ffy8TzGs
The key's randomart image is:
+---[RSA 3072]----+
|  o+             |
| ...o  .         |
|  .o .+ .        |
|    o+.+ o o     |
|  ... o.So* .    |
|  o+ = +.X=      |
| o oO + *..+     |
|. oo.o o .E .    |
| o... oo.. o     |
+----[SHA256]-----+

Create a Jenkins SSH credential

Go to your Jenkins dashboard;
Go to Manage Jenkins option in main menu and click on credentials button;
select the drop option Add Credentials from the global item;
Fill the form:
- Kind: SSH Username with private key;
- id: jenkins
- description: The jenkins ssh key
- username: jenkins
- Private Key: select Enter directly and press the Add button to insert your private key from ~/.ssh/jenkins_agent_key
- Passphrase: fill your passphrase used to generate the SSH key pair and then press OK

Creating your Docker agent

On Linux

Here we will use the docker-ssh-agent image to create the agent containers.

run the command to start your first agent:
```
docker run -d --rm --name=agent1 -p 22:22 \
-e "JENKINS_AGENT_SSH_PUBKEY=[your-public-key]" \
jenkins/ssh-agent:alpine
```
Remember to replace the tag [your-public-key] for your own SSH public key.

Your public key in this example is: cat ~/.ssh/jenkins_agent_key.pub
Now run the following command to update the container environment:
```
$ VARS1="HOME=|USER=|MAIL=|LC_ALL=|LS_COLORS=|LANG="
$ VARS2="HOSTNAME=|PWD=|TERM=|SHLVL=|LANGUAGE=|_="
$ VARS="${VARS1}|${VARS2}"
$ docker exec agent1 sh -c "env | egrep -v '^(${VARS})' >> /etc/environment"
```
The step 2 is necessary because the image is configured to reject changes to environment variables. when the issue #33 is fixed, we can ignore this step.
Now the container agent1 is running.
Hint: the command docker ps can be used to check if the container is running as expected.

On Windows

Here we will use the docker-ssh-agent image to create the agent containers.

run the command to start your first agent:
```
docker run -d --rm --name=agent1 --network jenkins -p 22:22 `
  -e "JENKINS_AGENT_SSH_PUBKEY=[your-public-key]" `
  jenkins/ssh-agent:jdk11
```
Remember to replace the tag [your-public-key] for your own SSH public key.

Your public key in this example is: Get-Content $Env:USERPROFILE\.ssh\jenkins_agent_key.pub
Now the container agent1 is running.
Hint: the command docker ps can be used to check if the container is running as expected. Additionally, the command docker container inspect agent1 | Select-String -Pattern '"IPAddress": "\d+\.\d+\.\d+\.\d+"' can be used to see the Host to be set in Jenkins for the agent.

Setup up the agent1 on jenkins.

Go to your Jenkins dashboard;
Go to Manage Jenkins option in main menu;
Go to Manage Nodes and clouds item;
Go to New Node option in side menu;
Fill the Node/agent name and select the type; (e.g. Name: agent1, Type: Permanent Agent)
Now fill the fields:
- Remote root directory; (e.g.: /home/jenkins )
- label; (e.g.: agent1 )
- usage; (e.g.: only build jobs with label expression…)
- Launch method; (e.g.: Launch agents by SSH )
  - Host; (e.g.: localhost or your IP address )
  - Credentials; (e.g.: jenkins )
  - Host Key verification Strategy; (e.g.: Manually trusted key verification … )
Press the button save and the agent1 will be registered, but offline. Click on it.
Now press the button Launch agent and wait some seconds, then you should receive
the message: Agent successfully connected and online on the last log line.

Delegating the first job to agent1

Go to your jenkins dashboard;
Select New Item on side menu;
Enter a name. (e.g.: First Job to Agent1)
Select the Freestyle project and press OK;
Check the option: Restrict where this project can be run;
Fill the field: label with the agent1 label; (e.g.: agent1)

Be careful with white spaces before or after the label.
Now Select the option Execute shell at Build Section;
Add the command: echo $NODE_NAME in the Command field of the Execute shell step and the name
of the agent will be printed inside the log when this job is run;
press the save button and then select the option Build Now;
Wait some seconds and then go to Console Output page

you should receive output similar to:

Started by user Admin User
Running as SYSTEM
Building remotely on agent1 in workspace /home/jenkins/workspace/First Job to Agent1
[First Job to Agent1] $ /bin/sh -xe /tmp/jenkins15623311211559049312.sh
+ echo $NODE_NAME
agent1
Finished: SUCCESS

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