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DevOps Day 11: Deploying a Java Application on Tomcat

Today's task involved deploying a Java application, where I set up the Apache Tomcat server, configured it, and deployed a pre-packaged web application. The deployment process included preparing the destination server, ensuring secure transfer, and executing the deployment, highlighting the multi-phase nature of application deployment.

Table of Contents

The Task

My objective was to deploy a Java web application on App Server 3. The process involved several specific requirements: 1. Install the Apache Tomcat application server. 2. Configure Tomcat to run on port 8084 instead of its default 8080. 3. Copy a pre-built application file, ROOT.war, from the /tmp directory on the Jump host. 4. Deploy this .war file to the Tomcat server. 5. Verify that the application was accessible directly at the base URL (e.g., http://stapp03:8084).

My Step-by-Step Solution

I broke my solution into three logical phases to ensure a smooth and error-free deployment.

Phase 1: Installing and Configuring Tomcat on App Server 3

First, I prepared the destination server.

  1. Connect and Install: I logged into App Server 3 (banner@stapp03) and used yum to install the Tomcat package. 

    sudo yum install -y tomcat

  2. Configure the Port: This was a critical configuration change. I edited Tomcat's main configuration file, server.xml, with vi. 

    sudo vi /etc/tomcat/server.xml

    Inside the file, I searched for the Connector tag and changed its port attribute from 8080 to 8084.

  3. Start and Enable the Service: Finally, I started the Tomcat service and enabled it to ensure it would restart automatically after a reboot. 

    sudo systemctl start tomcat
sudo systemctl enable tomcat

Phase 2: Preparing for Secure File Transfer

The application file was on a different server (the Jump host). To automate the copy, I needed to set up password-less SSH from the Jump host to App Server 3.

  1. Connect to Jump Host: I logged into the Jump host (thor@jump_host).

  2. Set up SSH Keys: I generated an SSH key pair (if one didn't exist) and then used ssh-copy-id to send my public key to the banner user on App Server 3. 

    ssh-keygen -t rsa
ssh-copy-id banner@stapp03
    I entered banner's password one last time to authorize the key transfer.

Phase 3: Deploying the Web Application

With all the prerequisites in place, I was ready for the final deployment.

  1. Copy the .war File: From the Jump host, I used scp to securely copy the application file to App Server 3. It transferred instantly without a password. 

    scp /tmp/ROOT.war banner@stapp03:

  2. Move to Tomcat's webapps Directory: I switched back to my App Server 3 terminal. The ROOT.war file was now in my home directory. I used sudo to move it into Tomcat's special auto-deployment directory. 

    sudo mv ROOT.war /usr/share/tomcat/webapps/
    Tomcat automatically detects new files in this directory and deploys them.

  3. Verification: I waited about 15 seconds for Tomcat to unpack and start the application. Then, I used curl to test the endpoint. 

    curl http://stapp03:8084
    I was greeted with the HTML of the deployed application, which was the final proof of a successful deployment.

Why Did I Do This? (The "What & Why")

- Apache Tomcat: This is one of the most popular application servers in the Java ecosystem. It provides a runtime environment that manages Java Servlet and JavaServer Pages (JSP) technologies, which are the foundation of many Java web applications. - .war (Web Application Archive) File: This is the standard for distributing and deploying Java web applications. It's a single file that bundles all the application's components (Java classes, libraries, static files like HTML/CSS, etc.) into a predictable structure that any compliant application server, like Tomcat, can understand and run. - Port Configuration (server.xml): Editing the server's configuration is a fundamental sysadmin skill. The server.xml file is the heart of Tomcat's configuration, and changing the Connector port is necessary to avoid conflicts with other services and adhere to project requirements.

Deep Dive: The Magic of ROOT.war

A key part of this task was understanding the special meaning of the filename ROOT.war.

Normally, if you deploy a file named my-app.war, Tomcat will make it available at the URL path /my-app. For example: http://stapp03:8084/my-app.

However, the filename ROOT.war (all caps) is a special convention. It tells Tomcat to deploy this application at the root context. This means the application becomes the default one for the server, and you can access it directly from the base URL: http://stapp03:8084/. This is perfect for when a server is dedicated to hosting a single, primary application.

Common Pitfalls

- Firewall Issues: If I had been unable to curl the application, the next step would have been to check the firewall on App Server 3 (sudo firewall-cmd --list-all) to ensure that traffic on the custom port 8084 was being allowed. - Forgetting to Start Tomcat: A simple but common mistake is to configure the server but forget to start and enable the service, leading to "Connection refused" errors. - Incorrect Permissions on webapps: If I had tried to scp the file directly into /usr/share/tomcat/webapps, it would have failed with a permission error. The two-stage copy (copy to home directory, then sudo mv) is the correct pattern to handle this. - Not Waiting for Deployment: Tomcat needs a few seconds to unpack the .war file and start the application. Trying to curl the URL immediately after moving the file might result in a 404 error or connection refused, leading to false-negative troubleshooting.

Exploring the Commands Used

- sudo yum install -y tomcat: Installs the Tomcat application server and its dependencies. - sudo vi /etc/tomcat/server.xml: Edits the main Tomcat configuration file. - sudo systemctl start tomcat: Starts the Tomcat service. - sudo systemctl enable tomcat: Ensures the Tomcat service starts on server boot. - ssh-copy-id user@host: Sets up password-less SSH for automated file transfers. - scp [source] [destination]: Securely copies the .war file from the jump host to the app server. - sudo mv [source] [destination]: Moves the .war file into Tomcat's auto-deployment directory. - curl [URL]: A command-line tool to make web requests, used here to verify that the application was running and accessible.