Load Balancing and Clustering in Apache Tomcat & HTTPD – Scale with mod_jk

Scaling Apache Tomcat efficiently requires load balancing and clustering, which help distribute traffic across multiple servers, ensuring high availability and performance. This guide covers how to set up clustering and configure Apache HTTPD with mod_jk to balance loads across Tomcat instances while integrating Apache HTTP Server for better request handling. ⚡

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✅ Why Use Load Balancing and Clustering in Tomcat & HTTPD?

• ⚡ High Availability (HA) – Prevents downtime if a server fails.
• 🚀 Better Performance – Distributes requests efficiently across multiple Tomcat instances.
• 🌍 Scalability – Handles more concurrent users without affecting response times.
• 🔄 Session Replication – Ensures users don’t lose session data if a server goes down.
• 🌐 Improved Static Content Handling – Apache HTTPD efficiently serves static files, reducing Tomcat’s workload.

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🔧 Setting Up Load Balancing with Apache HTTPD & mod_jk

1️⃣ Install Apache HTTP Server and mod_jk 📦

To balance traffic between multiple Tomcat servers, install Apache HTTPD and the mod_jk connector.

📌 On Ubuntu/Debian:

sudo apt update && sudo apt install apache2 libapache2-mod-jk

📌 On RHEL/CentOS:

sudo yum install httpd mod_jk

Enable mod_jk in Apache:

sudo a2enmod jk
sudo systemctl restart apache2

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2️⃣ Configure mod_jk Workers for Tomcat ⚙️

mod_jk requires a workers.properties file to define Tomcat instances.

📌 Create workers.properties file (e.g., /etc/libapache2-mod-jk/workers.properties):

worker.list=loadbalancer

worker.node1.type=ajp13
worker.node1.host=192.168.1.101
worker.node1.port=8009

worker.node2.type=ajp13
worker.node2.host=192.168.1.102
worker.node2.port=8009

worker.loadbalancer.type=lb
worker.loadbalancer.balance_workers=node1,node2

💡 Explanation:

• Defines two Tomcat instances (node1 & node2).
• Uses AJP protocol for communication (port 8009).
• loadbalancer worker distributes traffic between node1 & node2.

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3️⃣ Configure Apache HTTPD to Use mod_jk 🌐

Edit Apache’s configuration file to direct traffic to the Tomcat cluster while also serving static content efficiently.

📌 Modify /etc/apache2/sites-available/000-default.conf (or httpd.conf on CentOS):

<VirtualHost *:80>
    ServerName mydomain.com
    DocumentRoot /var/www/html

    # Forward dynamic requests to Tomcat
    JkMount /* loadbalancer
    
    # Serve static files from Apache instead of Tomcat
    <Directory "/var/www/html">
        Require all granted
    </Directory>

    ErrorLog ${APACHE_LOG_DIR}/error.log
    CustomLog ${APACHE_LOG_DIR}/access.log combined
</VirtualHost>

Restart Apache for changes to take effect:

sudo systemctl restart apache2

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🔄 Setting Up Tomcat Clustering for Session Replication

To prevent session loss when a server fails, configure session replication.

4️⃣ Enable Clustering in server.xml 🔗

Modify Tomcat’s server.xml to enable clustering on each Tomcat instance.

📌 Update <Engine> section in server.xml:

<Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster" />

📌 Ensure <Context> inside context.xml has session replication enabled:

<Context>
    <Manager className="org.apache.catalina.ha.session.DeltaManager" />
</Context>

💡 Explanation:

• SimpleTcpCluster enables Tomcat’s built-in clustering.
• DeltaManager replicates session data across instances.

Restart Tomcat after applying these changes:

sudo systemctl restart tomcat

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5️⃣ Verify Load Balancing and Clustering ✅

Test Load Balancing:

1️⃣ Deploy an application (myapp.war) on both Tomcat instances (node1 & node2). 2️⃣ Access the app via Apache: http://mydomain.com/myapp. 3️⃣ Check logs (access.log) to confirm requests are distributed.

Test Session Replication:

1️⃣ Log in to the application on http://mydomain.com/myapp. 2️⃣ Stop node1 (sudo systemctl stop tomcat). 3️⃣ Refresh the page – your session should persist on node2.

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🎯 Summary: Key Configurations ✅

✔️ Apache HTTPD + mod_jk – Handles load balancing and static content efficiently.
✔️ AJP Protocol – Used for fast communication between Apache and Tomcat.
✔️ Session Replication – Ensures users don’t lose session data.
✔️ High Availability – Distributes traffic across multiple Tomcat nodes.

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