Clustering and Load Balancing in Tomcat 5, Part 2
Tutorial Details:
This is the second part of a series on clustering and load balancing in Tomcat 5 server. In part 1, I provided an overview of large-scale J2EE system design as well as various factors to be considered when designing the system for scalability and high availability. I also discussed Tomcat\'s support for clustering, load-balancing, fault-tolerance, and session-replication capabilities. In this part, we\'ll cover the architecture of a proposed cluster setup and go over the installation and configuration details in deploying the cluster (by running multiple Tomcat server instances).
Proposed Cluster Setup
Listed below are the main objectives that I wanted to achieve in the proposed Tomcat cluster:
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The cluster should be highly scalable.
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It should be fault-tolerant.
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It should be dynamically configurable, meaning it should be easy to manage the cluster declaratively (changing a configuration file) rather than programmatically (changing Java code).
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It should provide automatic cluster member discovery.
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Fail-over and load-balancing features for session data with in-memory session state replication.
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Pluggable/configurable load-balancing policies.
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Group membership notification when a member of the cluster joins or leaves a group.
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No loss of message transmission through multicast.
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Clustering should be seamless to the web application and the server. It should provide both client and server transparency. Client Transparency means that the client is not aware of clustered services or how the cluster is set up. The cluster is identified and accessed as a single thing rather than individual services. And server transparency means that the application code in a server is not aware that it\'s in a cluster. The application code cannot communicate with the other members of cluster.
Four instances of Tomcat server were installed to set up the clustering environment. Tomcat was used for both load balancing and clustering requirements. The cluster setup was done using vertical scaling method (multiple Tomcat server instances running on a single machine). One server group and two clones were configured in the cluster (a server group is a logical presentation of the application server). The clones had same exact configuration (in terms of web application directory structure and contents) as the server group to optimize session replication. To follow are the main components in the proposed cluster setup:
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Load Balancer: A Tomcat instance is configured to distribute the traffic among cluster nodes. This instance is given a code name TC-LB.
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Clustering: Three Tomcat server instances were run as part of the cluster. The code names for these instances are TC01, TC02, and TC03.
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Session Persistence: In-memory session replication was chosen as the session persistence mechanism. The session data was copied to all three cluster members whenever the session object is modified.
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Fail-Over: The balancer application that comes with Tomcat installation is not designed to handle fail over. I wrote a utility class called ServerUtil to check the server status before forwarding any requests to it. It has two methods to verify the status of a cluster node. In the first method, it uses McastService to check if a specified server instance is currently running or not. The second method verifies a cluster node\'s availability by creating a URL object based on the web page URL passed in as a parameter. To use this class make sure catalina-cluster.jar (located in %TOMCAT_HOME%/server/lib directory) and commons-logging-api.jar (%TOMCAT_HOME%/bin directory) files are specified in the classpath.
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