Project mod_cluster/mod_proxy_cluster is an intelligent load balancer.
Like mod_jk and mod_proxy, mod_cluster uses a communication channel to forward requests from the load balancer to one of a set of application server nodes. Unlike mod_jk and mod_proxy, mod_cluster leverages an additional connection between the application server nodes and the load balancer.
The application server nodes use this connection to transmit server-side load balance factors and lifecycle events back to load balancer via a custom set of HTTP methods, affectionately called the Mod-Cluster Management Protocol (MCMP). This additional feedback channel allows mod_cluster to offer a level of intelligence and granularity not found in other load balancing solutions.
Within Apache httpd, mod_proxy_cluster is implemented as a set of modules for httpd with enabled mod_proxy. Much of the logic comes from mod_proxy, e.g. mod_proxy_ajp provides all the AJP logic needed by mod_proxy_cluster.
Implementation and naming
Project mod_cluster consists of two implementations: one in Java and one (native) in C.
Both formerly shared the same code repository under the name of mod_cluster. However, the different lifecycles and an effort for a simpler development resulted in their separation. Currently, under mod_cluster is only the Java implementation, whereas the C implementation is now in a separate repository as mod_proxy_cluster.
A pure-Java load balancer implementation is available as part of Undertow. Container integration modules are available for WildFly (formerly known as JBoss AS), JBoss EAP and Apache Tomcat.
mod_cluster boasts the following advantages over other load balancers:
Dynamic configuration of httpd workers
Traditional httpd-based load balancers require explicit configuration of the workers available to a proxy. In mod_cluster, the bulk of the proxy’s configuration resides on the application servers.
The set of proxies to which an application server will communicate is determined either by a static list or using dynamic discovery via the advertising mechanism. The application server relays lifecycle events (e.g. server startup/shutdown) to the proxies allowing them to effectively auto-configure themselves. Notably, the graceful shutdown of a server will not result in a fail-over response by a proxy, as is the case with traditional httpd-based load balancers.
Server-side load balance factor calculation
In contrast with traditional load balancers, mod_cluster uses load balance factors calculated and provided by the application servers rather than computing these in the proxy. Consequently, mod_cluster offers a more robust and accurate set of load metrics than is available from the proxy.
Fine grained web-app lifecycle control
Traditional load balancers do not handle web application undeployments particularly well. From the proxy’s perspective, requests to an undeployed web application are indistinguishable from a request for a non-existent resource and will result in 404 errors. In mod_cluster, each server forwards any web application context lifecycle events (e.g. web-app deploy/undeploy) to the proxy, informing it to start/stop routing requests for a given context to that server.
AJP is optional
Unlike mod_jk, mod_cluster does not require AJP. Connections to application server nodes can use HTTP, HTTPS, or AJP.
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