Load Balancing as an ESB Service
Most people, upon hearing the term "load balancing" immediately think of web and application servers deployed at the edge of the network. After all, that's where load balancing is most often used - to ensure that a public facing web site is always as available and fast as possible.
What many architects don't consider, however, is that in the process of deploying a SOA (Service Oriented Architecture) those same web and application servers end up residing deeper in the data center, away from the edge of the network. These web and application servers are hosting the services that make up a public facing application or site, but aren't necessarily afforded the same consideration in terms of availability as the initial entry point into that application.
This situation is compounded by the fact that there may be an ESB (Enterprise Service Bus) orchestrating that application, and that services critical to the application are not afforded the same measure of reliability as those closer to the edge of the network. While most ESBs are capable of load balancing these critical services, this capability is often limited and lacking the more robust and dynamic features offered by application delivery controllers (ADC).
Consider a public facing service that takes advantage of an ESB:
The Compliance and Shipping Services are critical to the public facing service, as are the other services provided by the ESB. And yet in a typical implementation only the public facing service (the Order Management Service) will be afforded the reliability and optimization provided by an application delivery controller.
It may be the case that the ESB is load balancing the Compliance and Shipping services, as simple load balancing capabilities are provided by most ESBs.
While you might find the load balancing capabilities adequate, consider that most ESBs lack the health monitoring capabilities of application delivery controllers in addition to being unable to balance the load based on real-time factors such as number of connections and response time, making it difficult to optimize your SOA. The lesson learned by application delivery vendors in the past have not yet been incorporated into ESBs. Just because a server responds to an ICMP ping or can successfully open a TCP socket does not mean the application - or service - is actually running and returning valid responses. An ADC is capable of monitoring services at the application level, ensuring that not only is the service running but that it's also returning valid responses. This is something most ESBs are not capable of providing, which reduces the effectiveness of such health checks and can result in a service being treated as available even if it's malfunctioning.
When it's really important, i.e. when services are critical to a critical application - such as a customer/public facing application - then it's likely you should ensure that those back-end services are always available and optimized.
An application delivery controller isn't limited to hanging out in the DMZ or on the edge of the network. In fact, an ADC can just as easily slide into your SOA and provide the same high availability, optimization, and failover services as it does for public facing applications.
By integrating an application delivery controller into the depths of the data center you can let the ESB do what it's best at doing: transformation, message enrichment, and reliable messaging and remove the burden of performing tasks that are better suited to an ADC than an ESB such as load balancing, protocol optimization, and health monitoring.
The addition of an ADC to assist the ESB and ensure reliability also provides a layer of abstraction that fits nicely into your SOA and aligns with one of SOA's primary goals: agility.
SOA implementations are by their nature distributed. Composing applications from distributed services achieves agility and reuse, but introduces potential show-stopping problems usually experienced at the edge of the network into the heart of the data center. It is important to consider the impact of failure of a service on the application(s) that may be using that service, and if that application is critical, then its dependent services should also be treated as critical and worthy of the protection of an application delivery controller.
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