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This white paper from Citrix discusses the evolution of load balancing solutions to more comprehensive application delivery controllers (ADCs). It highlights the need for enterprises to consider not just scalability and availability, but also application performance, security, and adaptability. The paper outlines 8 must-have features for today’s network demands, including Layer 4 and 7 load balancing, global server load balancing, application acceleration, comprehensive application security, a purpose-built platform, an integrated, modular design, and unified, simplified management.
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Overview Early-generation server load balancing technology has proven to be an invaluable asset, especially for organizations hosting widely utilized Web applications. By operating as a virtual entry point to such applications, load balancing provides an opportunity to execute a variety of algorithms for splitting the processing load among back-end servers. In addition, periodic polling to establish the status of participating nodes can be used not only to fine tune the load distribution but also to avoid directing traffic to servers that are actually offline. In other words, server load balancers (SLBs) are a simple yet highly effective means to scale an application environment while simultaneously ensuring its availability. Time marches on, however. Business requirements evolve, as do the processes and technologies used to fulfill them. In fact, the following are just a handful of the key changes and trends that have taken hold since SLBs were first introduced:
Citrix NetScaler is an enterprise-class solution for server and global server load balancing. However, it is much more than that. Because NetScaler also incorporates comprehensive application performance and security functionality, it is appropriately classified as a full-featured Application Delivery Controller. A market-proven solution, NetScaler is used by eight out of the 10 largest Web sites, with an estimated 75 percent of Internet users hitting a NetScaler daily.
1 Layer 4 load balancing 2 Layer 7 load balancing 3 Global server load balancing 4 Application acceleration 5 Comprehensive application security 6 A purpose-built platform – The key to superior scalability 7 An integrated, modular design – The key to superior agility 8 Unified, simplified management – The key to superior usability
Also referred to as content switching, L7 load balancing is essentially an extension of the traffic distribution, health monitoring and session persistence capabilities discussed above. The difference is that routing decisions can also be based on application layer data and attributes, such as HTTP header, uniform resource identifier, SSL session ID and HTML form data. This difference enables more-efficient utilization of resources because all of the services and components of an application no longer need to be implemented on all of the server nodes. As a result, each physical system can now be tailored to the functions it will be supporting. When evaluating solutions against this criterion, emphasis should be placed on the breadth and depth of L7 load balancing and content-switching policies that can be established, as well as the ease with which they can be constructed or configured. Organizations should also consider the value of a variety of advanced L7 content features not strictly associated with distributing traffic. For example, NetScaler enables content to be rewritten (e.g., to mask sensitive data) and includes a responder module for configuring custom responses (e.g., redirects, error messages) to specified types of inbound requests.
The general concept of global server load balancing is to extend the core L4 and L7 capabilities so that they are applicable across geographically distributed server farms. The primary objective is to provide an additional degree of availability by accounting for site level disruptions and outages. Secondary benefits include: (a) being able to further enhance performance for remote users by routing their sessions to the closest or best-performing datacenter; and (b) being able to balance and optimize resource utilization on an enterprise wide basis. Unlike many other solutions on the market, NetScaler incorporates global server load balancing as an optional feature. A separate, standalone device is not required. NetScaler’s other distinct advantage, once again, is that it offers an extensive array of options when it comes to the site level health attributes that can be monitored, as well as the mechanisms and algorithms that can be used to distribute sessions among an organization’s different datacenters. Stepping up to application delivery The point has already been made that simple, early generation load balancers are not sufficient. Overall, they leave organizations in the undesirable position of having to acquire and implement an additional set of products to achieve adequate levels of application performance and security. The deficiencies in these early load balancers also explain why leading industry analysts strongly encourage organizations to embrace advanced Application Delivery Controllers (ADCs) when replacing their server load balancers. The intent with ADCs in general, and NetScaler in particular, is to have a single device that incorporates not just a core set of load balancing capabilities but a comprehensive set of application performance and security services as well. The next two sections elaborate on what this means in terms of specific functionality.
Compensating for obvious deficiencies and otherwise enhancing application performance can be a tricky proposition. Sub-optimal application performance can be the result of resource constraints at virtually any point in the path that a user’s session traverses. A few of the more likely bottlenecks are inadequate client hardware, insufficient bandwidth at either the client or server end of the connection and overloaded server infrastructure. Alternately, there can be problems with the application itself. This is frequently the case when the underlying protocols or application logic have not been optimized for operation over a wide area network. The resulting condition, referred to as chattiness, is a highly inefficient behavior whereby it takes numerous back-and-forth exchanges between client and server to complete a single, user level action. The diversity of potential issues is why an ideal solution should incorporate an overlapping set of features that enhance application performance. These include caching, compression, TCP communications management and SSL offload.
Application delivery is substantially more compute intensive than ordinary load balancing. Not only is the scope of functionality greater, but so is the depth of processing that needs to be conducted to provide the requisite level of application visibility and control. Less clear, though, is how to account for this difference, especially in ensuring the solution is able to scale appropriately. One key is having a platform where the hardware—and more importantly, the system level software—has been constructed and optimized explicitly for the higher-level services that define an ADC. Some of the more significant features of such a platform are:
With NetScaler VPX—a full-featured virtual appliance version of NetScaler that can be deployed on any hardware platform running the Citrix® XenServer™^ server virtualization system—there is no physical appliance to deal with. As a result, IT departments can deploy application availability, security and accelerations services on-demand, anywhere within private, hosted or cloud-based networks and datacenters. Not only is a more thorough implementation of critical application delivery services possible, but it can be done in a way that takes full advantage of virtualized servers and off-the-shelf hardware that already in place—all while facilitating the longer-term objective of having a fully dynamic datacenter.
For most organizations, having options is a firm requirement. So is having a solution that is adaptable and, therefore, future proof. Consequently, a top consideration for an SLB replacement is that it feature a modular design. This way individual capabilities (e.g., application firewall, SSL VPN) can be added as needed when the organization is ready to take the next step in the evolution of its application delivery infrastructure. Furthermore, new modules that account for ever changing conditions can be developed and implemented over time without having to resort to deploying a fleet of additional, standalone devices. Equally important is that the modules be truly integrated components of the overall system. For instance:
Ultimately, the ability to unleash the full power of an ADC depends heavily on the strength and usability of the associated management capabilities. Three elements of the NetScaler solution are particularly helpful in identifying the specific features to look for when considering management capabilities.