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Analyze acquisition costs, maintenance costs, stability, performance, range of functions, high availability, security, configuration complexity, integration into your existing landscape and content caching when deciding between software and hardware load balancing.

F5 and SAP have a close and important partnership that involves engineers and business folks working to deliver the best customer experience for any given SAP landscape.  One of the questions that comes up over and over is how a company can make the decision between software load balancing and hardware high availability.  We recently published a document (that's available by emailing me ) about the logistics and considerations of making the choice or making the switch.

I will provide a summary of the document here, but please contact me if you need the document itself. 

In the most basic SAP Landscape SAP clients (either SAP GUI or web browser) make direct connections to SAP instances.  For example, a user wishing to file her travel expenses might click on the link from her company's portal, log into SAP (or be signed on automatically using single sign-on) and then she would fill out her expenses and submit them.

You can probably tell by the above scenario alone that this isn't a very reliable or scalable architecture.  Connecting directly to the instance means there's no load balancing.  All the users will end up at the same link.  What if our hypothetical user was part of a convention and there were thousands of users just like her trying to file the expenses on Monday morning?  The system would probably fail or come to a halt because of the load of traffic.

SAP with Web Dispatcher is the software load balancing solution to this problem.  In a software load balanced SAP Landscape, high availability is achieved when SAP clients connect directly to Web Dispatcher and then are load balanced to the back-end SAP servers.  Web Dispatchers can be installed either on the Central Instance or on its own server.  Web Dispatcher communicates with SAP Message Server to get health information about the SAP instances and Web Dispatcher also analyzes web browser cookies to create persistent connections (sending users to the same instances).

This is a big improvement over scenario one.  However, in a software load balanced SAP Landscape, high availability is achieved only as long as the SAP Message Server is aware of a failure in the Dialogue Instance  (Portal, for example) or if there is a graceful shutdown.  Many failures are, unfortunately, not detected by SAP Message Server today, creating a potential for Web Dispatcher to send traffic to Dialogue Instances that are down.  There is also the issue of Web Dispatcher as a single point of failure itself.  There are no out-of-the-box solutions for making software load balancers highly available.

To summarize, web Dispatcher supports HTTP, cookie load balancing and persistence as well as static round robin.  Web Dispatcher can also make decisions on routing based on the client's IP Address and finally, it can offload SSL.

SAP with BIG-IP Local Traffic Manager is the hardware high availability and performance enhancing solution to the SAP scaling problem.  In a hardware load balanced SAP Landscape, high availability, security, acceleration, health monitoring and intelligent session persistence can be achieved.  When an SAP client makes a direct connection with F5's BIG-IP Local Traffic Manager, the user is intelligently load balanced to only the back-end services that are available at that very moment through a health monitor that passively tests login and database.

BIG-IP Local Traffic Manager, out of the box, can also be configured to optimize the transport protocol between the client and the server, it can make intelligent persistence decisions based on cookies, it can off-load SSL, and most importantly, using web application server health monitoring, traffic can be guaranteed to be delivered.  With advanced scripting abilities, for e-commerce or other "five nines" environments, requests can even be re-submitted if a server crashes mid-transaction, without causing the user to re-submit their data or to lose any work. BIG-IP also has a variety of out-of-the-box high availability solutions that remove it as a single point of failure.

Both software and hardware solutions provide significant enhancements in contrast to a direct deployment with no load balancing or high availability.  When making the decision between hardware and software, SAP and F5 recommend that you frame the decision in the following way.  Ask about acquisition costs, maintenance costs, stability, performance, range of functions, high availability, security, configuration complexity, integration into your existing landscape and content caching.