The most popular load balancers or application delivery controllers mix traditional simple Layer 4 load balancing with the more advanced application sensitive Layer 7 (Taken from the OSI model) content switching technology.
Load Balancing, Layer 4 and Layer 7
Load balancing balances application requests across a number of servers, these servers are often in a pool known as a cluster and the load balancer presents them to the outside world as a virtual server, it accepts requests as the unique interface to the pool and directs the request to the most appropriate server based on algorithms that have been defined by the IT department. The key thing is that the servers are used need to contain the same content.
The most popular Layer 4 load balancing techniques are:
- weighted round-robin
- least connections
- weighted least connections
Layer 7 switching directs its requests at the application layer, this type of switching is also known as:
- Request switching
- Application switching
- Content based routing
Like Layer 4 load balancing the load balancer presents the server cluster as a single virtual server and distributes the requests based on the performance health check the network load balancer uses to assess the performance of each application.
How is Layer 7 Load Balancing different?
Where Layer 7 load balancing differs from Layer 4 is the staff who are responsible for the applications themselves need to ensure that their applications are perfectly tuned for optimal application performance.
Layer 7 load balancing differs from Layer 4 load balancing in a fundamental way because the servers do not replicate the same content, but effectively “pass the parcel” this allows for fine tuning , here is an example:
- Server 1 supplies images and graphics
- Server 2 delivers the content to the site visitor using scripting and content like CSS and HTML
- Server 3 allows the user to buy the content
- Server 4 delivers the purchased content
As can be seen from the example above Layer 7 Load Balancing allows for much more complex models of application and content delivery based on real resource usage.
Layer 7 and Kemp
At Kemp Technologies, the combination of the best of both Layer 4 and advanced Layer 7 content switching technologies, has resulted in our award winning LoadMaster load balancers. Using these highly optimised applicaiton delivery network devices our customers can develop extremely scalable solutions for a myriad of purposes.