Load Balancing (III)

Before we go any deeper into the abyss of all the techniques and algorithms used in the load balancing world it is important to clarify some concepts and notions and take a look at the most used load balancing terminology. The target audience of this blog is supposed to know what the OSI Model is and therefore I won’t even bother to explain what the layers are...  

Server health checking

Server health checking is the ability of the load balancer to run a test against the servers to determine if they are providing service:

• Ping: This is the most simple method, however it is not very reliable as the server can be up whilst the web service could be down;

• TCP connect:  This is a more sophisticated method which can check if a service is up and running like a service on port 80 for web. i.e. try and open a connection to that port on the real server;

• HTTP GET HEADER: This will make a HTTP GET request to the web server and typically check for a header response such as 200 OK;

• HTTP GET CONTENTS:  This will make a HTTP GET and check the actual content body for a correct response. Can be useful to check a dynamic web page that returns 'OK' only if some application health checks work i.e. backend database query validates. This feature is only available on some of the more advanced products but is the superior method for web applications as its will check that the actual application is available.

Layer-2 Load Balancing

Layer-2 load balancing (also referred as link aggregation, port aggregation, ether channel or gigabit ether channel port bundling) is to bond two or more links into a single, higher-bandwidth logical link. Aggregated links also provide redundancy and fault tolerance if each of the aggregated links follows a different physical path.
Read more »