Load Balancer Overview

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This is a general overview on how the load balancer works.

VIP or Virtual Server - The VIP (Virtual IP) or Virtual Server is the external facing IP address and port for a particular site or service.  This will normally be the IP that your A record(s) for a particular site or sites will point to.  You can have multiple VIP's created using the same IP as long as they are using a different port.  In most cases, you will have one VIP with a 69.63.xxx.xxx address on port 80 and one on port 443 for the same IP.  The VIP's you have will always be tied to a Pool.

Pool - A Pool is tied to one or more VIP's or Virtual Servers.  This contains one or more member servers that it balances traffic between.  These normally consist of internal IP's that point directly to a group of servers.  The traffic will hit a particular VIP and then get passed to the pool which distributes the traffic to individual members according to availability and responsiveness.  In most cases, the Health Check is also tied to the pool to make sure the member is responding to requests.

Member - A Member is an IP address and port combination tied to a particular Node (A.K.A Server).  You can have as many members as you'd like in a particular pool that traffic will be distributed between.  They normally are the internal IP's on the actual server corresponding to a particular port.  Health checks can also be assigned to members individually if need be.

Node - A Node is just another term for server.  These are the actual physical machines doing all of the request processing once the load balancer distributes traffic to it.  You can have multiple members tied to the same node as long as they are different IP and port combinations.

Health checks - The health check determines if a member or pool is down.  The load balancer checks every 5-10 seconds to ensure it can reach the members.  These can be customized to suit your needs.  For instance, if you have a function under the /probe/healthcheck.aspx page that runs some database queries and creates a particular response based upon the success of those queries, the health check will constantly hit that page to ensure it gets the response it is looking for.  If not, it will fail out the particular member that is not responding to avoid sending traffic to it.

SSL Offloading - SSL Offloading is very common among Virtual Servers (VIP's) running on port 443.  We install the SSL on the load balancer and apply it to the appropriate VIP.  When a request comes through to that VIP, the certificate is used to encrypt the traffic between the load balancer and the client computer.  The traffic being sent from the load balancer to the server however is not encrypted, taking the load off of the server and allowing you to use host headers on the server.  It also makes it so the certificate only needs to be licensed and installed to 1 physical machine.  The load balancer inserts an HTTP header namedX-FORWARDED-PROTO. The value is set to http or https depending on the clientrequest. Your application should use this to determine if the client's requestwas secure or not. Please note, some customers' VIPs might be configured to usea legacy header called BIGIPSSL, which is set to true if the client's requestwas secure.

X-FORWARDED-FOR - The load balancer modifies the source IPaddress of the packets in order to route traffic properly. As a result, the webserver sees the request as originating from the load balancer address, not theoriginal client IP address. The load balancer will insert the original clientIP address in an HTTP header named X-FORWARDED-FOR.



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Category: Load Balancer

Last Modified:Tuesday, February 11, 2014 10:14 PM

Type: INFO

Level: Intermediate

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