How does the Load Balancer in the Edge component work?
 Technote (FAQ)
 
Question
The Load Balancer can route traffic to several identical servers. Here is how it works.
 
Answer
The Load Balancer component is an IP-level load balancer. Load Balancer does not use DNS, even though static DNS is commonly used in front of the Load Balancer in solutions.
After installing and configuring the Load Balancer, the cluster address becomes the site IP address for all packets sent to your clients. You can define as many cluster addresses as you need. Once clusters are defined, define the ports you want to support inside each cluster. At this point, the actual servers that will provide the service on each of those ports. Also, you can conceal from your clients the real IP addresses of the servers in the cluster by filtering them at the gateway router. This concealment approach is recommended.

This object-oriented cluster-port-server structure provides a simple configuration interface that can be created and modified dynamically without stopping and restarting Load Balancer, permitting true 24 x 7 operation. Because Load Balancer is a truly generic TCP/IP application, its function can be applied to HTTP traffic, FTP traffic and other standard-compliant types of TCP and UDP traffic.


Start by outlining the simple steps of the request:
  1. The client obtains the "cluster" IP address for a site, and may or may not cache it (for example, ibm.com)
  2. The client sends the inbound packets for a request to the site (ibm.com)
  3. The site is represented by the Edge Server, which chooses a server to process the request, and forwards it to that server
  4. The chosen server accepts the request and responds to the client
  5. Outbound response packets go directly to the client, not through the Edge Server

    Note: This is for MAC forwarding, it is different for kcbr and nat.

How it really works:
  1. Client sees one IP address for the site, for example 9.37.38.39
  2. Load Balancer advertizes that address, 9.37.38.39, called a cluster address, usually as a standard alias
  3. Gateway router (and router network) knows how to route to it
  4. Packets for Load Balancer machine that are not to be balanced go to non-forwarding address of the machine and NOT the cluster address
  5. Load Balancer forwards traffic to the chosen server via the servers real address, but the addresses in the packet are not modified
  6. Real servers also carry the cluster address 9.37.38.39 but as a non-advertising alias, usually on the loopback interface, and therefore will accept these incoming packets
  7. Real servers respond directly to the client with the cluster address as source IP address (MAC forwarding)

Notes:
  • The client sees a single system image (such as, www.mycompany.com) even though there may be several "backend" servers
  • The client's packets are not modified when forwarded to the chosen server
  • Edge Server will not choose a server that is down
  • Completely transparent to both client and server
  • Edge Server only sees the inbound client-to-server flows. In other words, only 5% of the traffic (MAC only) is seen.
 
 
Product Alias/Synonym
Edge
LB
Load Balancer
dispatcher
network dispatcher
 
 
 


Document Information


Product categories: Software > Application Servers > Distributed Application & Web Servers > WebSphere Application Server > Edge Component
Operating system(s): Windows
Software version: Load Balancer 5.1.1
Software edition:
Reference #: 1231497
IBM Group: Software Group
Modified date: Feb 28, 2006