Dispatcher component
Overview
The Dispatcher component performs intelligent load balancing by using server availability, capability, workload, and other criteria you can define, to determine where to send a request. These weights and measurements are set dynamically by the Dispatcher.
Load balancing is provided at a specific service level, such as HTTP, FTP, SSL, NNTP, IMAP, POP3, SMTP, and Telnet. It does not use a domain name server to map domain names to IP addresses and it supports such forwarding methods as:
- Media Access Control (MAC)
- Network Address Translation (NAT)
- Network Address Port Translation (NAPT)
- CBR forwarding
The core function of Dispatcher is the Executor. It is a kernel-level function (or device driver) that examines only the header of each packet and decides whether the packet belongs to an existing connection, or represents a new connection request. The Executor uses a connection table in memory to achieve this. Note that the actual connection is never set up on the Dispatcher machine. The connection is between the client and server, just as it would be if the Dispatcher were not installed. But the connection table records its existence and the address of the server to which the connection was sent.
Based on information provided by the Manager, the Executor selects to which server it will forward a new connection packet. The Manager periodically sets weights for the servers and may use Advisors, which are lightweight clients that run inside the Dispatcher. Standard advisors execute a trivial command to determine the status of the servers. If the request succeeds, the server is considered up. A server that is very busy (or slower in responding to an advisor request) will receive a higher port load and lower overall weight than a server that has more capacity available and fewer new requests will be routed to the busy server. A down server is given a weight of zero, and packets will not be forwarded to it until the server responds to the advisor. A server that was manually marked down has a weight of -1.
You can also use Metric server, a system monitoring component of Load Balancer that must be installed in each load-balanced server within your configuration. Metric server can then provide customizable monitoring of the activity on a server, contributing to the weight value for the server. The script for Metric server must return a value from 0-100 (or -1 if you want to mark the server down) to define the weight for each server. In other words, one weight will be recorded for all ports configured to that server. This will be reported in the manager report under the system column.
The interaction of Executor, Manager, and other Load Balancer components are shown in Figure 4-2.
Figure 4-2 Dispatcher components interaction
The command-line interface for Dispatcher is dscontrol. Use lbadmin to start the GUI interface.
WebSphere is a trademark of the IBM Corporation in the United States, other countries, or both.
IBM is a trademark of the IBM Corporation in the United States, other countries, or both.