Advisors provide load-balancing information
Advisors are software agents that work within Load Balancer to provide information
about the load on a given server. A different advisor exists for each standard
protocol (HTTP, SSL, and others). Periodically, the Load Balancer base code performs
an advisor cycle, during which it individually evaluates the status of all
servers in its configuration.
By writing your own advisors for the Load Balancer, you can customize how your server
machines' load is determined.
Standard advisor function
In general, advisors work to enable load balancing in the following manner.
- Periodically, the advisor opens a connection with each server and sends
it a request message. The content of the message is specific to the protocol
running on the server; for instance, the HTTP advisor sends a HEAD request
to the server.
- The advisor listens for a response from the server. After getting the
response, the advisor calculates and reports the load value for that server.
Different advisors calculate the load value in different ways, but most standard
advisors measure the time the server takes to respond, then reports that value
in milliseconds as the load.
- The advisor reports the load to the Load Balancer's manager function. The load
appears in the Port column of the manager report. The manager uses the advisor's
reported load along with weights set by the administrator to determine how
to load balance incoming requests to the servers.
- If a server does not respond, the advisor returns a negative value (-1)
for the load. The manager uses this information to determine when to suspend
service for a particular server.
Standard advisors provided with the Load Balancer include advisors for the following
functions. Detailed information about these advisors is available in the WebSphere® Application Server Load Balancer Administration Guide
- Connect
- DB2
- DNS
- FTP
- HTTP
- HTTPS
- IMAP
- LDAP
- NNTP
- Ping
- POP3
- Reach
- Self
- SIP
- SMTP
- SSL
- Telnet
- WebSphere Application Server
- WebSphere Application Server Caching Proxy
- Workload Manager
To support proprietary protocols for which standard advisors are not
provided, you must write custom advisors.