Enabling advisors to manage load balancing

Advisors are software agents that work within Load Balancer to provide information about the load on a server. A different advisor exists for each standard protocol (HTTP, SSL, and others). Periodically, the Load Balancer base code runs an advisor cycle, during which it individually evaluates the status of all servers in its configuration.

Before you begin

Advisors are agents within Load Balancer. Their purpose is to assess the health and loading of server machines with a proactive client-like exchange with the servers. Advisors can be considered as lightweight clients of the application servers.

By writing your own advisors for the Load Balancer, you can customize how your server machines’ load is determined.

For more information on how advisors work, read Advisors.

If you are using IPv6 protocol on your machine and want to use advisors, you must modify the protocol file. To enable IPv6, insert the following line in the protocol file:

ipv6-icmp 58 IPv6-ICMP # IPv6 interface control message protocol

The protocol file is in the following directory:

/etc/protocols
C:\windows\system32\drivers\etc\

The product provides several protocol-specific advisors for the most popular protocols. However, it does not make sense to use all of the provided advisors with Load Balancer. Load Balancer also supports the concept of a "custom advisor" that allows users to write their own advisors.

Restriction: To use advisors on bind specific servers, start two instances of the server: One instance to bind on the cluster@port and the other instance to bind on the server@port. To determine whether the server is bind specific, issue the netstat -an command and look for the server@port. If the server is not bind specific, the result from this command is 0.0.0.0:80. If the server is bind specific, you see an address such as 192.168.15.103:80.

About this task

You can start an advisor for a particular port across all clusters (group advisor). Or, you can choose to run different advisors on the same port, but on different clusters (cluster specific advisor).

Note: If Load Balancer is running on a computer with multiple network adapters, you cannot force the source IP address of the packet to a specific address. This restriction is a concern when you want the advisor traffic to flow over a particular adapter.

Procedure

Start the advisor of your choice.
For a list of possible advisors, refer to the list of advisors, or create a custom advisor.
Cluster-specific advisor
To start an advisor on port 80 for clusterA, for example, specify both the cluster and port:
```
dscontrol advisor start ADV_name clusterA@80
```
This command starts an advisor on port 80 for clusterA. This advisor advises on all servers that are attached to port 80 for clusterA.
Group advisor
To start an advisor on port 80 for all other clusters, specify the port:
```
dscontrol advisor start ADV_name 80
```
This command starts the advisor on port 80 for all clusters and sites that do not currently have a cluster or site-specific advisor. Your advisor advises on all servers that are attached to port 80.
1. Optional: If you are starting the HTTP or HTTPS advisor, you might want to define a unique client URL string to allow the advisor to monitor individual services in the server. For more information, see Getting service-specific advice with the advisor request or response option.
Optional: Set the advisor interval.
The advisor interval sets how often an advisor asks for status from the servers on the port it is monitoring and then reports the results to the manager. If the advisor interval is too low, it can mean poor performance as a result of the advisor constantly interrupting the servers. If the advisor interval is too high, it can mean that the manager’s decisions about weighting is not based on accurate, up-to-date information.
Note: The advisor defaults work efficiently for most possible scenarios. Be careful when you enter values other than the defaults.
For example, to set the interval to 3 seconds for the HTTP advisor for port 80, enter the following command:
```
dscontrol advisor interval http 80 3
```
It does not make sense to specify an advisor interval that is smaller than the manager interval. The default advisor interval is 7 seconds.
Optional: Set the advisor report timeout.
The manager can make sure that out-of-date information is not used in its load-balancing decisions. The manager does not use information from the advisor whose timestamp is older than the time set in the advisor report timeout. The advisor report timeout must be larger than the advisor polling interval. If the timeout is smaller, the manager ignores reports that logically would be used. By default, advisor reports do not time out. The default value is unlimited.
For example, to set the advisor report timeout to 30 seconds for the HTTP advisor for port 80, enter the following command:
```
dscontrol advisor timeout http 80 30
```
For more information on setting the advisor report timeout, see dscontrol advisor.
Optional: Set the advisors connect and receive timeout values.
For Load Balancer, you can set the advisor’s timeout values at which it detects a particular port on the server (a service) is failed. The failed-server connecttimeoutand receivetimeout timeout values determine how long an advisor waits before it reports that either a connect or receive has failed.
To obtain the fastest failed-server detection, set the advisor connect and receive timeouts to the smallest value , which is 1 second, and set the advisor and manager interval time to the smallest value, which is 1 second.
Note: If your environment experiences a moderate to high volume of traffic such that server response time increases, be careful not to set the connecttimeout and receivetimeout values too small. The advisor might prematurely mark a busy server as failed.
For example, to set the connecttimeout and receivetimeout to 9 seconds for the HTTP advisor on port 80, type the following command:
```
dscontrol advisor connecttimeout http 80 9

dscontrol advisor receivetimeout http 80 9
```
The default for connect and receive timeout is three times the value that is specified for the advisor interval time.
Optional: Set the advisor retry value.
Advisors can retry a connection before they mark a server down. The advisor does not mark a server down until the server query fails the number of retries plus 1. The retry value must be no larger than 3.
The following command sets a retry value of 2 for the LDAP advisor on port 389:
```
dscontrol advisor retry ldap 389 2
```