Planning for RTMON application server group configuration

What you need to plan for

When you configure RTMON to work with application servers, you must

Identify the application servers to which the appliance will send data
Identify the data you want the appliance to send to specific groups of application servers

Application server group defined

When you configure RTMON to work with application servers, you need to configure one or more logical application server groups in the appliance RTMON configuration.

Application server groups have the following primary purposes:

To enable you to identify which data you want the appliance to send to which application servers
Each group is identified by a logical group name of your choosing. You could, for example, assign a group of billing servers a name that would identify them as billing servers. Mappings of the group name to IP addresses is handled by DNS.

You might have different types of application servers that need the same data. Alternatively, you might have two different types of application servers that require different data. You define, by logical group, the data that the appliance is to send to servers in the group. See Planning for data that RTMON can send to application servers.
To provide a failover scheme in case of application server unavailability
To provide a load-balancing scheme

Data that each server in an application server group receives

When multiple application servers are members of the same logical application server group, each server receives the same user events and user attributes. However, NetCache always sends a specific user's data to the same application server, which balances the load over the server as described for load balancing. See also What occurs during failover.

How load balancing works

To balance the load of data across application servers in the same application server group, NetCache uses a weighting technique based on a history of connection round trips. Server CPU load and other server resources are not considered, nor are differences such as bandwidth used by the streams that specific clients are watching.

Initially, all servers have the same chance of being selected. If a server is consistently slower in responding to the appliance over time, the chance decreases that the appliance will send data to the application server.

In practice, this scheme has resulted in a fairly even distribution of data across application servers in an application server group. However, you should monitor the load on your application servers resulting from data pushed by the appliance and make adjustments to your server size or number of servers as necessary.

What occurs during failover

When you define an application group, you are instructing the appliance to distribute the data over all servers in the group. If one server becomes unavailable, NetCache continues to automatically distribute the data over the remaining available servers in the group.

The following occurs if an application server in an application server group becomes unavailable:

RTMON sends data about new users to available servers in the same application server group.
For users whose data was being sent to the server that became unavailable, the following occurs:
- If HTTP POST and buffering are configured on the appliance, the appliance attempts to resend the events in the buffer after the server becomes available again.
  The NetCache appliance retains the data until the logs are deleted. By default, NetCache buffers data until it runs out of disk space. A rough estimate of when the appliance would run out of disk space is when the remaining disk space falls below 10 percent of the total NetCache volume size.
- If TCP socket is configured on the appliance, no buffering will occur. Therefore, no data is available for the appliance to resend to an application server after the server becomes available again.
Network Appliance recommends that you set up HTTP with buffering so that the appliance can buffer data for resending when the server becomes available.

Number of application servers communicating with NetCache

You can add application servers to application server groups as necessary to increase reliability and scalability. Network Appliance recommends that you do not exceed 10 application servers communicating with a NetCache appliance.

Considerations for defining application server groups

Consider the following when defining your application server groups:

You might want to include servers on different networks in the same application server group. That way, if one network becomes unavailable, the other servers in the application server group can handle the monitoring services.
You might want to use dedicated servers to reduce the possibility of the servers becoming bottlenecks.
You can group servers by the type of data they need the appliance to send to them. See Planning for data that RTMON can send to application servers.