Array Features

The array software offers the following features:

• reliability

• scalability

• load balancing

How these features are provided is described in the following subsections.

Reliability

The array software provides reliability by:

1. monitoring individual host hardware and software, and upon failure of some component...

2. moving the service address of a service instance on a host to a different host.

Existing clients of a moved service address continue to be served. The DNS within the Netra Proxy Cache array removes the moved service address from proxy cache service provided by the array, so that new clients are never connected to it.

Scalability

The array software provides scalability by implementing a protocol that responds dynamically to changes in array membership. At a frequent and regular interval, the array daemon multicasts information messages over the control network. These information messages are, at once, a heartbeat and a means of conveying "health" information about each host.

When a host is removed--for example, if a machine is receiving a software upgrade--within milliseconds, the array detects the machine's absence and removes the machine's service address(es) from availability. Similarly, if a machine is added to the array, nearly immediately, the array detects the new host and makes its service address available for incoming client requests.

Load Balancing

The array software provides load balancing through a modified DNS round robin. The Netra Proxy Cache array forms its own DNS zone that consists of the service addresses associated with the proxy cache service. This zone is identified by its own domain name.

When a host fails or becomes overloaded, the array software removes the host from the array's DNS round robin, so that the down host receives no new client requests. This process is discussed in greater detail in "The Role of DNS".