Coyote Point Systems Equalizer, Руководство по установке и администрированию

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Introducing Equalizer
Equalizer Installation and Administration Guide 3
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Static Weight—requests are distributed among the servers according to user-defined
relative weights. A server with a higher static weight will get a higher percentage of
the incoming requests.
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Adaptive— Equalizer monitors the performance of each server and adjusts the
percentage of requests routed to each server accordingly.
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Fastest Response—Equalizer optimizes its adaptive server selection to route requests
to the server with the fastest response time.
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Least Connections—Equalizer optimizes its adaptive server selection to route requests
to the server with the fewest connections.
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Server Agent—Equalizer optimizes its adaptive server selection based on actual
performance statistics collected by server agent daemons running on each server.
When you configure the servers in a virtual cluster, you assign a static weight between 20
and 200 for each server. When one of the adaptive load balancing algorithms is selected,
Equalizer uses the servers’ static weights as a starting point when determining what
percentage of requests to route to each server. Each server contributes to the total load
based on its percentage of the total weights in the server cluster. However, the static
weight assigned to a server doesn’t dictate the percentage of requests that will be routed to
the server when one of the adaptive algorithms is selected. Equalizer dynamically adjusts
server weights according to real-time conditions to ensure that requests are routed to the
server that is actually best able to respond. A server with a weight of zero (0) is considered
down or unavailable—Equalizer will not route new requests to servers in this state.
Real Time Server Status Information
Equalizer can gather real-time information about a server’s status using Server Agents and
Active Content Verification.
Server Agents can be installed on each server that provide Equalizer with real-time
performance statistics. This enables Equalizer to adjust the dynamic weights of the servers
in a cluster according to their actual performance characteristics. When server agents are
used, Equalizer periodically contacts the server agent daemon on each server and
downloads the server performance statistics. When queried, a server agent returns a value
in the range 0-100. Server agent values above 60-70 indicate that the server is overloaded.
If this persists and adaptive load balancing is enabled, Equalizer responds by reducing the
server’s dynamic weight so that fewer requests are routed to the server.
Equalizer’s Active Content Verification (ACV) provides a way to check the validity of a
server. When ACV is enabled for a cluster, Equalizer requests data from each server in the
cluster and verifies the returned data. ACV can be used with most network services that
support a text-based request/response protocol, such as HTTP. However, ACV cannot be
used with UDP-based services. You control what information is used to verify the servers
by specifying an ACV Probe String and an ACV Response string. Equalizer uses the probe
string to request data from each server. To verify the server’s content, Equalizer searches
the returned data for the response string. By default, Equalizer expects to receive a
response within .2 seconds when performing active content verification. If no response is
received or the response string is not found in the first 1024 characters of the response, the
verification fails and Equalizer will stop routing new requests to that server.