Overview
Real-Time Server Status Information
Equalizer gathers real-time information about a server’s status using ICMP Probes, TCP Probes,
Active Content Verification (ACV), and Server Agents. ICMP and TCP Probes are the default
probing methods.
ICMP Probes use Internet Control Message Protocol to send an "Echo request" to the server, and
then wait for the server to respond with an ICMP "Echo reply" message (i.e., the Unix
ping
command). ICMP is a Layer 3 protocol. ICMP probes can be disabled via a global flag.
TCP Probes establish (and tear down) a TCP connection between the appliance and the server in a
typical Layer 4 exchange of TCP SYN, ACK, and FIN packets. If the connection cannot be
completed, the appliance considers the server down and stops routing requests to it. TCP probes
cannot be disabled.
Active Content Verification (ACV) provides an optional method for checking the validity of a
server’s response using Layer 7 network services that support a text-based request/response
protocol, such as HTTP. When you enable ACV for a cluster, the appliance requests data from each
server in the cluster (using an ACV Probe string)and verifies the returned data (against anACV
Response string). If it receives no response or the response string is not in the response, the
verification fails and it stops routing new requests to that server. See
for more information.
Note
- ACV is not supported for Layer 4 UDP clusters.
Server Agent Probes enable the appliance to communicate with a user-written program (the
agent) running on the server. A server agent is written to open a server port and, when the
appliance connects to the port, the server agent responds with an indication of the current server
load and performance. This enables adjustment of the dynamic weights of the server according to
detailed performance measurements performed by the agent, based on any metrics available on
the server. If the server is overloaded and you have enabled
server agent
load balancing, the
appliance reduces the server’s dynamic weight so that the server receives fewer requests. The
interface between the appliance and server agents is simple and well-defined. Agents can be
written in any language supported on the server (e.g., perl, C, shell script, javascript, etc.). See
Simple Health Checks and Load Balancing Policies
for more information.
For those who have one or more VMware ESX Servers, VLB can be configured to use VMware’s
status reporting to determine server status, and can also be configured to automatically manage
VMware servers based on status information obtained from VMware.
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Copyright © 2014 Coyote Point Systems, A Subsidiary of Fortinet, Inc.
Summary of Contents for Equalizer GX Series
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