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If a replica set of SNMP subagents are all running on the same port on different hosts, then SNMP responses are ambiguous - only one "27017" entry is returned, and for which member appears to be arbitrary.
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MONGO-MIB::globalOpInsert."27017" = Counter32: 1
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MONGO-MIB::globalOpQuery."27017" = Counter32: 60
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MONGO-MIB::globalOpUpdate."27017" = Counter32: 1
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MONGO-MIB::globalOpDelete."27017" = Counter32: 0
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...
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MONGO-MIB::replSetName."27017" = STRING: "snmpd"
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MONGO-MIB::replSetVersion."27017" = INTEGER: 3
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MONGO-MIB::replIsMaster."27017" = INTEGER: 1
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MONGO-MIB::replIsSecondary."27017" = INTEGER: 0
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MONGO-MIB::replPrimary."27017" = STRING: "hostA:27017"
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MONGO-MIB::replMe."27017" = STRING: "hostA:27017"
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Furthermore, if the replica set member identified by the MONGO-MIB::replMe field is brought down (in the above example, say hostA:27017 becomes unavailable), then the snmpwalk becomes unresponsive - does not return any results from any of the other members.
This behavior would seem to limit the utility of the subagent model given a "typical" replica set deployment. However of course a workaround exists to always utilize a distinct set of ports when using SNMP.
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