[mongodb] Query DBs status and stats only for master and primary members

[kris-lab]

Since WiredTiger/mongo@3.0 there is no possibility to list databases and stats for databases on secondary members.

Only partial data is collected

I, [2017-01-27T10:36:43.914870 #18952]  INFO -- : Starting plugin mongodb with config {"hostname"=>"0.0.0.0", "port"=>27018, "user"=>"xxx", "password"=>"xxx"}
E, [2017-01-27T10:38:44.022819 #18952] ERROR -- mongodb rep1-db7.mongodb.xxx.net::mongodb::0_0_0_0: Measurement timeout of 120.0 seconds reached.
E, [2017-01-27T10:40:44.023639 #18952] ERROR -- mongodb rep1-db7.mongodb.xxx.net::mongodb::0_0_0_0: Measurement timeout of 120.0 seconds reached.

[kris-lab]

rep1:STARTUP2> db.adminCommand({serverStatus:1})
{
 (...)
    "repl" : {
        "setName" : "rep1",
        "setVersion" : 14,
        "ismaster" : true,
        "secondary" : false,
        "hosts" : [
            "rep1-db5.mongodb.xxx.net:27018",
            "rep1-db6.mongodb.xxx.net:27018",
            "rep1-db7.mongodb.xxx.net:27018"
        ],
        "arbiters" : [
            "rep1-arbiter3.mongodb.xxx.net:27018"
        ],
        "primary" : "rep1-db6.mongodb.xxx.net:27018",
        "me" : "rep1-db7.mongodb.xxx.net:27018",
        "rbid" : 2086099880
    },
 (...)
}

[kris-lab]

@njam please review

[njam]

Seems strange that we can't collect slow queries on secondaries. But since we plan to move to managed mongodb anyway it's not a big issue.

Is it okay to not set some fields for bipbip and Copperegg?

otherwise lgtm

[kris-lab]

Is it okay to not set some fields for bipbip and Copperegg?

seems to be ok! It runs like that for the moment.

Seems strange that we can't collect slow queries on secondaries. But since we plan to move to managed mongodb anyway it's not a big issue.

agree, it looks like the slave/secondary doesn't allow to browse any specific data. Maybe if you cannot really read, then slow queries metric doesn't make sense anyway. I am wondering how it works if you actually wanna read data from secondaries. Not sure if this is supported in 3.0+, should I investigate?

[kris-lab]

@njam also WiredTiger does not include backgroundFlushing section in servers status. It is because it has own and very detailed object with stats.

{
    (...)
    "storageEngine" : {
        "name" : "wiredTiger"
    },
    "wiredTiger" : {
            (many stats)
        }
    (...)
}

[njam]

We never read from secondaries so far. Let's keep it simple and remove unsupported stats for now?

[kris-lab]

@njam please re-review

[njam]

lgtm