MongoDB Docker Replica Set - Application Connection Timeout

Question

I was successfully was able to create the docker replica set using three containers:

CONTAINER ID        IMAGE               PORTS              NAMES
b530275d1958        mongo       0.0.0.0:30003->27017/tcp   mongo3
dca4fa2d6f93        mongo       0.0.0.0:30002->27017/tcp   mongo2
0b4823661cf1        mongo       0.0.0.0:27017->27017/tcp   mongo1

In addition, replica set is successfully configured:

rs0:PRIMARY> rs.status()
{
    "set" : "rs0",
    "date" : ISODate("2018-01-08T20:57:30.395Z"),
    "myState" : 1,
    "term" : NumberLong(16),
    "heartbeatIntervalMillis" : NumberLong(2000),
    "optimes" : {
        "lastCommittedOpTime" : {
            "ts" : Timestamp(1515445046, 1),
            "t" : NumberLong(16)
        },
        "readConcernMajorityOpTime" : {
            "ts" : Timestamp(1515445046, 1),
            "t" : NumberLong(16)
        },
        "appliedOpTime" : {
            "ts" : Timestamp(1515445046, 1),
            "t" : NumberLong(16)
        },
        "durableOpTime" : {
            "ts" : Timestamp(1515445046, 1),
            "t" : NumberLong(16)
        }
    },
    "members" : [
        {
            "_id" : 0,
            "name" : "mongo1:27017",
            "health" : 1,
            "state" : 1,
            "stateStr" : "PRIMARY",
            "uptime" : 898,
            "optime" : {
                "ts" : Timestamp(1515445046, 1),
                "t" : NumberLong(16)
            },
            "optimeDate" : ISODate("2018-01-08T20:57:26Z"),
            "electionTime" : Timestamp(1515444174, 1),
            "electionDate" : ISODate("2018-01-08T20:42:54Z"),
            "configVersion" : 1,
            "self" : true
        },
        {
            "_id" : 1,
            "name" : "mongo2:27017",
            "health" : 1,
            "state" : 2,
            "stateStr" : "SECONDARY",
            "uptime" : 877,
            "optime" : {
                "ts" : Timestamp(1515445046, 1),
                "t" : NumberLong(16)
            },
            "optimeDurable" : {
                "ts" : Timestamp(1515445046, 1),
                "t" : NumberLong(16)
            },
            "optimeDate" : ISODate("2018-01-08T20:57:26Z"),
            "optimeDurableDate" : ISODate("2018-01-08T20:57:26Z"),
            "lastHeartbeat" : ISODate("2018-01-08T20:57:29.056Z"),
            "lastHeartbeatRecv" : ISODate("2018-01-08T20:57:30.324Z"),
            "pingMs" : NumberLong(0),
            "syncingTo" : "mongo1:27017",
            "configVersion" : 1
        },
        {
            "_id" : 2,
            "name" : "mongo3:27017",
            "health" : 1,
            "state" : 2,
            "stateStr" : "SECONDARY",
            "uptime" : 871,
            "optime" : {
                "ts" : Timestamp(1515445046, 1),
                "t" : NumberLong(16)
            },
            "optimeDurable" : {
                "ts" : Timestamp(1515445046, 1),
                "t" : NumberLong(16)
            },
            "optimeDate" : ISODate("2018-01-08T20:57:26Z"),
            "optimeDurableDate" : ISODate("2018-01-08T20:57:26Z"),
            "lastHeartbeat" : ISODate("2018-01-08T20:57:29.055Z"),
            "lastHeartbeatRecv" : ISODate("2018-01-08T20:57:29.506Z"),
            "pingMs" : NumberLong(0),
            "syncingTo" : "mongo1:27017",
            "configVersion" : 1
        }
    ],
    "ok" : 1,
    "operationTime" : Timestamp(1515445046, 1),
    "$clusterTime" : {
        "clusterTime" : Timestamp(1515445046, 1),
        "signature" : {
            "hash" : BinData(0,"c3aBSVWElstPhnBx3c5NysBfdmk="),
            "keyId" : NumberLong("6504664228182360065")
        }
    }
}

It's important to note that it is one VM only connection at ports 27017 , 30002 AND 30003

Yet I seem to be having an issue when my PHP application connects to it using the mongo DSN string that I attach it to

mongodb://<username>:<pass>@172.31.6.177:27017,172.31.6.177:30002,172.31.6.177:30003/opsserver-main?replicaSet=rs0&amp;authSource=admin"

---

Error from my PHP application:

[Doctrine\MongoDB\Exception\ResultException]                                                  
  No suitable servers found: `serverSelectionTimeoutMS` expired: [connection timeout calling i  
  smaster on '172.31.6.177:27017'] [connection timeout calling ismaster on 'mongo1:27017'] [co  
  nnection timeout calling ismaster on 'mongo2:27017'] [connection timeout calling ismaster on  
   'mongo3:27017']  

It is weird to me that it knows the container names , or at least trying to connect to it. What am i doing wrong?

Answer 1

This is because inside the docker network, the containers mongo1, mongo2, and mongo3 can find each other. However, those names are not known by the host system, so your application cannot connect to the whole set.

You can try pinging mongo1 from your host system, where it should say Unknown host.

The easiest way to solve this is to change the hostnames in your replica set config (the hostnames you use when initiating the replica set with rs.initiate(...)), e.g.:

• mongo1:27017 --> <your local hostname>:27017
• mongo2:27017 --> <your local hostname>:30002
• mongo3:27017 --> <your local hostname>:30003

Replace <your local hostname> with the output of the hostname -f command.

This way, your local host can find all your replica set nodes, and all your replica set nodes can still find each other via your local host's port mappings.