Can mongos 6.0 connect to mongodb 4.4?

[AmitG (Amit Gupta)]

Hi, we currently run a mongodb 4.4 sharded cluster. The documentation says

The mongos binary will crash when attempting to connect to mongod instances whose feature compatibility version (fCV) is greater than that of the mongos

Does this mean that a mongos 6.0 instance should be able to connect to a mongodb 4.4 sharded cluster since the sharded cluster’s FCV (4.4) is less tha 6.0? Sorry if this is an obvious/dumb question.

[chris (Chris Dellaway)]

In fact no, this is not the case. mongos need to be the same version as the mongod.

mongos can be a major version below the rest of the sharded cluster, during a version upgrade mongos is the last component to be upgraded before upgrading the FCV.

If you upgrade or use a mongos version higher than that of the cluster you’re likely to see mongos not connecting to the shard and config replica sets.

{
  "t": {
    "$date": "2022-11-05T18:20:19.555+00:00"
  },
  "s": "I",
  "c": "NETWORK",
  "id": 4712102,
  "ctx": "ReplicaSetMonitor-TaskExecutor",
  "msg": "Host failed in replica set",
  "attr": {
    "replicaSet": "c0",
    "host": "mongo-c-a:27019",
    "error": {
      "code": 188,
      "codeName": "IncompatibleServerVersion",
      "errmsg": "remote host has incompatible wire version: Server min and max wire version (9,9) is incompatible with client min wire version (13,13).You (client) are attempting to connect to a node (server) with a binary version with which you (client) no longer accept connections. Please upgrade the server’s binary version."
    },
    "action": {
      "dropConnections": false,
      "requestImmediateCheck": false,
      "outcome": {
        "host": "mongo-c-a:27019",
        "success": false,
        "errorMessage": "IncompatibleServerVersion: remote host has incompatible wire version: Server min and max wire version (9,9) is incompatible with client min wire version (13,13).You (client) are attempting to connect to a node (server) with a binary version with which you (client) no longer accept connections. Please upgrade the server’s binary version."
      }
    }
  }
}