Hi,

We are using sharded cluster in our staging environment. Currently we are at version 4.0. We have upgraded from v3.6 to v4.0 successfully. But now facing issue while upgrade from v4.0 to v4.2. Following is the sequence of upgraded components.

1. Config servers - upgraded to v4.2 - No errors
2. Shards servers - upgraded to v4.2 - No errors
3. Mongos - upgraded to v4.2 with errors

When we upgrade the first mongos server to v4.2 and start the service, we will see below logs in mongos.log file. Also, further if we run db command like “show ** collections”, we will get same error on mongo shell.

*Mongos.log file:*
2021-07-23T11:16:08.823+0000 I  SH_REFR  [ConfigServerCatalogCacheLoader-0] Refresh for database tap took 2 ms and failed :: caused by :: InternalError: DatabaseVersion doesn't exist in database entry { _id: "tap", partitioned: false, primary: "int-gws1" } despite the config server being in binary version 4.2 or later.

**

Mongos shell:

mongos> use tap
switched to db tap
mongos> show collections
2021-07-23T11:16:08.825+0000 E  QUERY    [js] uncaught exception: Error: listCollections failed: {
        "ok" : 0,
        "errmsg" : "DatabaseVersion doesn't exist in database entry { _id: \"tap\", partitioned: false, primary: \"int-gws1\" } despite the config server being in binary version 4.2 or later.",
        "code" : 1,
        "codeName" : "InternalError",
        "operationTime" : Timestamp(1627038991, 1),
        "$clusterTime" : {
                "clusterTime" : Timestamp(1627038991, 1),
                "signature" : {
                        "hash" : BinData(0,"YseuzguVafN0/tLEPsnOSkBM/XY="),
                        "keyId" : NumberLong("6963542575651225631")
                }
        }
} :
_getErrorWithCode@src/mongo/shell/utils.js:25:13
DB.prototype._getCollectionInfosCommand@src/mongo/shell/db.js:835:15
DB.prototype.getCollectionInfos@src/mongo/shell/db.js:883:16
shellHelper.show@src/mongo/shell/utils.js:893:9
shellHelper@src/mongo/shell/utils.js:790:15
@(shellhelp2):1:1

• • 
    We also ignored this error and upgraded all mongos servers to v4.2 and restarted the mongos services, but error still exists in mongos.log file. Could you please help us to resolve this issue.