hello (database command) (original) (raw)

hello

New in version 4.2.

hello returns a document that describes the role of the mongod instance. If the optional fieldsaslSupportedMechs is specified, the command also returns anarray of SASL mechanisms used to create the specified user's credentials.

If the instance is a member of a replica set, then hello returns a subset of the replica set configuration and status including whether or not the instance is the primary of the replica set.

When sent to a mongod instance that is not a member of a replica set, hello returns a subset of this information.

MongoDB drivers and clients usehello to determine the state of the replica set members and to discover additional members of a replica set.

This command is available in deployments hosted in the following environments:

Note

This command is supported in all MongoDB Atlas clusters. For information on Atlas support for all commands, seeUnsupported Commands.

The command has the following syntax:


db.runCommand(

   {

     hello: 1

   }

)

The hello command accepts optional fieldssaslSupportedMechs: <db.user> to return an additional fieldhello.saslSupportedMechs in its result and comment <any> to add a log comment associated with the command.


db.runCommand(

   {

     hello: 1,

     saslSupportedMechs: "<db.username>",

     comment: <any>

   }

)

The db.hello() method in mongoshprovides a wrapper around hello.

If the client that issued hello disconnects before the operation completes, MongoDB marks hello for termination usingkillOp.

The following hello fields are common across all roles:

hello.isWritablePrimary

A boolean value that reports when this node is writable. Iftrue, then this instance is a primary in areplica set, or a mongos instance, or a standalonemongod.

This field will be false if the instance is asecondary member of a replica set or if the member is anarbiter of a replica set.

hello.topologyVersion

For internal use by MongoDB.

hello.maxBsonObjectSize

The maximum permitted size of a BSON object in bytes for this mongod process. If not provided, clients should assume a max size of "16 * 1024 * 1024".

hello.maxMessageSizeBytes

The maximum permitted size of a BSON wire protocol message. The default value is 48000000 bytes.

hello.maxWriteBatchSize

The maximum number of write operations permitted in a write batch. If a batch exceeds thislimit, the client driver divides the batch into smaller groups each with counts less than or equal to the value of this field.

The value of this limit is 100,000 writes.

hello.localTime

Returns the local server time in UTC. This value is anISO date.

hello.logicalSessionTimeoutMinutes

The time in minutes that a session remains active after its most recent use. Sessions that have not received a new read/write operation from the client or been refreshed withrefreshSessions within this threshold are cleared from the cache. State associated with an expired session may be cleaned up by the server at any time.

Only available when featureCompatibilityVersion is "3.6" or greater.

hello.connectionId

An identifier for the mongod / mongosinstance's outgoing connection to the client.

hello.minWireVersion

The earliest version of the wire protocol that thismongod or mongos instance is capable of using to communicate with clients.

Clients may use minWireVersion to help negotiate compatibility with MongoDB.

hello.maxWireVersion

The latest version of the wire protocol that this mongodor mongos instance is capable of using to communicate with clients.

Clients may use maxWireVersion to help negotiate compatibility with MongoDB.

hello.readOnly

A boolean value that, when true, indicates that themongod or mongos is running in read-only mode.

hello.compression

An array listing the compression algorithms used or available for use (i.e. common to both the client and the mongod ormongos instance) to compress the communication between the client and the mongod ormongos instance.

The field is only available if compression is used. For example:

"compression": [ "zlib" ]  
"compression": [ "zlib", "snappy" ]  
"compression": [ "snappy" ]

That is, if the client does not specify compression or if the client specifies a compressor not enabled for the connectedmongod or mongos instance, the field does not return.

hello.saslSupportedMechs

An array of SASL mechanisms used to create the user's credential or credentials. Supported SASL mechanisms are:

The field is returned only when the command is run with thesaslSupportedMechs field:


db.runCommand( { hello: 1, saslSupportedMechs: "<db.username>" } )

mongos instances add the following field to thehello response document:

hello.msg

Contains the value isdbgrid when helloreturns from a mongos instance.

hello contains these fields when returned by a member of a replica set:

hello.hosts

An array of strings in the format of "[hostname]:[port]" that lists all members of the replica set that are neitherhidden, passive, nor arbiters.

Drivers use this array and the hello.passives to determine which members to read from.

hello.setName

The name of the current :replica set.

hello.setVersion

The current replica set config version.

hello.secondary

A boolean value that, when true, indicates if themongod is a secondary member of a replica set.

hello.passives

An array of strings in the format of "[hostname]:[port]"listing all members of the replica set which have amembers[n].priority of 0.

This field only appears if there is at least one member with amembers[n].priority of 0.

Drivers use this array and the hello.hosts to determine which members to read from.

hello.arbiters

An array of strings in the format of "[hostname]:[port]"listing all members of the replica set that arearbiters.

This field only appears if there is at least one arbiter in the replica set.

hello.primary

A string in the format of "[hostname]:[port]" listing the current primary member of the replica set.

hello.arbiterOnly

A boolean value that , when true, indicates that the current instance is an arbiter. The arbiterOnlyfield is only present, if the instance is an arbiter.

hello.passive

A boolean value that, when true, indicates that the current instance is passive. Thepassive field is only present for members with amembers[n].priority of 0.

hello.hidden

A boolean value that, when true, indicates that the current instance is hidden. Thehidden field is only present for hidden members.

hello.tags

A tags document contains user-defined tag field and value pairs for the replica set member.


{ "<tag1>": "<string1>", "<tag2>": "<string2>",... }

For more information, seeConfigure Replica Set Tag Sets.

Tip

hello.me

The [hostname]:[port] of the member that returnedhello.

hello.electionId

A unique identifier for each election. Included only in the output of hello for the primary. Used by clients to determine when elections occur.

hello.lastWrite

A document containing optime and date information for the database's most recent write operation.

hello.lastWrite.opTime

An object giving the optime of the last write operation.

hello.lastWrite.lastWriteDate

A date object containing the time of the last write operation.

hello.lastWrite.majorityOpTime

An object giving the optime of the last write operation readable by majority reads.

hello.lastWrite.majorityWriteDate

A date object containing the time of the last write operation readable bymajority reads.

For details on the ok status field, the operationTime field, and the $clusterTime field, seeCommand Response.