hello

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Definition

Syntax
Behavior
Output

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Definition

hello

New in version 4.4.2: (and 4.2.10, 4.0.21, and 3.6.21)

hello returns a document that describes the role of the mongod instance. If the optional field saslSupportedMechs is specified, the command also returns an array 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 use hello to determine the state of the replica set members and to discover additional members of a replica set.

Syntax

The command takes the following form:

db.runCommand( { hello: 1 } )

The hello command accepts optional fields saslSupportedMechs: <db.user> to return an additional field hello.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 the mongo shell provides a wrapper around hello.

Behavior

Client Disconnection

Starting in MongoDB 4.2, if the client that issued hello disconnects before the operation completes, MongoDB marks hello for termination using killOp.

Output

All Instances

The following hello fields are common across all roles:

hello.isWritablePrimary

A boolean value that reports when this node is writable. If true, then this instance is a primary in a replica set, or a mongos instance, or a standalone mongod.

This field will be false if the instance is a secondary member of a replica set or if the member is an arbiter 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 this limit, 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 an ISO 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 with refreshSessions 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. See Backwards Incompatible Features.

hello.connectionId: An identifier for the mongod / mongos instance's outgoing connection to the client.

hello.minWireVersion

The earliest version of the wire protocol that this mongod 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 mongod or 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 the mongod 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 or mongos instance) to compress the communication between the client and the mongod or mongos instance.

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

If the mongod is enabled to use both the snappy,zlib compressors and a client has specified zlib, the compression field would contain:
```
"compression": [ "zlib" ]
```
If the mongod is enabled to use both the snappy,zlib compressors and a client has specified zlib,snappy, the compression field would contain:
```
"compression": [ "zlib", "snappy" ]
```
If the mongod is enabled to use the snappy compressor and a client has specified zlib,snappy, the compression field would contain :
```
"compression": [ "snappy" ]
```
If the mongod is enabled to use the snappy compressor and a client has specified zlib or the client has specified no compressor, the field is omitted.

That is, if the client does not specify compression or if the client specifies a compressor not enabled for the connected mongod 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:

GSSAPI
SCRAM-SHA-256
SCRAM-SHA-1

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

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

Sharded Instances

mongos instances add the following field to the hello response document:

hello.msg: Contains the value isdbgrid when hello returns from a mongos instance.

Replica Sets

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 neither hidden, 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 the mongod 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 a members[n].priority of 0.

This field only appears if there is at least one member with a members[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 are arbiters.

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 arbiterOnly field is only present, if the instance is an arbiter.

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

hello.hidden: A boolean value that, when true, indicates that the current instance is hidden. The hidden 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 read operations, you can specify a tag set in the read preference to direct the operations to replica set member(s) with the specified tag(s).
For write operations, you can create a customize write concern using settings.getLastErrorModes and settings.getLastErrorDefaults.

For more information, see Configure Replica Set Tag Sets.