hello
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Definition
hello
New in version 5.0.
hello
returns a document that describes the role of themongod
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 use
hello
to determine the state of the replica set members and to discover additional members of a replica set.
Compatibility
This command is available in deployments hosted in the following environments:
MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud
Note
This command is supported in all MongoDB Atlas clusters. For information on all commands, see Unsupported Commands.
MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB
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 mongosh
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 amongos
instance, or a standalonemongod
.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.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.
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
ormongos
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
ormongos
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
ormongos
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 themongod
ormongos
instance.The field is only available if compression is used. For example:
If the
mongod
is enabled to use both thesnappy,zlib
compressors and a client has specifiedzlib
, thecompression
field would contain:"compression": [ "zlib" ] If the
mongod
is enabled to use both thesnappy,zlib
compressors and a client has specifiedzlib,snappy
, thecompression
field would contain:"compression": [ "zlib", "snappy" ] If the
mongod
is enabled to use thesnappy
compressor and a client has specifiedzlib,snappy
, thecompression
field would contain :"compression": [ "snappy" ] If the
mongod
is enabled to use thesnappy
compressor and a client has specifiedzlib
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
ormongos
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:
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.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
of0
.This field only appears if there is at least one member with a
members[n].priority
of0
.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. ThearbiterOnly
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. Thepassive
field is only present for members with amembers[n].priority
of0
.
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 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
andsettings.getLastErrorDefaults
.
For more information, see Configure Replica Set Tag Sets.
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.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 by
majority
reads.
For details on the ok
status field, the operationTime
field,
and the $clusterTime
field, see
Command Response.