- Reference >
- Database Commands >
- Replication Commands >
- isMaster
isMaster¶
On this page
Definition¶
-
isMaster
¶ isMaster
returns a document that describes the role of themongod
instance.If the instance is a member of a replica set, then
isMaster
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,isMaster
returns a subset of this information.MongoDB drivers and clients use
isMaster
to determine the state of the replica set members and to discover additional members of a replica set.The
db.isMaster()
method in themongo
shell provides a wrapper aroundisMaster
.The command takes the following form:
See also
Output¶
All Instances¶
The following isMaster
fields are common across all
roles:
-
isMaster.
ismaster
¶ A boolean value that reports when this node is writable. If
true
, then this instance is a primary in a replica set, or a master in a master-slave configuration, 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.
-
isMaster.
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
”.
-
isMaster.
maxMessageSizeBytes
¶ New in version 2.4.
The maximum permitted size of a BSON wire protocol message. The default value is
48000000
bytes.
-
isMaster.
localTime
¶ New in version 2.2.
Returns the local server time in UTC. This value is an ISO date.
-
isMaster.
minWireVersion
¶ New in version 2.6.
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.
-
isMaster.
maxWireVersion
¶ New in version 2.6.
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.
Replica Sets¶
isMaster
contains these fields when returned by a member
of a replica set:
-
isMaster.
setName
¶ The name of the current :replica set.
-
isMaster.
secondary
¶ A boolean value that, when
true
, indicates if themongod
is a secondary member of a replica set.
-
isMaster.
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
isMaster.passives
to determine which members to read from.
-
isMaster.
passives
¶ An array of strings in the format of
"[hostname]:[port]"
listing all members of the replica set which have apriority
of0
.This field only appears if there is at least one member with a
priority
of0
.Drivers use this array and the
isMaster.hosts
to determine which members to read from.
-
isMaster.
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.
-
isMaster.
primary
¶ A string in the format of
"[hostname]:[port]"
listing the current primary member of the replica set.
-
isMaster.
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.
-
isMaster.
passive
¶ A boolean value that, when
true
, indicates that the current instance is passive. Thepassive
field is only present for members with apriority
of0
.
A boolean value that, when
true
, indicates that the current instance is hidden. Thehidden
field is only present for hidden members.
A document that lists any tags assigned to this member. This field is only present if there are tags assigned to the member. See Configure Replica Set Tag Sets for more information.