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mongos for “MongoDB Shard,” is a routing service for MongoDB shard configurations that processes queries from the application layer, and determines the location of this data in the sharded cluster, in order to complete these operations. From the perspective of the application, a mongos instance behaves identically to any other MongoDB instance.


Changed in version 2.1.

Some aggregation operations using the aggregate will cause mongos instances to require more CPU resources than in previous versions. This modified performance profile may dictate alternate architecture decisions if you use the aggregation framework extensively in a sharded environment.


--help, -h

Returns a basic help and usage text.


Returns the version of the mongod daemon.

--config <filename>, -f <filename>

Specifies a configuration file, that you can use to specify runtime-configurations. While the options are equivalent and accessible via the other command line arguments, the configuration file is the preferred method for runtime configuration of mongod. See the “Configuration File Options” document for more information about these options.

Not all configuration options for mongod make sense in the context of mongos.

--verbose, -v

Increases the amount of internal reporting returned on standard output or in the log file specified by --logpath. Use the -v form to control the level of verbosity by including the option multiple times, (e.g. -vvvvv.)


Runs the mongos instance in a quiet mode that attempts to limit the amount of output.

--port <port>

Specifies a TCP port for the mongos to listen for client connections. By default mongos listens for connections on port 27017.

UNIX-like systems require root access to access ports with numbers lower than 1024.

--bind_ip <ip address>

The IP address that the mongos process will bind to and listen for connections. By default mongos listens for connections all interfaces. You may attach mongos to any interface; however, when attaching mongos to a publicly accessible interface ensure that you have implemented proper authentication and/or firewall restrictions to protect the integrity of your database.

--maxConns <number>

Specifies the maximum number of simultaneous connections that mongos will accept. This setting will have no effect if the value of this setting is higher than your operating system’s configured maximum connection tracking threshold.

This is particularly useful for mongos if you have a client that creates a number of collections but allows them to timeout rather than close the collections. When you set maxConns, ensure the value is slightly higher than the size of the connection pool or the total number of connections to prevent erroneous connection spikes from propagating to the members of a shard cluster.


You cannot set maxConns to a value higher than 20000.


Forces the mongos to validate all requests from clients upon receipt to ensure that invalid objects are never inserted into the database. This option has a performance impact, and is not enabled by default.

--logpath <path>

Specify a path for the log file that will hold all diagnostic logging information.

Unless specified, mongos will output all log information to the standard output. Additionally, unless you also specify --logappend, the logfile will be overwritten when the process restarts.


Specify to ensure that mongos appends additional logging data to the end of the logfile rather than overwriting the content of the log when the process restarts.


New in version 2.1.0.

Sends all logging output to the host’s syslog system rather than to standard output or a log file as with --logpath.


You cannot use --syslog with --logpath.

--pidfilepath <path>

Specify a file location to hold the “PID” or process ID of the mongos process. Useful for tracking the mongos process in combination with the mongos --fork option.

Without a specified --pidfilepath option, mongos creates no PID file.

--keyFile <file>

Specify the path to a key file to store authentication information. This option is only useful for the connection between mongos instances and components of the sharded cluster.


Disables listening on the UNIX socket. mongos always listens on the UNIX socket, unless --nounixsocket is set, --bind_ip is not set, or --bind_ip specifies

--unixSocketPrefix <path>

Specifies a path for the UNIX socket. Unless this option has a value mongos creates a socket with /tmp as a prefix.

MongoDB will always create and listen on a UNIX socket, unless --nounixsocket is set, --bind_ip is not set, or --bind_ip specifies


Enables a daemon mode for mongos which forces the process to the background. This is the normal mode of operation, in production and production-like environments, but may not be desirable for testing.

--configdb <config1>,<config2><:port>,<config3>

Set this option to specify a configuration database (i.e. config database) for the sharded cluster. You must specify either 1 configuration server or 3 configuration servers, in a comma separated list.


mongos instances read from the first config server in the list provided. All mongos instances must specify the hosts to the --configdb setting in the same order.

If your configuration databases reside in more that one data center, order the hosts in the --configdb argument so that the config database that is closest to the majority of your mongos instances is first servers in the list.


Never remove a config server from the --configdb parameter, even if the config server or servers are not available, or offline.


This option is for internal testing use only, and runs unit tests without starting a mongos instance.


This option updates the meta data format used by the config database.

--chunkSize <value>

The value of the --chunkSize determines the size of each chunk, in megabytes, of data distributed around the sharded cluster. The default value is 64 megabytes, which is the ideal size for chunks in most deployments: larger chunk size can lead to uneven data distribution, smaller chunk size often leads to inefficient movement of chunks between nodes. However, in some circumstances it may be necessary to set a different chunk size.

This option only sets the chunk size when initializing the cluster for the first time. If you modify the run-time option later, the new value will have no effect. See the “Modify Chunk Size” procedure if you need to change the chunk size on an existing sharded cluster.


Enables IPv6 support to allow clients to connect to mongos using IPv6 networks. MongoDB disables IPv6 support by default in mongod and all utilities.


Permits JSONP access via an HTTP interface. Consider the security implications of allowing this activity before enabling this option.


Disables the scripting engine.


New in version 2.1.2.

Disables the HTTP interface.


New in version 2.2.

--localThreshold affects the logic that mongos uses when selecting replica set members to pass read operations to from clients. Specify a value to --localThreshold in milliseconds. The default value is 15, which corresponds to the default value in all of the client drivers.

When mongos receives a request that permits reads to secondary members, the mongos will:

  • find the member of the set with the lowest ping time.

  • construct a list of replica set members that is within a ping time of 15 milliseconds of the nearest suitable member of the set.

    If you specify a value for --localThreshold, mongos will construct the list of replica members that are within the latency allowed by this value.

  • The mongos will select a member to read from at random from this list.

The ping time used for a set member compared by the --localThreshold setting is a moving average of recent ping times, calculated, at most, every 10 seconds. As a result, some queries may reach members above the threshold until the mongos recalculates the average.

See the Member Selection section of the read preference documentation for more information.


New in version 2.0.7.

--noAutoSplit prevents mongos from automatically inserting metadata splits in a sharded collection. If set on all mongos, this will prevent MongoDB from creating new chunks as the data in a collection grows.

Because any mongos in a cluster can create a split, to totally disable splitting in a cluster you must set --noAutoSplit on all mongos.


With --noAutoSplit enabled, the data in your sharded cluster may become imbalanced over time. Enable with caution.

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