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Manage mongod Processes

On this page

  • Start mongod Processes
  • Stop mongod Processes
  • Troubleshoot mongod Processes
  • Stop a Replica Set

MongoDB runs as a standard program. You can start MongoDB from a command line by issuing the mongod command and specifying options. For a list of options, see the mongod reference.

The following examples assume the directory containing the mongod process is in your system paths. The mongod process is the primary database process that runs on an individual server. mongos provides a coherent MongoDB interface equivalent to a mongod from the perspective of a client. The mongosh binary provides the administrative shell.

This document discusses the mongod process; however, some portions of this document may be applicable to mongos instances.

Start mongod Processes

By default, MongoDB listens for connections from clients on port 27017, and stores data in the /data/db directory.

On Windows, this path is on the drive from which you start MongoDB. For example, if you do not specify a --dbpath, starting a MongoDB server on the C:\ drive stores all data files in C:\data\db.

To start MongoDB using all defaults, issue the following command at the system shell:

mongod

Specify a Data Directory

If you want mongod to store data files at a path other than /data/db you can specify a dbPath. The dbPath must exist before you start mongod. If it does not exist, create the directory and the permissions so that mongod can read and write data to this path. For more information on permissions, see the security operations documentation.

To specify a dbPath for mongod to use as a data directory, use the --dbpath option. The following invocation will start a mongod instance and store data in the /srv/mongodb path

mongod --dbpath /srv/mongodb/

Note

mongod includes a Full Time Diagnostic Data Capture mechanism to assist MongoDB engineers with troubleshooting deployments. If this thread fails, it terminates the originating process. To avoid the most common failures, confirm that the user running the process has permissions to create the FTDC diagnostic.data directory. For mongod the directory is within storage.dbPath. For mongos it is parallel to systemLog.path.

Specify a TCP Port

Only a single process can listen for connections on a network interface at a time. If you run multiple mongod processes on a single machine, or have other processes that must use this port, you must assign each a different port to listen on for client connections.

To specify a port to mongod, use the --port option on the command line. The following command starts mongod listening on port 12345:

mongod --port 12345

Use the default port number when possible, to avoid confusion.

Start mongod as a Daemon

To run a mongod process as a daemon (i.e. fork), and write its output to a log file, use the --fork and --logpath options. You must create the log directory; however, mongod creates the log file if it does not exist.

The following command starts mongod as a daemon and records log output to /var/log/mongodb/mongod.log.

mongod --fork --logpath /var/log/mongodb/mongod.log

Additional Configuration Options

For an overview of common configurations and deployments for common use cases, see Run-time Database Configuration.

Stop mongod Processes

In a clean shutdown a mongod completes all pending operations, flushes all data to data files, and closes all data files. Other shutdowns are unclean and can compromise the validity of the data files.

To ensure a clean shutdown, always shutdown mongod instances using one of the following methods:

Use shutdownServer()

Shut down the mongod from mongosh using the db.shutdownServer() method as follows:

use admin
db.shutdownServer()

Calling the same method from a init script accomplishes the same result.

For systems with authorization enabled, users may only issue db.shutdownServer() when authenticated to the admin database or via the localhost interface on systems without authentication enabled.

Use --shutdown

Supported on Linux only. From the command line, shut down the mongod using the --shutdown option:

mongod --shutdown

Use CTRL-C

When running the mongod instance in interactive mode (i.e. without --fork), issue Control-C to perform a clean shutdown.

Use kill

Supported on Linux and macOS only. From the command line, shut down a specific mongod instance using one of the following commands:

kill <mongod process ID>
kill -2 <mongod process ID>

SIGTERM and Replica Sets

If a replica set primary receives a SIGTERM, the primary attempts to step down before shutting down.

  • If the step down succeeds, the instance does not vote in the ensuing election of the new primary, and continues its shutdown.

  • If the step down fails, the instance continues its shutdown.

SIGKILL

Warning

Never use kill -9 (i.e. SIGKILL) to terminate a mongod instance.

Troubleshoot mongod Processes

Generate a Backtrace

For MongoDB instances running on Linux:

  • When the mongod and mongos processes receive a SIGUSR2 signal, backtrace details are added to the logs for each process thread.

  • Backtrace details show the function calls for the process, which can be used for diagnostics and provided to MongoDB Support if required.

The backtrace functionality is available for these architectures:

  • x86_64

  • arm64 (starting in MongoDB 5.0.10, and 6.0)

To issue a SIGUSR2 signal to a running mongod process, use the following command:

kill -SIGUSR2 <mongod process ID>

The resulting backtrace data is written to the mongod logfile as configured with --logpath.

Stop a Replica Set

Procedure

If the mongod is the primary in a replica set, the shutdown process for this mongod instance has the following steps:

  1. Check how up-to-date the secondaries are.

  2. If no secondary is within 10 seconds of the primary, mongod will return a message that it will not shut down. You can pass the shutdown command a timeoutSecs argument to wait for a secondary to catch up.

  3. If there is a secondary within 10 seconds of the primary, the primary will step down and wait for the secondary to catch up.

  4. After 60 seconds or once the secondary has caught up, the primary will shut down.

Force Replica Set Shutdown

If there is no up-to-date secondary and you want the primary to shut down, issue the shutdown command with the force argument, as in the following mongosh operation:

db.adminCommand({shutdown : 1, force : true})

To keep checking the secondaries for a specified number of seconds if none are immediately up-to-date, issue shutdown with the timeoutSecs argument. MongoDB will keep checking the secondaries for the specified number of seconds if none are immediately up-to-date. If any of the secondaries catch up within the allotted time, the primary will shut down. If no secondaries catch up, it will not shut down.

The following command issues shutdown with timeoutSecs set to 5:

db.adminCommand({shutdown : 1, timeoutSecs : 5})

Alternately you can use the timeoutSecs argument with the db.shutdownServer() method:

db.shutdownServer({timeoutSecs : 5})

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