Configure a systemd unit file for mongos

mongos provides the entry point for an application to connect to a sharded cluster. To automate the mongos process management, you can use a system unit file. This file defines how the mongos service should behave when the system boots, shuts down, or encounters an issue.

This document provides a sample configuration for a mongos systemd unit file that you can use and/or modify to meet your specific needs. For security considerations, cluster components use a keyfile for internal authentication.

Before you start

1. Ensure you have a working config server replica set and shards. Refer to the deployment documentation for guidelines

2. Check that you have fulfilled all prerequisites in your system:

   • /var/log/mongo directory is created
   • If SELinux is in use, /var/run/mongos.pid is added to the policy so mongos process can create it

3. Get the shared key file from any existing member of the cluster. Refer to the MongoDB documentation for how to create keyfiles.

Procedure

The steps are the following:

1. Create a mongos user and a group. This user will own the mongos process. Use the following command:

   $ groupadd mongos && sudo useradd -r -s /bin/false -g mongos mongos
   

2. Create the environment file at the path /etc/sysconfig/mongos and specify the following environment variables within:

   /etc/sysconfig/mongos

   OPTIONS="-f /etc/mongos.conf"
   STDOUT="/var/log/mongo/mongos.stdout"
   STDERR="/var/log/mongo/mongos.stderr"
   

3. Create a mongos configuration file at the path /etc/mongos.conf. In the following example configuration, replace the security.keyfile with the path to your keyfile and specify the name of the config server replica set and its members in the format hostname:port:

   /etc/mongos.conf

   # where to write logging data.
   systemLog:
     destination: file
     logAppend: true
     path: /var/log/mongo/mongos.log
   
   processManagement:
     fork: true
     pidFilePath: /var/run/mongos.pid
   
   # network interfaces
   net:
     port: 27017
     bindIp: 127.0.0.1
   
   security:
     keyFile: /etc/mongos.key
   
   sharding:
     configDB: configRS/cfg1.example.com:27017,cfg2.example.com:27017,cfg3.example.com:27017
   

4. Create the systemd unit file at the path /usr/lib/systemd/system/mongos.service. Specify the following configuration:

   $ tee /usr/lib/systemd/system/mongos.service <<EOF
   [Unit]
   Description=High-performance, schema-free document-oriented database
   After=time-sync.target network.target    
   
   [Service]
   Type=forking
   User=mongos
   Group=mongos
   PermissionsStartOnly=true
   LimitFSIZE=infinity
   LimitCPU=infinity
   LimitAS=infinity
   LimitNOFILE=64000
   LimitNPROC=64000
   EnvironmentFile=/etc/sysconfig/mongos
   ExecStart=/usr/bin/env bash -c "/usr/bin/mongos $OPTIONS > ${STDOUT} 2> ${STDERR}"
   PIDFile=/var/run/mongos.pid    
   
   [Install]
   WantedBy=multi-user.target
   EOF
   

5. Grant read/write access for the mongos user to the following directories and files:

   $ sudo chown -R mongos:mongos /var/log/mongo \
   /var/run/mongos.pid \
   /etc/mongos.conf \
   /etc/sysconfig/mongos \
   <path-to-keyfile>
   

   6. Reload the systemd daemon to apply the changes:

   $ sudo systemctl daemon-reload
   

6. Start the mongos service:

   $ sudo systemctl start mongos