PSMDB-158 Documented how to configure systemd service for mongos (#1019) (#1020)

PSMDB-158 Documented how to configure systemd service for mongos

new file:   docs/mongos-config.md
	modified:   mkdocs-base.yml

---------

Co-authored-by: Ivan Groenewold <[email protected]>

Author: nastena1606

docs/mongos-config.md

@@ -0,0 +1,108 @@
+# 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 :octicons-link-external-16:](https://www.mongodb.com/docs/manual/tutorial/deploy-sharded-cluster-with-keyfile-access-control/#create-the-config-server-replica-set) 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 :octicons-link-external-16:](https://www.mongodb.com/docs/manual/reference/configuration-options/#mongodb-setting-security.keyFile) for how to create keyfiles. 
+
+## Procedure
+
+The steps are the following:
+{.power-number}
+
+1. Create a `mongos` user and a group. This user will own the `mongos` process. Use the following command:
+
+    ```{.bash data-prompt="$"}
+    $ 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:
+
+    ```ini title="/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`:
+
+    ```yaml title="/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:
+
+	```{.bash data-prompt="$"}
+	$ 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> ${STDERR}"
+    PIDFile=/var/run/mongos.pid    
+
+    [Install]
+    WantedBy=multi-user.target
+    EOF
+    ```
+
+5. Grant read/write access for the `mongos` user to the following directories and files:
+
+    ```{.bash data-prompt="$"}
+	$ 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:
+
+	```{.bash data-prompt="$"}
+	$ sudo systemctl daemon-reload
+	```
+
+7. Start the `mongos` service:
+
+	```{.bash data-prompt="$"}
+	$ sudo systemctl start mongos
+	```
+

mkdocs-base.yml

@@ -212,6 +212,7 @@ nav:
       - ngram-full-text-search.md
   - Administration:
       - "Tune parameters": "set-parameter.md"
+      - mongos-config.md
       - Upgrade:
         - "Upgrade from 6.0 to 7.0": "install/upgrade-from-60.md"
         - "Upgrade from MongoDB Community": "install/upgrade-from-mongodb.md"