Daemon, Service?

Daemon

Process running in background to perform system tasks.

Service

1 or more daemon/processes combined together for servicing the user. Eg: File server, Apache/httpd service(shows 5-6 processes in ps), sshd, systemd

# ps -aux|grep systemd
PID   %cpu  %mem   vsz    rss  tty  state  start-time   command
root     1   0.0  0.0  193844  5808   ?    Ss    2020  1:11  /usr/lib/systemd/systemd --switched-root --system
root    632  0.0  0.0  390601  5782   ?    Ss    2020  1:11  /usr/lib/systemd/systemd-journald     //Event Logging with journald
root    669  0.0  0.0  112121  1212   ?    Ss    2020  1:11  /usr/lib/systemd/systemd-udevd
root    904  0.0  0.0  129492  8589   ?    Ss    2020  1:11  /usr/lib/systemd/systemd-logind
Converting Process to service

# g++ test.cpp -o AmitService -std=c++11
int main(){
    cout<<"Sleeping for 9 sec";
    this::thread::sleep_for(chrono::microseconds(9*10pow6));
    cout<<"Awake";
}
# vim /etc/systemd/system/AmitService.service
[Unit]
Description=Test Service
After=network.target
StartLimitIntervalSec=0
[Service]
Type=simple
Restart=always
RestartSec=1
User=amit
ExecStart=/usr/bin/env /home/amit/code/test.cpp
[Install]
WantedBy=multi-user.target
# systemctl start|enable|status|disable AmitService
# cat /var/log/message                        //Check Logs

Systemd (Parent of All processes)

(In fedora, RHEL, CentOS) This is Replacement of init process. System and Service manager
Advantages of systemd over init?
1. systemd can start multiple services parallely(while init cannot) which reduces boot time of a system.
2. Ability to remove unneccessary services.
3. systemd itself takes less booting time wrt init.

Units in systemd

This is systemd specific Object that perform/controls particular task. systemd unit have Name,Type,unit-file(ini/configuration file).
Example: consider systemd as a bus driver and the units such as gear, clutch, lever, brake, accelerator, etc. Bus driver uses controllers to drive/control bus, systemd uses units to control the system processes and services.
Commands to List all units, 1 type of units
Types of Units
Type extension Description
Target *.target To define the various system states, the SysV uses numbers, known as run-levels, while the Systemd uses target-units
Service *.service To start/stop/restart/reload a service daemon Eg Apache webserver
Socket *.service For IPC majorly
Service unit
Service units have a .service extension and represent system services.
Dependent services: A Service may be dependent on 0 or more services, stopping/restarting 1 may require start/stop others also. # systemctl list-dependencies
Masking Services: A system may have conflicting services installed. Eg: iptables, firewalld. Masking service means if service is started by mistake nothing will happen. #systemctl mask
Socket Unit
Socket units have a .socket extension and represent inter-process communication (IPC) sockets that systemd should monitor. 

            # systemctl list-units --type=socket --all                   //List all socket units, active and inactive
Target Unit
systemd units named as *.target. Every target has a configuration file at /usr/lib/systemd/*target.
To define the various system states, the SysV uses numbers, known as run-levels, while the Systemd uses keyword, known as target-units. Eg: Graphical, Multiuser, Rescue, Emergency
One target can be part of another target. Eg: graphical.target includes multi-user.target which in turn depends on basic.target. 

// Listing all available targets and check which current?
$ systemctl    list-units    --type=target    --all    
$ vim    /usr/lib/systemd/*.target
# systemctl    get-default
    graphical.target            

// Setting the mode/target    
# systemctl     set-default    graphical.target
# systemctl    isolate    multi-user.target            //This changes system to Text-mode without booting the system. System is still in graphical mode.    #systemctl isolate 
# systemctl    ioslate    graphical.target            //Brings back Graphical.target    

// Booting kernel in rescue.target/emergency.target
- Interrupt boot loader by pressing any key
- Press e to edit command line>    At end of Line starting with linux16 enter       systemd.unit=rescue.target 
- Press Ctrl+x to save and exit
    systemd.unit=emergency.target        //This boots root system in RO mode.

Booting up with systemd

Power On > systemd process > Reads default.target(This is symlink to graphical.target) > Reads /etc/system-d/system/multi-user.target (Sets env for multi-user support. Starts firewall services) > Reads basic.target (Starts graohical manager service) > Reads sysinit.target (Mounts system mounts, swap spaces, devices. Kernel additional options etc.) > Reads local-fs.target (Handles low level services. Performs actions on /etc/fstab, /etc/inittab)
Troubleshooting booting process

//Built-in command to examine boot process. You can find out the units which are facing errors 
during boot up and can further trace and correct boot component issues.
    # systemd-analyze    

//shows the time spent in kernel,  and normal user space
    # systemd-analyze time
        Startup finished in 1440ms (kernel) + 3444ms (userspace)

//Prints a list of all running units, sorted  by the time taken by then to initialize, in this way you can have idea of 
    which services are taking long time to start during boot up.          
    $ systemd-analyze blame        
    2001ms mysqld.service
    234ms httpd.service
    191ms vmms.service

//Shows if there are any syntax errors in the system units. Systemd-analyze plot can be used to write down whole startup process to a SVG formate file.
# systemd-analyze plot > boot.svg