Systemd

Pengantar Systemd

Systemd adalah sistem init dan service manager modern yang digunakan di hampir semua distribusi Linux utama (Debian, Ubuntu, Fedora, Arch). Systemd menggantikan SysV init dan Upstart, membawa paralelisasi booting, dependency-based startup, dan manajemen service yang lebih canggih.

PID 1 di Linux modern adalah systemd:

ps -p 1 -o comm=         # Output: systemd

Unit Files

Systemd menggunakan unit files - file konfigurasi yang mendefinisikan resource sistem. Jenis unit utama:

Unit Type	Ekstensi	Fungsi
Service	`.service`	Daemon / aplikasi latar belakang
Socket	`.socket`	IPC atau network socket (aktivasi berbasis socket)
Timer	`.timer`	Jadwal eksekusi (pengganti cron)
Mount	`.mount`	Mount point filesystem
Path	`.path`	Aktivasi berdasarkan perubahan file
Target	`.target`	Grup unit (pengganti runlevel)

Lokasi unit files:

/lib/systemd/system/ - Unit bawaan paket
/etc/systemd/system/ - Unit kustom/admin (lebih prioritas)

Contoh Service Unit

File /etc/systemd/system/myapp.service:

[Unit]
Description=My Custom Application
After=network.target
Wants=redis.service

[Service]
Type=simple
User=myapp
WorkingDirectory=/opt/myapp
ExecStart=/opt/myapp/app --config /etc/myapp/config.yml
Restart=on-failure
RestartSec=5
Environment=NODE_ENV=production

[Install]
WantedBy=multi-user.target

systemctl - Manajemen Service

systemctl start nginx           # Start service
systemctl stop nginx            # Stop service
systemctl restart nginx         # Restart
systemctl reload nginx          # Reload konfigurasi (tanpa mati)
systemctl enable nginx          # Auto-start saat boot
systemctl disable nginx         # Nonaktifkan auto-start
systemctl status nginx          # Status + log terkini
systemctl is-active nginx       # Cek apakah aktif (active/inactive)
systemctl is-enabled nginx      # Cek enable/disable

systemctl daemon-reload         # Reload unit files setelah edit manual

List semua service:

systemctl list-units --type=service          # Service aktif
systemctl list-units --type=service --all    # Semua service
systemctl list-unit-files --type=service     # Status enable/disable

journalctl - Log Management Systemd

Systemd mengelola log sendiri melalui journald:

journalctl -u nginx                  # Log service nginx
journalctl -u nginx -f               # Follow (real-time)
journalctl -u nginx --since "1 hour ago"  # Log 1 jam terakhir
journalctl -u nginx -p err           # Filter level: emerg, alert, crit, err, warning, info, debug
journalctl --list-boots              # Riwayat boot
journalctl -b -1                     # Log dari boot sebelumnya
journalctl -k                        # Kernel log (dmesg)
journalctl --disk-usage              # Cek ukuran log

Target (Runlevel)

Systemd target menggantikan runlevel SysV:

Runlevel	Target Systemd
0	poweroff.target
1 / S	rescue.target
2, 3, 4	multi-user.target
5	graphical.target
6	reboot.target

systemctl get-default                   # Lihat default target
sudo systemctl set-default multi-user.target  # Set default (no GUI)
sudo systemctl isolate graphical.target       # Pindah target sekarang

Drop-in Configuration

Tanpa mengedit unit file asli (yang bisa tertimpa update), systemd mendukung drop-in konfigurasi:

mkdir -p /etc/systemd/system/nginx.service.d/
cat > /etc/systemd/system/nginx.service.d/override.conf << 'EOF'
[Service]
LimitNOFILE=65536
Environment=MY_CUSTOM_VAR=value
EOF

systemctl daemon-reload
systemctl restart nginx

Keuntungan drop-in: konfigurasi kustom tetap ada meskipun paket nginx di-upgrade.

Dependency dan Ordering

Systemd menyelesaikan dependency antar unit dengan direktif:

Direktif	Fungsi
`Requires=`	Unit target harus berhasil start
`Wants=`	Opsional - start jika bisa, tetap lanjut jika gagal
`After=`	Unit ini start setelah unit target
`Before=`	Unit ini start sebelum unit target
`BindsTo=`	Jika unit target mati, unit ini ikut mati

[Unit]
Description=Web App
Requires=postgresql.service redis.service
After=network.target postgresql.service redis.service

systemd-analyze

Tool analisis boot time dan dependency:

systemd-analyze                       # Total waktu boot
systemd-analyze blame                 # Waktu start per unit (descending)
systemd-analyze critical-chain        # Chain kritis boot
systemd-analyze dot nginx.service | dot -Tsvg > graph.svg   # Dependency graph
systemd-analyze verify /etc/systemd/system/myapp.service    # Validasi unit file

systemd-analyze blame sangat berguna untuk mengidentifikasi service yang memperlambat boot - optimasi bisa berupa parallel startup atau delayed start dengan After=.

Timer Unit - Pengganti Cron

# /etc/systemd/system/backup.timer
[Unit]
Description=Daily Backup Timer

[Timer]
OnCalendar=daily
Persistent=true

[Install]
WantedBy=timers.target

# /etc/systemd/system/backup.service
[Unit]
Description=Daily Backup

[Service]
Type=oneshot
ExecStart=/usr/local/bin/backup.sh

Aktifkan:

systemctl enable --now backup.timer
systemctl list-timers

Timer lebih unggul dibanding cron: logging terintegrasi journald, bisa mengejar jadwal yang terlewat (Persistent=true), dan dependency management.