Service Dependencies

Service Dependencies

Service dependencies in systemd control two independent things: ordering (which unit starts first) and requirements (whether a unit must succeed for another to start). These are separate concepts and must be understood independently. A common mistake is thinking After= implies a requirement relationship — it does not. After= only says "if we both start, I start after you."

Why dependencies matter at boot

Without dependencies:
  systemd starts everything in parallel → mysql starts before disk is mounted
  → mysql cannot find data directory → mysql fails → web app fails

With dependencies:
  mysql.service: After=local-fs.target Requires=local-fs.target
  → systemd mounts filesystems first
  → then starts mysql
  → then starts web app

Ordering directives: After and Before

# After=: "I should start AFTER these units have activated"
# Does NOT mean the listed units are required — if they are not running, this unit still starts

[Unit]
After=network.target mysql.service

# Before=: "I should start BEFORE these units"
# Reverse of After=
# Example: use Before= in a unit that sets up a prerequisite
[Unit]
Before=mysql.service    # "Start me before mysql"

Requirement directives

# Example: web app that requires mysql, should start after network
[Unit]
Description=My Web Application
After=network.target mysql.service
Requires=mysql.service    # If mysql is not running, don't start us

# Example: service that should NOT run at same time as another
[Unit]
Conflicts=apache2.service    # Cannot coexist with apache2

Viewing the dependency tree

# Show what units a service depends on (direct + transitive)
systemctl list-dependencies nginx.service

# Show what depends ON a service (reverse)
systemctl list-dependencies --reverse nginx.service

# Check ordering relationships specifically
systemctl list-dependencies --before nginx.service
systemctl list-dependencies --after nginx.service

# View all dependency info for a specific unit
systemctl show mysql.service -p Requires,Wants,After,Before

list-dependencies output

nginx.service
● ├─system.slice
● ├─sysinit.target
● │ ├─apparmor.service
● │ ├─dev-hugepages.mount
● │ └─... (transitive deps)
● └─network.target
    └─network-online.target

Common dependency patterns

# Pattern 1: Web app that needs database
[Unit]
Description=Web Application
After=network.target postgresql.service
Requires=postgresql.service

# Pattern 2: Service that needs network to be fully up (not just online)
[Unit]
After=network-online.target
Wants=network-online.target

# Pattern 3: Multiple services in a stack (docker compose alternative)
# redis.service
[Unit]
After=network.target

# api.service
[Unit]
After=network.target redis.service
Requires=redis.service

# nginx.service (included by apt) automatically Wants multi-user.target

# Check that your dependency declarations are correct
systemd-analyze verify /etc/systemd/system/myapp.service

Conclusion

After= and Before= control only startup ordering. Requires= and Wants= control whether a dependency must succeed. Most custom services need both: After=mysql.service to start in the right order AND Requires=mysql.service so that the web app does not start without its database. Use Wants= instead of Requires= for optional dependencies where degraded operation is acceptable. Use systemctl list-dependencies --reverse servicename to understand what stops if your service stops.