Service startup ordering race conditions in Debian's systemd parallel initialization

Service startup ordering race conditions in Debian's systemd parallel initialization

Debian's systemd service manager resolves dependencies using After=, Before=, Wants=, and Requires= directives in unit files. When services have no direct interdependencies, systemd may start them in parallel, which can cause race conditions if services expect sequential startup. The Type= directive (simple, forking, oneshot, notify, dbus, idle) affects startup behavior, and the Restart= directive controls failure responses.

However, the interaction between systemd's parallel startup behavior and services that implicitly depend on other services being fully initialized remains an unresolved design question. For instance, a service might assume a database socket exists without explicitly declaring the dependency.

Questions:
1. How can administrators reliably detect services that have implicit ordering dependencies not declared in unit files?
2. What is the recommended approach for handling services that need to wait for filesystem mounts or network availability before starting?
3. When should After= be used versus Wants= or Requires= for different dependency scenarios?