Random Musings

The Wants and Requires directives in SystemD unit files define dependencies between services, but they behave differently in terms of how they enforce and handle dependencies.

Requires=

• Purpose: Specifies a hard dependency on another unit. If the unit listed in Requires= is not available, the current unit will fail to start.
• Behavior:
• If the unit specified in Requires= fails to start, the current unit will not start either.
• When the current unit stops, any unit listed in Requires= is also stopped automatically.
• Use case: For dependencies that are critical for the service to function. For example, a database service might Require a network service to be up and running. Example:

  [Unit]
  Requires=network.target

In this case, the service won’t start unless the network.target is active.

Wants=

• Purpose: Specifies a weak dependency on another unit. The system will try to start the unit listed in Wants=, but it is not a failure if that unit cannot start.
• Behavior:
• If the unit specified in Wants= fails to start, the current unit will still proceed to start.
• When the current unit stops, the Wants= dependency is not stopped automatically.
• Use case: For optional dependencies that enhance functionality but are not critical. For example, a service might Want a logging service to be active, but it can run without it. Example:

  [Unit]
  Wants=remote-fs.target

In this case, the service will try to start remote-fs.target, but it won’t fail if remote-fs.target cannot be activated.

Key Differences:

Summary:

• Use Requires= for must-have dependencies that are critical for the service to work.
• Use Wants= for nice-to-have dependencies that are optional but can enhance functionality.