What is the purpose of the "requisite" directive in systemd and how is it different from "required by"?
The "requisite" directive in systemd serves a important purpose in managing dependencies and ordering of services in a Linux system. It is used to define a dependency relationship between two units, where the second unit is required for the successful execution of the first unit. This directive ensures that the dependent unit is started before the requiring unit, guaranteeing the correct order of execution.
Unlike the "required by" directive, which specifies that a unit requires another unit to be started, the "requisite" directive takes the dependency relationship a step further. When a unit has a "requisite" dependency, it not only requires the dependent unit to be started but also mandates that the dependent unit must succeed in its execution. If the dependent unit fails to start or encounters any other error, the requiring unit will not be started.
To illustrate this, let's consider an example. Suppose we have two units, A and B, where B has a "requisite" dependency on A. If A fails to start or encounters an error during its execution, B will not be started. This ensures that the dependent unit B is not started if its prerequisite A is not functioning correctly. This behavior is particularly useful in scenarios where the successful execution of a prerequisite unit is critical for the operation of the dependent unit.
In contrast, the "required by" directive only specifies that a unit requires another unit to be started, but it does not enforce the success of the dependent unit. If the dependent unit fails to start, the requiring unit will still be started, potentially leading to unexpected behavior or system instability.
The "requisite" directive in systemd is used to establish a dependency relationship between units, ensuring that the dependent unit is started and successfully executed before the requiring unit. This directive goes beyond the "required by" directive by enforcing the success of the dependent unit. By utilizing the "requisite" directive effectively, system administrators can ensure proper ordering and dependencies between services in a Linux system, enhancing the overall stability and reliability of the system.
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