Exercise 1: A Logging Service That Requires a Collector, In Order — Possible Solution
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[Unit]
Requires=log-collector.service
After=log-collector.service

Explanation:

The exercise specifically says the logging service "should only run
if" the log-collector service is available -- this is a genuine hard
dependency, not just a nice-to-have, matching the chapter's own
definition of Requires= as appropriate "only when the unit truly
cannot function at all without the other one." Since the logging
service is described as fundamentally dependent on the collector being
present, Requires= (not the softer Wants=) is the correct choice here
-- if log-collector.service fails to start, systemd should treat the
logging service's own startup as a failure too, exactly Requires='s
own defined behavior.

After=log-collector.service is included separately because, per the
chapter's own central point, Requires= alone only controls WHETHER the
collector gets started -- it says nothing about ORDER. Without this
second line, both units could theoretically be told to start at the
same time, with no guarantee the collector is actually up first. The
exercise explicitly requires "must not start until after it," which
is exactly what the separate After= line guarantees. Both lines are
needed together, matching the chapter's own repeated point that
whether-to-start and when-to-start are two independent axes.

WHY THIS WORKS AS AN ANSWER
------------------------------
This selects Requires= (not Wants=) based on the exercise's own "should
only run if" wording matching the chapter's own hard-dependency
definition, and explicitly includes the separate After= line with a
justification for why Requires= alone wouldn't satisfy the ordering
requirement.