What can a sequence diagram show that a component diagram cannot, and what is the rule for drawing one worth having?
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What is being tested
Whether you know that time and ordering are architectural properties, and that the happy path is the least useful thing to draw.
What only a sequence diagram shows
Time and ordering. A component diagram shows that A talks to B and C. A sequence diagram shows whether it talks to them sequentially or in parallel — which is the difference between 40 ms and 240 ms on a high-latency link.
Round trip count, which is the dominant term in cross-region latency and is invisible in a static diagram.
Temporal coupling. A flow requiring six services to be simultaneously available to complete is obvious here and hidden everywhere else. Availability becomes the product of six numbers, and the diagram makes that visible.
Where state changes relative to other actions, which is what reveals a missing compensation or an irreversible step placed too early.
The rule that makes them worth drawing
Draw the failure sequences, not just the happy path.
A happy-path diagram usually tells the reader what they already assumed. The valuable ones answer: what happens when the third call times out? Who retries? What is the user shown? What state is left behind, and who cleans it up?
Those questions have architectural answers, and drawing them is frequently how a design's missing compensation logic is discovered — before it is built rather than during an incident.
How many to draw
One or two per system, for the interactions that genuinely matter: the critical user journey, the distributed transaction with compensation, the asynchronous flow whose ordering is subtle.
Not one per endpoint. That is documentation the code expresses better, it will drift, and the set will not be maintained.
The practical limits
Six participants is about the readable maximum. Include timeouts and expected durations on a diagram whose entire subject is time — a sequence diagram without timing information has discarded half its value.