Question 1

How do I draw a sequence diagram for a user login flow?

Accepted Answer

Place actors left to right starting with the initiator (USER, BROWSER, API, DB), draw vertical lifelines, then add horizontal labeled arrows for each message in time order. Use dashed arrows for returns like the token response and a self-message on the API lifeline for password hash verification.

Question 2

What is the difference between synchronous and asynchronous arrows in sequence diagrams?

Accepted Answer

Synchronous calls use a solid arrow with a filled head, while asynchronous events use an open arrowhead or dashed line. Returns are always dashed. This visual distinction lets readers tell call-and-response from fire-and-forget messaging.

Question 3

When should I use a sequence diagram instead of an architecture diagram?

Accepted Answer

Use a sequence diagram when the question is about what happens in what order, timing, or async coordination, such as walking through a request flow. Use an architecture diagram when the question is about what components exist and how they connect.

Question 4

Should I always draw activation bars on sequence diagrams?

Accepted Answer

No, activation bars are optional. Most engineers skip them on whiteboards. Use them when timing or concurrency is the topic of discussion, since they show when a component is actively processing.

Question 5

How do I show a failure path with retries in a sequence diagram?

Accepted Answer

Draw the failure path on a separate diagram or in a different color from the happy path. Show each retry as a repeated call with self-messages labeled with wait durations, and include the compensating action such as a PaymentFailed event after retries exhaust.

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