What do alt and else mean?

alt starts alternative cases, else begins the next case and end closes the block. It is drawn as a labeled frame with one compartment per case.

Should the database be on the diagram?

Add a "User store" participant if looking up or creating the user matters. More participants make the diagram wider, so include only what a reviewer needs.

How long should the code be and how long should it last?

That is a security policy decision based on your risk and applicable guidance. The 5 minutes here is only an example.

SMS one-time code login sequence diagram template

Previewed in the “Light” theme; you can switch themes in the editor

When to use it

Use it to design a login module, document the API, or produce the login sequence diagram for a course or capstone project. It includes three details that are easy to miss and that reviewers ask about: checking the send rate first, giving the code an expiry, and deleting it as soon as it is used.

Make it yours

If the first sign-in creates an account, add "Create account if none exists" in the correct-code branch.
With a CAPTCHA step, insert its messages before the code request.
Set the expiry and rate limits from your own security policy.

Mermaid code

sms-login-sequence.mmd

sequenceDiagram
    actor U as User
    participant C as App
    participant A as Auth service
    participant R as Cache
    participant S as SMS gateway
    U->>C: Enter phone number and request code
    C->>A: Request one-time code
    A->>R: Check send rate
    R-->>A: Within limit
    A->>R: Store code for 5 minutes
    A->>S: Send SMS
    S-->>U: Code arrives
    A-->>C: Code sent
    U->>C: Enter code and sign in
    C->>A: Submit phone number and code
    A->>R: Read and compare code
    alt Code is correct
        A->>R: Delete code
        A-->>C: Return session token
        C-->>U: Open home screen
    else Code is wrong or expired
        A-->>C: Return error and count the attempt
        C-->>U: Ask to try again
    end

Ask an AI for one with this structure

Send the prompt below to ChatGPT or Claude, then paste the whole reply into the editor; the code inside is detected automatically.

prompt.txt

Draw an SMS one-time code login sequence diagram with Mermaid sequenceDiagram syntax. Participants are User, App, Auth service, Cache and SMS gateway. First the code request: the auth service checks the send rate, stores the code with an expiry and calls the SMS gateway. Then sign-in: the app submits the phone number and code, and the auth service reads and compares. Use alt for two cases: a correct code is deleted and a session token is returned; a wrong or expired code returns an error and counts the attempt. Output only the mermaid code block.

FAQ

What do alt and else mean?: alt starts alternative cases, else begins the next case and end closes the block. It is drawn as a labeled frame with one compartment per case.
Should the database be on the diagram?: Add a "User store" participant if looking up or creating the user matters. More participants make the diagram wider, so include only what a reviewer needs.
How long should the code be and how long should it last?: That is a security policy decision based on your risk and applicable guidance. The 5 minutes here is only an example.

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Updated 2026-11-11