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C4, sequence, flowchart, use case, ER or dictionary — .alab, arch-lab JSON, or Mermaid, auto-detected and rendered live. Nothing leaves your browser. Syntax reference

How .alab, JSON and Mermaid relate

.alab is the format to write: it is what the syntax reference documents, what share links carry, and what reads cleanly in a code review. .archlab.json is the same C4 model on disk — the interchange form any other tool can read without implementing a grammar; the two are lossless twins in both directions (proved on every build), so you never have to write the JSON by hand. Mermaid is read and written too, lossily — the format toggle above the pane converts in place and states what each direction drops. Nothing you type is uploaded or stored.

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archlab 1.0 er
title "Course catalogue"
description "A join table whose key is entirely foreign, a course that requires another course, and enrolments that outlive a term."

@er
  entity student "Student" [PostgreSQL]
    desc "Anyone who has ever enrolled, including alumni."
    attr id uuid pk
    attr email string uk
    attr full_name string
  entity course "Course"
    attr id uuid pk
    attr code string uk
      desc "The printed code, e.g. CS101 — unique, but never the primary key, because codes get reused across faculties."
    attr title string
    attr prerequisite_id uuid fk
  entity term "Term"
    attr id uuid pk
    attr name string
    attr starts_on date
  entity enrolment "Enrolment"
    desc "A join table: its primary key is the three foreign keys together, so a student can take a course once per term and no more."
    attr student_id uuid pk fk
    attr course_id uuid pk fk
    attr term_id uuid pk fk
    attr grade string

student ||--o{ enrolment : takes
  course ||--o{ enrolment : "is taken as"
  term ||--o{ enrolment : "runs in"
  course |o--o{ course : requires

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