Draw systems by typing

Proschi is a small text language for architecture diagrams and high-level designs. You write the services, stores and queues of a system and the requests that flow between them; Proschi lays out the diagram, plays every request step by step, and checks the design against the traffic and requirements you write next to it.

title "Notes"

user "User"      [Actor]
api  "Notes API" [REST API]   @backend
db   "Notes DB"  [PostgreSQL] @backend

user -> api : HTTPS
api  -> db  : SQL

usecase "Create a note" {
  user -> api : POST /notes json {"text": "Buy milk"}
  alt "Saved" {
    api  -> db   : INSERT note
    api --> user : 201 {"id": 42}
  } alt "DB down" {
    api  -x db   : INSERT note
    api --> user : 503
  }
}

Why text #

• It diffs. A design is a file next to the code, reviewed in pull requests like the code. No exported PNG that drifts from the system.
• It lays itself out. You never drag a box. Add a line and the diagram follows; pin a node with pos only when you want to.
• It plays. A use case is a real request flow: each step names its endpoint, payload and status, and alt keeps the happy path and every way it fails side by side.
• It checks itself. With traffic and requirements, a deterministic simulation tells you which node saturates first, what p99 to expect, whether the design survives a node failure and what it costs.
• It stays yours. The editor runs entirely in your browser. Your diagrams are saved there and travel in share links, never on a server. See Privacy.

Start here #

1. Quickstart: your first diagram in two minutes.
2. Language reference: every statement, with the grammar.
3. How the simulation works: the formulas, the default numbers, and how far to trust them.
4. Editor support: VS Code, IntelliJ, Neovim, Helix, Sublime Text, the command line and CI.
5. Practice: system design problems with tests instead of opinions, and how to write one.

Proschi is free and open source under the MIT license. The code is on GitHub.