Messages

Messages show the interactions between participants over time.

Syntax

source->target:label
source-->target:label

Message Types

Syntax	Style	Use For
->	Solid arrow	Synchronous request
-->	Dashed arrow	Response / async

Basic Example

@seq
[participants]
  client:Client
  api:API
  db:Database

[messages]
  client->api:GET /users
  api->db:SELECT * FROM users
  db-->api:rows
  api-->client:200 OK [users]

Rendered diagram

Self-Messages

A participant calling itself:

[messages]
  api->api:validate input
  api->api:transform data

Message Labels

Labels describe the interaction:

[messages]
  # HTTP methods
  client->api:POST /orders

  # SQL operations
  api->db:INSERT INTO orders

  # Events
  api-->queue:OrderCreated event

  # Return values
  db-->api:order_id = 123

Numbered Messages (Coming Soon)

In a future version, you'll be able to add step numbers:

[messages]
  1:client->api:Login request
  2:api->auth:Validate credentials
  3:auth->db:Check user

For now, message order is determined by sequence in the file.

Full Example

@seq
[participants]
  user:User|actor
  app:Mobile App|client
  api:API Gateway|service
  auth:Auth Service|service
  users:User Service|service
  db:PostgreSQL|database

[messages]
  user->app:Tap Login
  app->api:POST /auth/login
  api->auth:Authenticate
  auth->db:SELECT user
  db-->auth:user record
  auth->auth:Generate JWT
  auth-->api:JWT token
  api-->app:200 OK
  app->api:GET /profile
  api->users:Get user
  users->db:SELECT profile
  db-->users:profile
  users-->api:profile data
  api-->app:200 OK
  app-->user:Show profile

Rendered diagram

Next Steps

• Control Flow - Loops and conditions

• Examples - Real-world sequences