| name | c4-model |
| description | Create C4 Model architecture diagrams (Context, Container, Component, Code) using PlantUML, Mermaid, or Structurizr — invoked when documenting system architecture at any zoom level |
C4 Model — Architecture Diagramming
Simon Brown's C4 Model gives a common vocabulary for describing software architecture at four levels of abstraction. The key insight: different audiences need different zoom levels. One diagram does not serve all.
The Four Levels of Abstraction
ZOOM OUT ──────────────────────────────────────────────── ZOOM IN
Level 1: System Context
"What does this system do and who uses it?"
Audience: PMs, business stakeholders, new joiners
No tech details.
Level 2: Container
"What are the deployable units and how do they communicate?"
Audience: Developers, architects, DevOps
Technology is visible here.
Level 3: Component
"What are the major building blocks inside a container?"
Audience: Developers working on that container
Maps to packages / modules / namespaces.
Level 4: Code (optional)
"What do the classes/functions look like?"
Audience: Developers; usually auto-generated from IDE
Only draw manually for complex algorithms or onboarding.
Level 1 — System Context Diagram
Purpose
Show the system in the center, the humans who use it, and the external systems it integrates with. No technology, no internals.
Key question: "What does this system do and who uses it?"
What NOT to show:
- Technology choices (no "React", "PostgreSQL", "Kafka" here)
- Internal structure (no services, modules, or components)
- Implementation details of any kind
Notation:
[Person] Stick-figure shape. Someone who uses the system.
[Software System] Box. The system you are describing OR an external one.
[Relationship] Labeled arrow. What data/interaction flows, and why.
ASCII Example: E-Commerce System Context
+------------------+
| <<Person>> |
| Customer |
| Browses catalog, |
| places orders |
+--------+---------+
|
| Uses (HTTPS)
v
+------------------+ +-------------------------+
| <<Software Sys>> | | <<Software System>> |
| E-Commerce +-------->| Payment Gateway (Stripe)|
| Platform | Charges | Handles card processing |
| | via API | |
+--------+---------+ +-------------------------+
|
| Sends order confirmations via
v
+------------------+
| <<Software Sys>> |
| Email Service |
| (SendGrid) |
+------------------+
<<Person>>
Warehouse Staff
Fulfills orders, updates
stock levels
|
| Uses (HTTPS)
v
[E-Commerce Platform] (same system, different actor)
PlantUML — System Context
@startuml C4_Context_ECommerce
!include https://raw.githubusercontent.com/plantuml-stdlib/C4-PlantUML/master/C4_Context.puml
LAYOUT_WITH_LEGEND()
title System Context — E-Commerce Platform
Person(customer, "Customer", "Browses the catalog and places orders online.")
Person(warehouse, "Warehouse Staff", "Fulfills orders and manages inventory.")
System(ecommerce, "E-Commerce Platform", "Allows customers to browse products, place orders, and track deliveries.")
System_Ext(payment, "Payment Gateway", "Handles payment processing via Stripe API.")
System_Ext(email, "Email Service", "Sends transactional emails via SendGrid.")
System_Ext(shipping, "Shipping Provider API", "FedEx/UPS integration for label generation and tracking.")
Rel(customer, ecommerce, "Uses", "HTTPS")
Rel(warehouse, ecommerce, "Uses", "HTTPS")
Rel(ecommerce, payment, "Processes payments via", "HTTPS/REST")
Rel(ecommerce, email, "Sends order confirmations via", "HTTPS/SMTP")
Rel(ecommerce, shipping, "Generates labels and tracks shipments via", "HTTPS/REST")
@enduml
Mermaid — System Context
C4Context
title System Context — E-Commerce Platform
Person(customer, "Customer", "Browses catalog and places orders")
Person(warehouse, "Warehouse Staff", "Fulfills orders, manages inventory")
System(ecommerce, "E-Commerce Platform", "Core order and catalog management")
System_Ext(payment, "Payment Gateway (Stripe)", "Card processing")
System_Ext(email, "Email Service (SendGrid)", "Transactional email")
System_Ext(shipping, "Shipping Provider API", "Label generation, tracking")
Rel(customer, ecommerce, "Uses", "HTTPS")
Rel(warehouse, ecommerce, "Uses", "HTTPS")
Rel(ecommerce, payment, "Processes payments", "REST/HTTPS")
Rel(ecommerce, email, "Sends confirmations", "SMTP")
Rel(ecommerce, shipping, "Requests labels", "REST/HTTPS")
Level 2 — Container Diagram
Purpose
Zoom into the system. Show every separately deployable/runnable unit: web apps, APIs, mobile apps, databases, message brokers, CDNs, background workers.
Key question: "What are the high-level technology decisions and how do containers communicate?"
Technology is visible here. "Spring Boot API" not just "API". "PostgreSQL 16" not just "Database".
What counts as a container:
- Web application (React SPA, Next.js server)
- API (Spring Boot, FastAPI, Express)
- Database (PostgreSQL, MongoDB, Redis)
- Message broker (Kafka cluster, RabbitMQ)
- Background worker / job runner
- Mobile app (iOS, Android)
- Static file CDN / object storage
Notation:
[Container] Box with technology label in brackets: [Spring Boot]
[Person] Same as Context level
[Ext System] Same as Context level — treated as a black box
[Relationship] Arrow with: what is exchanged + protocol
ASCII Example: E-Commerce Container Diagram
Customer Warehouse Staff
<<Person>> <<Person>>
| |
| HTTPS | HTTPS
v v
+---------------------+ +---------------------+
| <<Container>> | | <<Container>> |
| React SPA | | React Admin SPA |
| [TypeScript/Vite] | | [TypeScript/Vite] |
| Customer-facing | | Inventory & order |
| storefront | | management UI |
+----------+----------+ +----------+----------+
| |
| REST/HTTPS (JSON) | REST/HTTPS (JSON)
v v
+----------------------------------------------------+
| <<Container>> |
| Order API |
| [Spring Boot 3, Java 21] |
| Handles catalog, orders, auth, inventory. |
+---+----------------+------------------+------------+
| | |
| SQL/JDBC | Publishes events | Reads/Writes
v v v
+----------+ +------------------+ +-----------+
|<<Contain>>| | <<Container>> | |<<Contain>>|
| PostgreSQL| | Kafka Cluster | | Redis |
| [v16] | | [Apache Kafka] | | [v7] |
| Orders, | | order.placed, | | Session |
| customers,| | order.shipped | | cache, |
| inventory | | events | | rate limit |
+-----------+ +--------+---------+ +-----------+
|
| Consumes events
v
+------------------+
| <<Container>> |
| Notification |
| Worker |
| [Python/Celery] |
| Sends emails and |
| push notifs |
+------------------+
|
| SMTP / FCM
v
[SendGrid] [Firebase Cloud Messaging]
<<Ext Sys>> <<Ext Sys>>
PlantUML — Container Diagram
@startuml C4_Container_ECommerce
!include https://raw.githubusercontent.com/plantuml-stdlib/C4-PlantUML/master/C4_Container.puml
LAYOUT_WITH_LEGEND()
title Container Diagram — E-Commerce Platform
Person(customer, "Customer", "Places orders online.")
Person(warehouse, "Warehouse Staff", "Manages inventory and fulfillment.")
System_Boundary(ecommerce, "E-Commerce Platform") {
Container(spa, "React SPA", "TypeScript, Vite", "Customer storefront. Catalog browsing, cart, checkout.")
Container(admin_spa, "Admin SPA", "TypeScript, Vite", "Inventory and order management for warehouse staff.")
Container(api, "Order API", "Spring Boot 3, Java 21", "Core business logic: catalog, orders, auth, inventory.")
ContainerDb(db, "Order Database", "PostgreSQL 16", "Stores orders, customers, products, inventory.")
Container(broker, "Message Broker", "Apache Kafka", "Publishes domain events: order.placed, order.shipped.")
Container(worker, "Notification Worker", "Python, Celery", "Consumes Kafka events. Sends emails and push notifications.")
ContainerDb(cache, "Cache", "Redis 7", "Session store, rate limiting, catalog cache.")
}
System_Ext(payment, "Stripe", "Payment processing.")
System_Ext(email, "SendGrid", "Transactional email delivery.")
System_Ext(push, "Firebase Cloud Messaging", "Mobile push notifications.")
Rel(customer, spa, "Uses", "HTTPS")
Rel(warehouse, admin_spa, "Uses", "HTTPS")
Rel(spa, api, "Calls", "REST/HTTPS, JSON")
Rel(admin_spa, api, "Calls", "REST/HTTPS, JSON")
Rel(api, db, "Reads/writes", "JDBC/SQL")
Rel(api, cache, "Reads/writes", "Redis protocol")
Rel(api, broker, "Publishes events to", "Kafka protocol")
Rel(api, payment, "Processes payments via", "REST/HTTPS")
Rel(worker, broker, "Consumes events from", "Kafka protocol")
Rel(worker, email, "Sends emails via", "SMTP/HTTPS")
Rel(worker, push, "Sends push notifications via", "FCM REST API")
@enduml
Mermaid — Container Diagram
C4Container
title Container Diagram — E-Commerce Platform
Person(customer, "Customer", "Places orders")
Person(warehouse, "Warehouse Staff", "Manages inventory")
System_Boundary(ec, "E-Commerce Platform") {
Container(spa, "React SPA", "TypeScript/Vite", "Customer storefront")
Container(admin, "Admin SPA", "TypeScript/Vite", "Warehouse management UI")
Container(api, "Order API", "Spring Boot 3", "Core business logic")
ContainerDb(db, "PostgreSQL 16", "Relational DB", "Orders, customers, inventory")
Container(kafka, "Kafka", "Apache Kafka", "Domain event bus")
Container(worker, "Notification Worker", "Python/Celery", "Email and push delivery")
ContainerDb(redis, "Redis 7", "Cache", "Sessions, rate limits")
}
System_Ext(stripe, "Stripe", "Payment processing")
System_Ext(sendgrid, "SendGrid", "Email delivery")
Rel(customer, spa, "Uses", "HTTPS")
Rel(warehouse, admin, "Uses", "HTTPS")
Rel(spa, api, "REST", "HTTPS/JSON")
Rel(admin, api, "REST", "HTTPS/JSON")
Rel(api, db, "JDBC/SQL")
Rel(api, kafka, "Publishes events")
Rel(api, redis, "Cache reads/writes")
Rel(api, stripe, "Payment API", "HTTPS")
Rel(worker, kafka, "Consumes events")
Rel(worker, sendgrid, "Sends email", "HTTPS")
Level 3 — Component Diagram
Purpose
Zoom into a single container and show its major logical components. A component maps to a package, namespace, or module — not a single class.
Key question: "What are the major components inside this container and what are their responsibilities?"
Audience: Developers actively working on that specific container.
Natural alignment with Hexagonal Architecture:
Inbound Adapters (left) Ports (center) Outbound Adapters (right)
HTTP Controller -----> PlaceOrderUseCase -----> OrderRepository
Kafka Consumer -----> ShipOrderUseCase -----> PaymentGatewayAdapter
GraphQL Resolver -----> -----> EmailNotificationAdapter
ASCII Example: Order API Component Diagram
<<Container: Order API — Spring Boot 3>>
┌─────────────────────────────────────────────────────────────────────┐
│ │
│ ┌─────────────────┐ ┌──────────────────┐ │
│ │ <<Component>> │ │ <<Component>> │ │
│ │ Order │────>│ Place Order │ │
│ │ Controller │ │ Use Case │ │
│ │ REST endpoints │ │ Orchestrates │ │
│ │ /orders/** │ │ order creation │ │
│ └─────────────────┘ └────────┬─────────┘ │
│ │ │
│ ┌─────────────────┐ │uses │
│ │ <<Component>> │ v │
│ │ Inventory │ ┌──────────────────┐ ┌─────────────── │
│ │ Controller │────>│ <<Component>> │ │ <<Component>> │
│ │ REST endpoints │ │ Order Domain │ │ Order │
│ │ /inventory/** │ │ Model │ │ Repository │
│ └─────────────────┘ │ Order, LineItem,│────>│ Spring Data │
│ │ Address, Money │ │ JPA │
│ ┌─────────────────┐ └────────┬─────────┘ └───────┬─────── │
│ │ <<Component>> │ │ │SQL │
│ │ Auth Filter │ │uses v │
│ │ JWT validation │ ┌────────┴─────────┐ [PostgreSQL 16] │
│ │ Spring Security│ │ <<Component>> │ │
│ └─────────────────┘ │ Kafka Event │ │
│ │ Publisher │──> [Kafka Broker] │
│ │ order.placed, │ │
│ │ order.shipped │ │
│ └──────────────────┘ │
└─────────────────────────────────────────────────────────────────────┘
PlantUML — Component Diagram
@startuml C4_Component_OrderAPI
!include https://raw.githubusercontent.com/plantuml-stdlib/C4-PlantUML/master/C4_Component.puml
LAYOUT_WITH_LEGEND()
title Component Diagram — Order API (Spring Boot 3)
Container_Ext(spa, "React SPA", "TypeScript")
ContainerDb_Ext(db, "PostgreSQL 16", "Relational DB")
Container_Ext(kafka, "Kafka Broker", "Apache Kafka")
Container_Boundary(api, "Order API") {
Component(order_ctrl, "Order Controller", "Spring MVC @RestController", "Exposes REST endpoints for order lifecycle: POST /orders, GET /orders/{id}, PATCH /orders/{id}/status.")
Component(inventory_ctrl, "Inventory Controller", "Spring MVC @RestController", "Exposes REST endpoints for stock checks and reservations.")
Component(auth_filter, "Auth Filter", "Spring Security, JWT", "Validates JWT bearer tokens. Extracts user identity and roles.")
Component(place_order_uc, "Place Order Use Case", "Spring @Service", "Orchestrates order creation: validates stock, persists order, publishes event.")
Component(ship_order_uc, "Ship Order Use Case", "Spring @Service", "Marks order as shipped, updates inventory, publishes shipped event.")
Component(domain, "Order Domain Model", "Plain Java records/classes", "Order, LineItem, Address, Money. Pure business logic, no framework dependencies.")
Component(order_repo, "Order Repository", "Spring Data JPA", "Persistence adapter. Maps domain Order to JPA entities. Executes SQL queries.")
Component(event_publisher, "Domain Event Publisher", "Spring Kafka @KafkaTemplate", "Publishes OrderPlaced and OrderShipped events to Kafka topics.")
}
Rel(spa, order_ctrl, "Calls", "REST/HTTPS")
Rel(spa, inventory_ctrl, "Calls", "REST/HTTPS")
Rel(order_ctrl, auth_filter, "Filtered by", "Spring Security chain")
Rel(order_ctrl, place_order_uc, "Delegates to")
Rel(order_ctrl, ship_order_uc, "Delegates to")
Rel(inventory_ctrl, place_order_uc, "Delegates to")
Rel(place_order_uc, domain, "Creates/modifies")
Rel(ship_order_uc, domain, "Creates/modifies")
Rel(place_order_uc, order_repo, "Persists via")
Rel(ship_order_uc, order_repo, "Persists via")
Rel(place_order_uc, event_publisher, "Publishes OrderPlaced via")
Rel(ship_order_uc, event_publisher, "Publishes OrderShipped via")
Rel(order_repo, db, "SQL/JDBC")
Rel(event_publisher, kafka, "Kafka protocol")
@enduml
Level 4 — Code (Use Sparingly)
Level 4 shows classes, interfaces, and their relationships — the UML class diagram level. Do not draw this for routine work. IDEs auto-generate it. Reserve it for:
- Explaining a complex algorithm where class interactions matter
- Onboarding to legacy code with no other documentation
- Design pattern implementation (explaining how Strategy or Chain of Responsibility is wired)
- Public API / SDK design review
When you do draw Level 4, use UML Class Diagram notation (see the uml-diagrams skill).
Diagram Rules — Checklist
Every element must have:
Every relationship must have:
Every diagram must have:
When to Draw Which Level
Situation → Recommended Levels
─────────────────────────────────────────────────────────
Starting a new project → Level 1 (align stakeholders)
+ Level 2 (tech decisions)
Onboarding a new developer → Level 1 + Level 2
+ Level 3 if codebase is large
Reviewing architecture in a PR → Level 3 (verify structure vs intent)
Incident post-mortem → Level 2 (trace failure path across containers)
Explaining a complex subsystem → Level 3 (component breakdown)
Documenting a tricky algorithm → Level 4 (class/sequence diagram)
C4 + Structurizr (Living Diagrams)
Structurizr DSL defines the model once and renders multiple views. Diagrams stay in sync because they come from a single source of truth.
workspace "E-Commerce Platform" "Online retail system" {
model {
customer = person "Customer" "Browses catalog, places orders."
ecommerce = softwareSystem "E-Commerce Platform" {
spa = container "React SPA" "Customer storefront" "TypeScript, Vite"
api = container "Order API" "Core business logic" "Spring Boot 3"
db = container "PostgreSQL 16" "Order persistence" "PostgreSQL" {
tags "Database"
}
kafka = container "Kafka" "Domain event bus" "Apache Kafka"
}
payment = softwareSystem "Stripe" "Payment processing" {
tags "External System"
}
customer -> spa "Uses" "HTTPS"
spa -> api "REST calls" "HTTPS/JSON"
api -> db "Reads/writes" "JDBC/SQL"
api -> kafka "Publishes events" "Kafka protocol"
api -> payment "Charges card" "REST/HTTPS"
}
views {
systemContext ecommerce "SystemContext" {
include *
autoLayout
}
container ecommerce "Containers" {
include *
autoLayout
}
styles {
element "Person" { shape Person }
element "Database" { shape Cylinder }
element "External System" { background #999999 }
}
}
}
Structurizr Lite (free, self-hosted):
docker run -it --rm -p 8080:8080 \
-v $(pwd)/workspace:/usr/local/structurizr \
structurizr/lite
CI integration:
structurizr-cli push -id WORKSPACE_ID -key KEY -secret SECRET -workspace workspace.dsl
Anti-Patterns
| Anti-Pattern | Problem | Fix |
|---|
| Unlabeled arrows | Viewer must guess what flows between elements | Every arrow gets a label: what + why + protocol |
| Mixing levels | Showing a class inside a Container diagram confuses the narrative | One diagram = one level of abstraction |
| Mega-Container diagram | 40 containers, 200 arrows — impossible to read | Split by bounded context; one diagram per domain area |
| Generic names | "Service A", "Handler B", "Manager C" communicate nothing | Use business names: "Order Placement Service", "Inventory Reservation Handler" |
| Diagram rot | Diagrams describe the architecture from 18 months ago | Store source in repo, generate images in CI, or use Structurizr |
| UML notation for C4 | Different symbols, different audience expectations | Use C4-PlantUML library or Structurizr; do not mix with UML class notation |
| Missing external systems | System appears to operate in isolation | Always show the external dependencies — they're the biggest failure points |
Quick Reference — C4-PlantUML Macros
' Context level
Person(alias, "Name", "Description")
Person_Ext(alias, "Name", "Description") ' External user
System(alias, "Name", "Description")
System_Ext(alias, "Name", "Description") ' External system
System_Boundary(alias, "Name") { ... }
' Container level
Container(alias, "Name", "Tech", "Description")
ContainerDb(alias, "Name", "Tech", "Description")
Container_Ext(alias, "Name", "Tech")
ContainerDb_Ext(alias, "Name", "Tech")
' Component level
Component(alias, "Name", "Tech", "Description")
ComponentDb(alias, "Name", "Tech", "Description")
' Relationships
Rel(from, to, "Label")
Rel(from, to, "Label", "Technology")
Rel_Back(from, to, "Label") ' Arrow direction reversed
Rel_Neighbor(from, to, "Label") ' Hint for layout
' Layout helpers
LAYOUT_WITH_LEGEND()
LAYOUT_LEFT_RIGHT()
LAYOUT_TOP_DOWN()
