Getting Started with Atlas

5-minute tutorial to understand Atlas basics.

What You'll Build

A simple user management system with:

• A database component
• A function to fetch users
• An API endpoint

Prerequisites

• Clojure CLI tools installed
• Basic Clojure knowledge

Step 1: Setup

Add Atlas to your deps.edn:

{:deps {com.github.semantic-namespace/atlas {:mvn/version "0.0.1-SNAPSHOT"}}

Start a REPL:

clojure -M:repl

Step 2: Initialize the Registry

(require '[atlas.registry :as registry])
(require '[atlas.ontology :as ont])

;; Register built-in entity types
(ont/register-entity-types!)

The registry is a global atom that holds all your semantic entities.

Step 3: Register a Component

Components are foundational building blocks (databases, caches, external services):

(registry/register!
  :component/user-db                    ; dev-id: human-readable identifier
  :atlas/structure-component            ; type: what kind of entity
  #{:tier/foundation                    ; aspects: semantic tags
    :domain/users}
  {:component/provides [:db/find-user   ; what this component offers
                        :db/save-user]})

What happened?

• Created an entity with compound identity #{:atlas/structure-component :tier/foundation :domain/users}
• Stored properties describing what it provides
• The :domain/users aspect links it semantically to user-related entities

Step 4: Register a Function

Functions contain business logic and declare their data flow:

(registry/register!
  :fn/get-user-by-id
  :atlas/execution-function
  #{:tier/service
    :domain/users
    :operation/read}
  {:execution-function/context [:user/id]           ; data this function needs
   :execution-function/response [:user/data]        ; data this function produces
   :execution-function/deps #{:component/user-db}}) ; dependencies

Key insight: The function declares:

• Context: What data flows in (:user/id)
• Response: What data flows out (:user/data)
• Dependencies: What components it needs

Step 5: Register an Endpoint

Endpoints expose functions via HTTP:

(registry/register!
  :endpoint/get-user
  :atlas/interface-endpoint
  #{:tier/api
    :domain/users
    :http/get}
  {:endpoint/context [:http/path-params]
   :endpoint/response [:http/json-response]
   :endpoint/deps #{:fn/get-user-by-id}})

Step 6: Query the Registry

Now the powerful part - querying your architecture:

(require '[atlas.query :as query])

;; Find all entities in the users domain
(query/find-by-aspect @registry/registry :domain/users)
;; => Returns all three entities we registered

;; Find only service-tier entities
(query/find-by-aspect @registry/registry #{:tier/service :domain/users})
;; => Returns just :fn/get-user-by-id

;; Get an entity's full semantic identity
(registry/identity-for :fn/get-user-by-id)
;; => #{:atlas/execution-function :tier/service :domain/users :operation/read}

Step 7: Check Invariants

Atlas can validate your architecture:

(require '[atlas.invariant :as inv])

;; Check all invariants
(inv/check-all)
;; => {:valid? true, :violations []}

;; Get a readable report
(println (inv/report))

What gets checked:

• All dependencies reference existing entities
• No circular dependencies
• Data flows are satisfiable (inputs are produced somewhere)
• Tier boundaries respected
• And more...

Step 8: Explore (Optional)

For a visual view of your registry:

(require '[atlas.atlas-ui.server :as ui])
(ui/start!)
;; Opens browser at http://localhost:8082

What You Learned

1. Compound identities - Entities are sets of semantic aspects
2. Entity types - Different kinds of entities (functions, components, endpoints)
3. Data flow - Functions declare context (in) and response (out)
4. Querying - Find entities by any aspect combination
5. Invariants - Validate architectural rules automatically

Next Steps

• Read Core Concepts for deeper understanding
• See Examples for real-world patterns
• Check API Reference for all functions

Common Questions

Q: Why not just use namespaces?

Namespaces give you one dimension (where code lives). Compound identities give you N dimensions (what code means). You can query by domain, tier, operation type, etc. - all at once.

Q: Do I need to use the UI?

No. The core value is in the registry and query APIs. The UI is a bonus for exploration.

Q: Can I define custom entity types?

Yes! Register them with :atlas/type:

(registry/register!
  :my-project/workflow
  :atlas/type
  #{}
  {:registry-definition/keys [:workflow/steps :workflow/transitions]})

Q: What about actual function implementations?

Atlas describes architecture, not implementations. Your actual functions live in regular Clojure namespaces. Atlas tells you what they mean and how they relate.