Build a microservice with magic¶

You will build todo-service — the canonical reference service for magic — and come away understanding it layer by layer. todo-service is a small but complete microservice: a CRUD API for todo items that exercises magic's features end to end. The source lives at tink3rlabs/todo-service.

Every snippet below is included live from tink3rlabs/todo-service at the release tag v0.9.0 — pulled straight from the repo at build time, not copied into this page.

Pinned to a release tag

The code blocks below are not copied — they're included live from tink3rlabs/todo-service at the pinned tag v0.9.0 via mkdocs pymdownx.snippets URL includes. mkdocs build --strict fetches and validates every include, so the tutorial can no longer silently drift from the real repo. When todo-service cuts a new release, bump this pin to the new tag (find/replace v0.9.0 in this file).

todo-service is a properly layered service, and that layering is the spine of this tutorial. We walk it in the order the request flows — and the order you'd build it:

migrations → types → features → routes → server

migrations — the database schema, applied at startup.
types — the data types and their OpenAPI annotations.
features — the business logic.
routes — the HTTP routing that wires features to URLs.
server — the bootstrap that assembles everything: storage, observability, health probes, auth, and the router.

When you're done you have a running service you can curl: health probes answer, full CRUD on /todos works, and a Lucene ?filter= query returns matching todos.

Get the code¶

git clone https://github.com/tink3rlabs/todo-service.git
cd todo-service
go mod download

Here's the map before the walk:

todo-service/
  main.go              # entrypoint — embeds config, hands off to the cobra CLI
  cmd/                  # cobra commands — root.go wires viper; server.go runs the service
  pkg/
    types/              # data types (Todo) plus their OpenAPI annotations
    features/           # business logic — the todo feature package
    routes/             # HTTP routing — maps /todos verbs onto the feature
  config/
    development.yaml    # service configuration (storage, auth, observability, ...)
    migrations/         # startup migrations, one set per SQL provider
    openapi.json        # generated OpenAPI spec artifact
  build/
    generate.go         # generates config/openapi.json from the type annotations

It runs with no external services

todo-service defaults to the in-memory storage adapter, with auth and pub/sub disabled. go run . server --config config/development.yaml starts it with no database, no tokens, and no AWS credentials.

Migrations¶

The first layer is the schema — the shape of the data the rest of the service sits on. magic's migrations describe that schema as ordered SQL, applied automatically at startup.

todo-service has exactly one table, created by one migration:

config/migrations/postgresql/01__base.yaml

---
description: Create the todos table
migrations:
  - migrate: >
      CREATE TABLE IF NOT EXISTS todos (
        id TEXT PRIMARY KEY,
        summary TEXT NOT NULL,
        done BOOLEAN NOT NULL DEFAULT FALSE
      )
    rollback: DROP TABLE IF EXISTS todos

The file format¶

Each migration file is YAML with two top-level keys:

description — a human-readable summary of what the file does.
migrations — an ordered list of migration steps. Each step has:
- migrate — the SQL that applies the change (here, creating the todos table).
- rollback — the SQL that undoes it (dropping the table).

Both statements use IF NOT EXISTS / IF EXISTS so they're idempotent: re-running a migration that's already applied — or rolling back one that's already gone — is a no-op rather than an error.

The filename prefix (01__) orders the files. Add a schema change later as 02__add_due_date.yaml and it runs after 01__base.yaml.

One directory per provider¶

config/migrations/
  postgresql/01__base.yaml
  mysql/01__base.yaml
  sqlite/01__base.yaml

SQL dialects differ — Postgres spells the id column TEXT, MySQL wants VARCHAR(255), SQLite stores done as INTEGER — so each provider gets its own directory. magic picks the directory matching the configured storage provider. The three columns (id, summary, done) map directly onto the Todo type the next section covers.

How the schema gets applied¶

The migration files ship inside the binary. main.go embeds the whole config tree and hands it to magic's storage package:

main.go

//go:embed config
var configFS embed.FS

func main() {
    storage.ConfigFs = configFS

At startup, runServer builds the storage adapter and then runs any pending migrations against it:

cmd/server.go

    storage.NewDatabaseMigration(storageAdapter).Migrate()

The Server section later covers the full bootstrap; for now the point is just that migrations run automatically, before the first request is served. This holds even for the default in-memory adapter used in local dev — it runs the same migrations, so the service behaves identically whether it's backed by Postgres or an in-process store. For the adapter details, see Storage Adapters.

Migrations run on every start

Migrate() applies only the migrations that haven't run yet and is safe to call on every boot. A fresh database gets the full schema; an up-to-date one is left untouched.

With the schema defined, the next layer is the types — the Go structs that model a todo and carry the OpenAPI annotations magic generates the spec from.

Types¶

The second layer is the data shapes the rest of the service is built around. Every layer above this one — features, routes, the generated OpenAPI spec — refers back to these structs. todo-service defines all three in a single file:

pkg/types/todo.go

package types

// @openapi
// components:
//
//  schemas:
//    Todo:
//      type: object
//      properties:
//        id:
//          type: string
//          description: The Todo's identifier
//          example: 01909c42-cc90-75dc-a943-2d87a16e787d
//        summary:
//          type: string
//          description: The Todo's summary
//          example: Pick up the groceries
//        done:
//          type: boolean
//          description: An indicator that tells if the Todo item is complete
//          example: false
type Todo struct {
    Id      string `json:"id"`
    Summary string `json:"summary"`
    Done    bool   `json:"done"`
}

// @openapi
// components:
//
//  schemas:
//    TodoUpdate:
//      type: object
//      properties:
//        summary:
//          type: string
//          description: The Todo's summary
//          example: Pick up the groceries
//        done:
//          type: boolean
//          description: An indicator that tells if the Todo item is complete
//          example: false
type TodoUpdate struct {
    Summary string `json:"summary"`
    Done    bool   `json:"done"`
}

// @openapi
// components:
//
//  schemas:
//    TodoList:
//      type: object
//      properties:
//        todos:
//          type: array
//          items:
//            $ref: '#/components/schemas/Todo'
//        next:
//          type: string
//          description: An identifier to use when requesting the next set of todos
//          example: MDE5MDlhOGUtNjcwNi03NWY1LWJjMjUtNWM0MjY0ZjUwZTQ1
type TodoList struct {
    Todos []Todo `json:"todos"`
    Next  string `json:"next"`
}

The `json` tags do double duty¶

The json struct tags are not just for serialization. magic's storage layer reads them too — they are the field and column names it uses, not the Go field names. The Todo.Id field is json:"id", so storage knows it as id. That's the same id you saw as the primary key in the Migrations section, and it's the literal string passed as the sort key in the feature layer:

    next, err := t.storage.List(&todos, "id", map[string]any{}, limit, cursor)

The same tags decide what's searchable. When a Lucene ?filter= query names a field, it names the json tag — summary:groceries, not Summary:groceries. magic introspects the struct once and builds the set of searchable fields from the tagged fields and their Go types. The Features section puts this to work; for the exact rules — which types are implicitly searchable, how json:"-" excludes a field — see Search (Lucene).

The `@openapi` annotation blocks¶

Each struct is preceded by an @openapi comment block holding a fragment of OpenAPI YAML. These are not documentation for humans — they're the source of the generated spec. build/generate.go runs openapi-godoc, which scans the package for @openapi comments, then calls types.MergeOpenAPIDefinitions to fold in magic's shared definitions. The result is written to config/openapi.json.

The schema names declared in these blocks — Todo, TodoList, TodoUpdate — are the contract. The route handlers reference them by name in their own @openapi annotations (request bodies, responses), and those references only resolve because the names are defined here. The Routes section covers that side.

Keep the struct and the annotation in sync

The @openapi block and the Go struct are maintained by hand, side by side. If you add a field to a struct, add it to the annotation too — nothing cross-checks them, and the generated spec is only as accurate as the comment.

Three types, three roles¶

Todo — the full record: id, summary, done. This is what storage persists and what list/get endpoints return.
TodoUpdate — the create/replace request body. It's Todo without id — the server owns identity, so the client never sends it.
TodoList — the list response shape: a todos array plus a next cursor for pagination.

With the shapes defined, the next layer is features — the business logic that creates, reads, updates, and searches todos.

Features¶

The features layer is where the business logic lives. It sits between the types and the routes: it knows nothing about HTTP — no request parsing, no status codes — and everything about what a todo operation means. Every method here is a storage-backed operation expressed in terms of the structs from the Types section.

todo-service keeps the whole layer in one file:

pkg/features/todo/todoService.go

package todo

import (
    "encoding/json"
    "log/slog"

    "github.com/google/uuid"
    "github.com/spf13/viper"

    "todo-service/pkg/types"

    "github.com/tink3rlabs/magic/logger"
    "github.com/tink3rlabs/magic/pubsub"
    "github.com/tink3rlabs/magic/storage"
    "github.com/tink3rlabs/magic/telemetry"
)

type TodoService struct {
    storage   storage.StorageAdapter
    created   telemetry.Counter
    publisher pubsub.Publisher
    topic     string
}

// WithPublisher attaches a pub/sub publisher; todo lifecycle events are published to topic.
func (t *TodoService) WithPublisher(p pubsub.Publisher, topic string) *TodoService {
    t.publisher = p
    t.topic = topic
    return t
}

// WithCreatedCounter attaches a metrics counter incremented on each successful create.
func (t *TodoService) WithCreatedCounter(c telemetry.Counter) *TodoService {
    t.created = c
    return t
}

func NewTodoService() *TodoService {
    storageAdapter, err := storage.StorageAdapterFactory{}.GetInstance(
        storage.StorageAdapterType(viper.GetString("storage.type")),
        viper.GetStringMapString("storage.config"),
    )

    if err != nil {
        logger.Fatal("failed to create TodoService instance", slog.Any("error", err.Error()))
    }
    t := TodoService{storage: storageAdapter}
    return &t
}

func (t *TodoService) ListTodos(limit int, cursor string) ([]types.Todo, string, error) {
    todos := []types.Todo{}
    next, err := t.storage.List(&todos, "id", map[string]any{}, limit, cursor)

    return todos, next, err
}

// SearchTodos returns todos matching a Lucene filter string, cursor-paginated.
// An empty filter returns everything (subject to limit/cursor).
func (t *TodoService) SearchTodos(filter string, limit int, cursor string) ([]types.Todo, string, error) {
    todos := []types.Todo{}
    next, err := t.storage.Search(&todos, "id", filter, limit, cursor)
    return todos, next, err
}

func (t *TodoService) GetTodo(id string) (types.Todo, error) {
    todo := types.Todo{}
    err := t.storage.Get(&todo, map[string]any{"id": id})
    return todo, err
}

func (t *TodoService) DeleteTodo(id string) error {
    return t.storage.Delete(&types.Todo{}, map[string]any{"id": id})
}

func (t *TodoService) UpdateTodo(todoToUpdate types.Todo) error {
    err := t.storage.Update(todoToUpdate, map[string]any{"id": todoToUpdate.Id})
    if err == nil {
        t.publishEvent("todo.updated", todoToUpdate)
    }
    return err
}

func (t *TodoService) CreateTodo(todoToCreate types.TodoUpdate) (types.Todo, error) {
    todo := types.Todo{}

    // Using UUIDv7 in order to easily support cursor based pagination without extra fields
    //
    // From the RFC (https://datatracker.ietf.org/doc/rfc9562/)
    //
    // UUIDv7 features a time-ordered value field derived from the widely
    // implemented and well-known Unix Epoch timestamp source, the number of
    // milliseconds since midnight 1 Jan 1970 UTC, leap seconds excluded.
    // Generally, UUIDv7 has improved entropy characteristics over UUIDv1
    // (Section 5.1) or UUIDv6 (Section 5.6).
    //
    // UUIDv7 values are created by allocating a Unix timestamp in
    // milliseconds in the most significant 48 bits and filling the
    // remaining 74 bits, excluding the required version and variant bits,
    // with random bits for each new UUIDv7 generated to provide uniqueness
    // as per Section 6.9.
    id, err := uuid.NewV7()
    if err != nil {
        return todo, err
    }

    todo.Id = id.String()
    todo.Summary = todoToCreate.Summary
    todo.Done = todoToCreate.Done

    err = t.storage.Create(todo)
    if err != nil {
        return todo, err
    }

    if t.created != nil {
        t.created.Add(1)
    }

    t.publishEvent("todo.created", todo)

    return todo, nil
}

func (t *TodoService) publishEvent(eventType string, todo types.Todo) {
    if t.publisher == nil {
        return
    }
    payload, err := json.Marshal(todo)
    if err != nil {
        slog.Error("failed to marshal todo event", slog.String("error", err.Error()))
        return
    }
    if err := t.publisher.Publish(t.topic, string(payload), map[string]any{"event_type": eventType}); err != nil {
        slog.Error("failed to publish todo event", slog.String("error", err.Error()), slog.String("event_type", eventType))
    }
}

Constructing the service¶

NewTodoService() builds the one thing the layer can't work without: a storage.StorageAdapter. It doesn't pick an adapter itself — it reads storage.type and storage.config from viper and hands them to storage.StorageAdapterFactory, which returns the matching adapter (in-memory, SQL, DynamoDB, CosmosDB). The feature code is written against the StorageAdapter interface, so swapping storage.type from memory to postgres in config changes nothing in this file. See Storage Adapters for the factory and the per-adapter config.

The CRUD methods¶

CreateTodo, GetTodo, UpdateTodo, and DeleteTodo are thin — each one is a single storage call wrapped in just enough logic to be meaningful:

CreateTodo mints an id, copies the summary/done from the TodoUpdate body, and persists the full Todo.
GetTodo and DeleteTodo key off map[string]any{"id": id} — the same id json tag the storage layer reads from the struct.
UpdateTodo replaces the record by id, and on success publishes a todo.updated event.

Identity is owned here, not by the client: CreateTodo generates the id with uuid.NewV7(). UUIDv7 is time-ordered — the most significant 48 bits are a Unix-millisecond timestamp — so rows sort chronologically by primary key. That's the property cursor pagination depends on, and it's why ListTodos and SearchTodos can paginate on id alone, with no separate created_at sort column.

Two list paths: plain list vs. Lucene search¶

This service exposes two ways to read a collection, and the difference between them is the core lesson of this layer.

ListTodos is the plain list. It calls storage.List with a structured, exact-match filter map and cursor pagination:

    next, err := t.storage.List(&todos, "id", map[string]any{}, limit, cursor)

The third argument is a map[string]any of field/value pairs ANDed together as exact matches — here it's empty, so every todo is returned, a page at a time.

SearchTodos is the search path. It calls storage.Search with a single Lucene filter string:

    next, err := t.storage.Search(&todos, "id", filter, limit, cursor)

The caller passes one expressive query — summary:groceries AND done:1 — and magic compiles it to safe, parameterized SQL. No string concatenation, no injection surface. An empty filter returns everything, just like List.

Both share the same shape: sort on id, page with limit/next. This mirrors the Blox-style convention — one filter Lucene parameter, no typed query params (?done=true&summary=...), and cursor pagination via limit/next. Callers compose their own filters instead of the service growing a query parameter per field.

Boolean filters on the in-memory/SQLite adapter: use done:1

As of magic v0.17.1, the Lucene parser passes a filter term through as a string without coercing it to the struct field's Go type. Against the in-memory adapter (a SQLite-backed database), done:true becomes the SQL string param "true", which never matches SQLite's INTEGER column — it stores 1/0. Write done:1 instead: SQLite's integer affinity coerces the "1" string and the match works. On Postgres, done:true works as written because the boolean column accepts it. todo-service's todoService_test.go asserts exactly this behaviour. See Search (Lucene) for the full filter syntax.

Optional seams: counter and publisher¶

TodoService has two optional collaborators, attached through fluent setters:

WithCreatedCounter plugs in a telemetry.Counter. CreateTodo increments it on each successful create — but only if t.created != nil, so leaving it unset is a no-op.
WithPublisher plugs in a pubsub.Publisher and a topic. publishEvent is the helper that uses it: it marshals the todo to JSON and publishes a todo.created or todo.updated event with an event_type attribute — and returns immediately if t.publisher == nil.

Both are nil by default, so the service is fully functional with neither. They're the seams where observability and pub/sub plug in; the Server section covers how cmd/server.go actually wires them up.

With the business logic in place, the next layer is routes — the HTTP handlers that parse requests, call these methods, and shape the responses.

Routes¶

The routes layer is the HTTP boundary. It turns the feature methods — which know nothing about HTTP — into a chi router: URLs and verbs map onto TodoService calls, request bodies are validated, and returned errors become status codes. todo-service keeps the router, its validation schemas, and all six handlers in one file.

The first half declares the JSON-schema validation maps, the two wiring structs, and NewTodoRouter — the constructor that assembles the router:

pkg/routes/todo.go

package routes

import (
    "encoding/json"
    "io"
    "net/http"
    "strconv"

    jsonpatch "github.com/evanphx/json-patch/v5"
    "github.com/go-chi/chi/v5"
    "github.com/go-chi/render"

    "todo-service/pkg/features/todo"
    "todo-service/pkg/types"

    "github.com/tink3rlabs/magic/errors"
    "github.com/tink3rlabs/magic/middlewares"
    "github.com/tink3rlabs/magic/pubsub"
    "github.com/tink3rlabs/magic/telemetry"
)

type TodoRouter struct {
    Router  *chi.Mux
    service *todo.TodoService
}

var createSchema = map[string]string{
    "body": `{
        "type": "object",
        "properties": {
            "summary": { "type": "string" },
            "done": { "type": "boolean" }
        },
        "required": ["summary"],
        "additionalProperties": false
    }`,
}

var replaceSchema = map[string]string{
    "body": `{
        "type": "object",
        "properties": {
            "summary": { "type": "string" },
            "done": { "type": "boolean" }
        },
        "required": ["summary", "done"],
        "additionalProperties": false
    }`,
    "params": `{
        "type": "object",
        "properties": {
            "id": { "type": "string" }
        },
        "required": ["id"]
    }`,
}

var idSchema = map[string]string{
    "params": `{
        "type": "object",
        "properties": {
            "id": { "type": "string" }
        },
        "required": ["id"]
    }`,
}

// AuthConfig carries the auth wiring for the todo routes.
type AuthConfig struct {
    Middleware func(http.Handler) http.Handler
    Enabled    bool
    WriteRole  string
}

// PubSubConfig carries the pub/sub wiring for the todo routes.
type PubSubConfig struct {
    Publisher pubsub.Publisher
    TopicARN  string
}

func NewTodoRouter(created telemetry.Counter, auth AuthConfig, pubSub PubSubConfig) *TodoRouter {
    t := TodoRouter{}
    h := middlewares.ErrorHandler{}
    v := middlewares.Validator{}

    router := chi.NewRouter()

    // Public reads.
    router.Get("/{id}", v.ValidateRequest(idSchema, h.Wrap(t.GetTodo)))
    router.Get("/", h.Wrap(t.ListTodos))

    // Protected writes — require a valid token (and the write role when auth is enabled).
    router.Group(func(r chi.Router) {
        r.Use(auth.Middleware)
        r.Use(middlewares.UserRequestContext)
        if auth.Enabled {
            r.Use(middlewares.RequireRole(auth.WriteRole))
        }
        r.Post("/", v.ValidateRequest(createSchema, h.Wrap(t.CreateTodo)))
        r.Put("/{id}", v.ValidateRequest(replaceSchema, h.Wrap(t.ReplaceTodo)))
        r.Patch("/{id}", v.ValidateRequest(idSchema, h.Wrap(t.UpdateTodo)))
        r.Delete("/{id}", v.ValidateRequest(idSchema, h.Wrap(t.DeleteTodo)))
    })

    t.Router = router
    service := todo.NewTodoService().WithCreatedCounter(created)
    if pubSub.Publisher != nil {
        service = service.WithPublisher(pubSub.Publisher, pubSub.TopicARN)
    }
    t.service = service

    return &t
}

Public reads, protected writes¶

NewTodoRouter builds a single chi.Mux and splits it into two access tiers:

Public reads — GET /{id} and GET / are registered straight on the router. Anyone can read a todo or list the collection; no token required.
Protected writes — POST, PUT, PATCH, and DELETE live inside a router.Group, which applies middleware to just that subtree. Every write goes through auth.Middleware (magic's EnsureValidToken middleware, populated by the Server section) and middlewares.UserRequestContext, which lifts the caller's identity off the validated token into the request context. middlewares.RequireRole(auth.WriteRole) is added only when auth.Enabled is true — so local dev with auth disabled still accepts writes, while a deployed service enforces the write role.

AuthConfig and PubSubConfig are the wiring structs the constructor takes as input. The routes layer declares what it needs — an auth middleware, an optional publisher — and the Server section populates them from configuration.

Schema validation before the handler¶

Each route is wrapped in v.ValidateRequest, where v is a middlewares.Validator{}. It takes a JSON-schema map and the handler, and validates the request before the handler runs:

createSchema validates the POST body — summary required, done optional, no extra properties.
replaceSchema validates both the PUT body (summary and done required) and the id path param.
idSchema validates just the id path param, used by GET /{id}, PATCH /{id}, and DELETE /{id}.

A request that fails its schema never reaches the handler — the validator rejects it with a 400.

Errors become status codes¶

Every handler has the signature func(w http.ResponseWriter, r *http.Request) error — it returns an error instead of writing a status code itself. h.Wrap, from middlewares.ErrorHandler{}, bridges that to a standard http.HandlerFunc: when a handler returns a typed error from magic's errors package, Wrap maps it to the matching HTTP status. &errors.BadRequest{} becomes a 400, &errors.NotFound{} becomes a 404. The handler just returns the error; the middleware owns the translation.

The handlers themselves make up the second half of the file:

pkg/routes/todo.go (continued)

// @openapi
// paths:
//
//  /todos:
//    get:
//      tags:
//        - todos
//      summary: Get all Todos
//      description: Returns all Todos
//      operationId: listTodos
//      parameters:
//        - name: limit
//          in: query
//          description: The number of todo items to return (defaults to 10)
//          required: false
//          schema:
//            type: number
//        - name: next
//          in: query
//          description: The next page identifier
//          required: false
//          schema:
//            type: string
//        - name: filter
//          in: query
//          description: A Lucene query string to filter todos (e.g. done:1)
//          required: false
//          schema:
//            type: string
//      responses:
//        '200':
//          description: successful operation
//          content:
//            application/json:
//              schema:
//                $ref: '#/components/schemas/TodoList'
//        '500':
//           $ref: '#/components/responses/ServerError'
func (t *TodoRouter) ListTodos(w http.ResponseWriter, r *http.Request) error {
    cursor := r.URL.Query().Get("next")
    filter := r.URL.Query().Get("filter")

    limit, err := strconv.ParseInt(r.URL.Query().Get("limit"), 10, 64)
    if err != nil || limit <= 0 {
        limit = 10
    }

    var todos []types.Todo
    var next string
    if filter != "" {
        todos, next, err = t.service.SearchTodos(filter, int(limit), cursor)
    } else {
        todos, next, err = t.service.ListTodos(int(limit), cursor)
    }
    if err != nil {
        return &errors.BadRequest{Message: err.Error()}
    }
    render.JSON(w, r, types.TodoList{Todos: todos, Next: next})
    return nil
}

// @openapi
// paths:
//
//  /todos/{id}:
//    get:
//      tags:
//        - todos
//      summary: Get a single Todo
//      description: Returns a Todos with the identifier {id} if exists
//      operationId: getTodo
//      parameters:
//        - name: id
//          in: path
//          description: The identifier of the Todo
//          required: true
//          schema:
//            type: string
//      responses:
//        '200':
//          description: successful operation
//          content:
//            application/json:
//              schema:
//                $ref: '#/components/schemas/Todo'
//        '404':
//           $ref: '#/components/responses/NotFound'
//        '500':
//           $ref: '#/components/responses/ServerError'
func (t *TodoRouter) GetTodo(w http.ResponseWriter, r *http.Request) error {
    id := chi.URLParam(r, "id")
    todo, err := t.service.GetTodo(id)
    if err != nil {
        return err
    }
    render.JSON(w, r, todo)
    return nil
}

// @openapi
// paths:
//
//  /todos/{id}:
//    delete:
//      tags:
//        - todos
//      summary: Delete a single Todo
//      description: Deletes a Todos with the identifier {id} if exists
//      operationId: deleteTodo
//      parameters:
//        - name: id
//          in: path
//          description: The identifier of the Todo
//          required: true
//          schema:
//            type: string
//      responses:
//        '204':
//          description: successful operation
//        '500':
//           $ref: '#/components/responses/ServerError'
func (t *TodoRouter) DeleteTodo(w http.ResponseWriter, r *http.Request) error {
    id := chi.URLParam(r, "id")
    err := t.service.DeleteTodo(id)
    if err != nil {
        return err
    }
    render.NoContent(w, r)
    return nil
}

// @openapi
// paths:
//
//  /todos:
//    post:
//      tags:
//        - todos
//      summary: Create a Todo
//      description: Create a new Todo
//      operationId: createTodo
//      requestBody:
//        description: Create a new Todo
//        content:
//          application/json:
//            schema:
//              $ref: '#/components/schemas/TodoUpdate'
//      responses:
//        '201':
//          description: successful operation
//        '400':
//           $ref: '#/components/responses/BadRequest'
//        '500':
//           $ref: '#/components/responses/ServerError'
func (t *TodoRouter) CreateTodo(w http.ResponseWriter, r *http.Request) error {
    var todoToCreate types.TodoUpdate

    decodeErr := json.NewDecoder(r.Body).Decode(&todoToCreate)
    if decodeErr != nil {
        return decodeErr
    }

    todo, err := t.service.CreateTodo(todoToCreate)
    if err != nil {
        return err
    }

    render.Status(r, http.StatusCreated)
    render.JSON(w, r, todo)
    return nil
}

// @openapi
// paths:
//
//  /todos/{id}:
//    put:
//      tags:
//        - todos
//      summary: Replace a Todo
//      description: Replace a Todo
//      operationId: replaceTodo
//      parameters:
//        - name: id
//          in: path
//          description: The identifier of the Todo
//          required: true
//          schema:
//            type: string
//      requestBody:
//        description: Updated Todo
//        content:
//          application/json:
//            schema:
//              $ref: '#/components/schemas/TodoUpdate'
//      responses:
//        '204':
//          description: successful operation
//        '400':
//           $ref: '#/components/responses/BadRequest'
//        '404':
//           $ref: '#/components/responses/NotFound'
//        '500':
//           $ref: '#/components/responses/ServerError'
func (t *TodoRouter) ReplaceTodo(w http.ResponseWriter, r *http.Request) error {
    id := chi.URLParam(r, "id")
    var todoToUpdate types.TodoUpdate

    decoder := json.NewDecoder(r.Body)
    err := decoder.Decode(&todoToUpdate)
    if err != nil {
        return err
    }

    currentRecord, err := t.service.GetTodo(id)
    if err != nil {
        return &errors.NotFound{Message: "Todo not found"}
    }

    todo := types.Todo{Id: currentRecord.Id, Summary: todoToUpdate.Summary, Done: todoToUpdate.Done}
    err = t.service.UpdateTodo(todo)
    if err != nil {
        return err
    }

    render.NoContent(w, r)
    return nil
}

// @openapi
// paths:
//
//  /todos/{id}:
//    patch:
//      tags:
//        - todos
//      summary: Update a Todo
//      description: Update a Todo using [JSON Patch](https://jsonpatch.com/)
//      operationId: updateTodo
//      parameters:
//        - name: id
//          in: path
//          description: The identifier of the Todo
//          required: true
//          schema:
//            type: string
//      requestBody:
//        description: JSON Patch operations to perform in order to update the Todo item
//        content:
//          application/json-patch+json:
//            schema:
//              type: array
//              items:
//                $ref: "#/components/schemas/PatchBody"
//              example:
//                - {"op": "replace", "path": "/summary", "value": "An updated TODO item summary"}
//                - {"op": "replace", "path": "/done", "value": true}
//      responses:
//        '204':
//          description: successful operation
//        '400':
//           $ref: '#/components/responses/BadRequest'
//        '404':
//           $ref: '#/components/responses/NotFound'
//        '500':
//           $ref: '#/components/responses/ServerError'
func (t *TodoRouter) UpdateTodo(w http.ResponseWriter, r *http.Request) error {
    id := chi.URLParam(r, "id")

    body, err := io.ReadAll(r.Body)
    if err != nil {
        return err
    }

    patch, err := jsonpatch.DecodePatch(body)
    if err != nil {
        return &errors.BadRequest{Message: err.Error()}
    }

    currentRecord, err := t.service.GetTodo(id)
    if err != nil {
        return &errors.NotFound{Message: "Todo not found"}
    }

    currentBytes, err := json.Marshal(currentRecord)
    if err != nil {
        return err
    }

    modifiedBytes, err := patch.Apply(currentBytes)
    if err != nil {
        return &errors.BadRequest{Message: err.Error()}
    }

    var modified types.Todo
    err = json.Unmarshal(modifiedBytes, &modified)
    if err != nil {
        return err
    }

    if modified.Id != currentRecord.Id {
        return &errors.BadRequest{Message: "Id field can't be changed"}
    }

    err = t.service.UpdateTodo(modified)
    if err != nil {
        return err
    }

    render.NoContent(w, r)
    return nil
}

The `GET /todos` query surface¶

ListTodos is where the two list paths from the Features section surface as one endpoint. It reads three query params:

?filter=<lucene> — if present, the handler calls t.service.SearchTodos, the Lucene search path. If absent, it falls through to t.service.ListTodos, the structured list. One endpoint, one decision: if filter != "".
?limit= — the page size, defaulting to 10 when missing or invalid.
?next= — the opaque cursor for the next page.

Together limit and next drive cursor pagination, and ?filter= selects which underlying path produces the page. There are no typed query params — a caller filtering on done writes ?filter=done:1, not ?done=true. See Search (Lucene) for the full filter syntax.

JSON Patch and the OpenAPI annotations¶

UpdateTodo is the one handler that doesn't take a plain JSON body. It expects a application/json-patch+json document — a list of JSON Patch operations — reads the current record, applies the patch, and persists the result. It rejects any patch that tries to change id.

Every handler is preceded by an @openapi annotation block describing its path, parameters, request body, and responses. Just like the type annotations from the Types section, these feed build/generate.go — the route blocks reference the Todo, TodoUpdate, and TodoList schemas by name, and those references resolve because the Types section defined them. The handlers and the type structs together produce the complete config/openapi.json.

With the router assembled, the last layer is the server — the bootstrap that builds storage, observability, health probes, and auth, populates the AuthConfig/PubSubConfig structs, and mounts this router.

Server¶

The server is the final layer — the wiring that assembles everything below it into a running process. Migrations, types, features, and routes each do one job; the server is what builds the storage adapter, runs the migrations against it, initialises observability, populates the AuthConfig/PubSubConfig structs the Routes section declared, mounts the router, and serves HTTP. todo-service splits this across three small files plus one larger one: main.go, cmd/root.go, and cmd/server.go.

`main.go` — embed the config, hand off to the CLI¶

main.go

package main

import (
    "embed"

    "todo-service/cmd"

    "github.com/tink3rlabs/magic/storage"
)

//go:generate go run build/generate.go
//go:embed config
var configFS embed.FS

func main() {
    storage.ConfigFs = configFS
    cmd.ConfigFS = configFS
    cmd.Execute()
}

main.go does almost nothing itself. The //go:embed config directive bakes the entire config/ tree — development.yaml, the generated openapi.json, and crucially the migrations/ directory — into the binary as an embed.FS. That filesystem is then handed to two places: storage.ConfigFs, which is where magic's storage package looks for the migration files at startup (this is what makes the Migrations section's SQL available at runtime, with no files to ship alongside the binary), and cmd.ConfigFS, which the server command reads openapi.json from to serve /api-docs. Then cmd.Execute() hands control to cobra.

`cmd/root.go` — the cobra root and viper config¶

cmd/root.go

package cmd

import (
    "embed"
    "fmt"
    "os"
    "strings"

    homedir "github.com/mitchellh/go-homedir"
    "github.com/spf13/cobra"
    "github.com/spf13/viper"

    "github.com/tink3rlabs/magic/logger"
)

var ConfigFS embed.FS
var cfgFile string
var rootCmd = &cobra.Command{
    Use:   "",
    Short: "ToDo is a reference implementaion of a common service architecture",
    Long: `ToDo is a reference implementaion of a common service architecture brought to you with love by tink3rlabs.
Complete documentation is available at https://github.com/tink3rlabs/todo-service`,
}

func Execute() {
    if err := rootCmd.Execute(); err != nil {
        fmt.Println(err)
        os.Exit(1)
    }
}

func init() {
    cobra.OnInitialize(initConfig)
    rootCmd.PersistentFlags().StringVar(&cfgFile, "config", "", "config file (default is $HOME/.todo.yaml)")
    if viperBindFlagsErr := viper.BindPFlags(rootCmd.Flags()); viperBindFlagsErr != nil {
        fmt.Println(viperBindFlagsErr)
        os.Exit(1)
    }
    rootCmd.AddCommand(serverCommand)
}

func initConfig() {
    // Don't forget to read config either from cfgFile or from home directory!
    if cfgFile != "" {
        // Use config file from the flag.
        viper.SetConfigFile(cfgFile)
    } else {
        // Find home directory.
        home, err := homedir.Dir()
        if err != nil {
            fmt.Println(err)
            os.Exit(1)
        }

        // Search config in home directory with name ".cobra" (without extension).
        viper.AddConfigPath(home)
        viper.SetConfigName(".todo")
    }

    viper.SetEnvPrefix("TODO")
    viper.SetEnvKeyReplacer(strings.NewReplacer("_", "."))
    viper.AutomaticEnv()
    if err := viper.ReadInConfig(); err != nil {
        fmt.Println("Can't read config:", err)
        os.Exit(1)
    }

    config := loggerConfig()
    logger.Init(config)
}

func loggerConfig() *logger.Config {
    // Fetch the log level and format from the config file
    levelStr := viper.GetString("logger.level")
    json := viper.GetBool("logger.json")

    return &logger.Config{
        Level: logger.MapLogLevel(levelStr),
        JSON:  json,
    }
}

root.go defines the cobra root command and registers the server subcommand. The work happens in initConfig, run by cobra.OnInitialize before any command executes: it points viper at the config file (the --config flag, or ~/.todo.yaml by default), enables TODO_-prefixed environment overrides, and reads the file. Every viper.GetString(...) call you'll see in server.go resolves against the config loaded here. Finally it calls logger.Init with the level and format from the config, so magic's structured logger is ready before the server starts.

`cmd/server.go` — the `server` command¶

server.go is the centrepiece. Its runServer function does the full bootstrap in wiring order. Here it is whole:

cmd/server.go

package cmd

import (
    "context"
    "fmt"
    "log/slog"
    "net/http"
    "os"
    "os/signal"
    "syscall"
    "time"

    "github.com/go-chi/chi/v5"
    "github.com/go-chi/chi/v5/middleware"
    "github.com/go-chi/cors"
    "github.com/go-chi/render"
    "github.com/go-co-op/gocron/v2"
    "github.com/spf13/cobra"
    "github.com/spf13/viper"
    "github.com/tink3rlabs/magic/errors"
    "github.com/tink3rlabs/magic/health"
    "github.com/tink3rlabs/magic/leadership"
    "github.com/tink3rlabs/magic/logger"
    "github.com/tink3rlabs/magic/middlewares"
    "github.com/tink3rlabs/magic/observability"
    "github.com/tink3rlabs/magic/pubsub"
    "github.com/tink3rlabs/magic/storage"
    "github.com/tink3rlabs/magic/telemetry"

    "todo-service/pkg/routes"
)

var serverCommand = &cobra.Command{
    Use:   "server",
    Short: "Run the ToDo server",
    RunE:  runServer,
}

func init() {
    serverCommand.Flags().StringP("port", "p", "8080", "The port on which the Todo server will listen on")
}

func initRoutes(obs *observability.Observer, todosCreated telemetry.Counter, auth routes.AuthConfig, pubSub routes.PubSubConfig) *chi.Mux {
    router := chi.NewRouter()
    router.Use(
        render.SetContentType(render.ContentTypeJSON), // Set content-Type headers as application/json
        middleware.Logger,          // Log API request calls
        middleware.RedirectSlashes, // Redirect slashes to no slash URL versions
        middleware.Recoverer,       // Recover from panics without crashing server
        middlewares.ObservabilityWithOptions(obs, middlewares.ObservabilityOptions{
            SkipPaths:        []string{"/metrics"},
            SkipPathPrefixes: []string{"/health/"},
        }),
        cors.Handler(cors.Options{
            AllowedOrigins:   []string{"https://*", "http://*"},
            AllowedMethods:   []string{"GET", "POST", "PUT", "PATCH", "DELETE", "OPTIONS"},
            AllowedHeaders:   []string{"Accept", "Authorization", "Content-Type", "X-CSRF-Token"},
            ExposedHeaders:   []string{"Link"},
            AllowCredentials: false,
            MaxAge:           300, // Maximum value not ignored by any of major browsers
        }),
    )

    t := routes.NewTodoRouter(todosCreated, auth, pubSub)
    router.Route("/", func(r chi.Router) {
        r.Mount("/todos", t.Router)
    })

    return router
}

func createScheduler() {
    slog.Info("strating scheduler")
    // create a scheduler
    s, err := gocron.NewScheduler()
    if err != nil {
        logger.Fatal("failed to create scheduler", slog.Any("error", err))
    }
    // add a job to the scheduler
    _, err = s.NewJob(
        gocron.DurationJob(30*time.Second),
        gocron.NewTask(
            func(param string) {
                slog.Info("scheduled job says", slog.String("param", param))
            },
            "hello",
        ),
    )
    if err != nil {
        logger.Fatal("failed to create scheduled job", slog.Any("error", err))
    }

    // start the scheduler
    s.Start()
}

func runServer(cmd *cobra.Command, args []string) error {
    openApiSpec, err := ConfigFS.ReadFile("config/openapi.json")

    if err != nil {
        return fmt.Errorf("failed to load OpenAPI definition, did you forget to run go generate?: %v", err)
    }

    ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
    defer stop()

    obsCfg := observability.DefaultConfig()
    obsCfg.ServiceName = viper.GetString("observability.service_name")
    if obsCfg.ServiceName == "" {
        obsCfg.ServiceName = "todo-service"
    }
    switch viper.GetString("observability.metrics_mode") {
    case "otlp":
        obsCfg.MetricsMode = observability.MetricsModeOTLP
        obsCfg.MetricsOTLPEndpoint = viper.GetString("observability.metrics_otlp_endpoint")
    default:
        obsCfg.MetricsMode = observability.MetricsModePrometheus
    }
    obsCfg.EnableTracing = viper.GetBool("observability.enable_tracing")
    obsCfg.TracesOTLPEndpoint = viper.GetString("observability.traces_otlp_endpoint")

    obs, err := observability.Init(ctx, obsCfg)
    if err != nil {
        logger.Fatal("failed to initialise observability", slog.String("error", err.Error()))
    }
    defer func() { _ = obs.Shutdown(context.Background()) }()

    todosCreated, err := obs.Counter(telemetry.MetricDefinition{
        Name: "todo_service_todos_created_total",
        Help: "Total number of todo items created.",
        Kind: telemetry.KindCounter,
    })
    if err != nil {
        logger.Fatal("failed to register todos_created counter", slog.String("error", err.Error()))
    }

    storageAdapter, err := storage.StorageAdapterFactory{}.GetInstance(
        storage.StorageAdapterType(viper.GetString("storage.type")),
        viper.GetStringMapString("storage.config"),
    )

    if err != nil {
        panic("failed to get storage adapter instance")
    }

    storage.NewDatabaseMigration(storageAdapter).Migrate()

    var publisher pubsub.Publisher
    if viper.GetBool("pubsub.enabled") {
        publisher, err = pubsub.PublisherFactory{}.GetInstance(pubsub.SNS, map[string]string{
            "region": viper.GetString("pubsub.region"),
        })
        if err != nil {
            logger.Fatal("failed to create pub/sub publisher", slog.String("error", err.Error()))
        }
    }

    electionProps := leadership.LeaderElectionProps{
        HeartbeatInterval: viper.GetDuration("leadership.heartbeat"),
        StorageAdapter:    storageAdapter,
        AdditionalProps: map[string]any{
            "global": viper.GetBool("storage.config.global"),
            "region": viper.GetString("storage.config.region"),
            "regios": viper.GetStringSlice("storage.config.regions"),
        },
    }
    election := leadership.NewLeaderElection(electionProps)
    election.Start()

    go func() {
        for result := range election.Results {
            if result == leadership.RESULT_ELECTED {
                createScheduler()
            }
        }
    }()

    authEnabled := viper.GetBool("auth.enabled")
    authMiddleware := middlewares.EnsureValidToken(middlewares.EnsureValidTokenConfig{
        Enabled:   authEnabled,
        IssuerURL: viper.GetString("auth.issuer_url"),
        Audience:  viper.GetStringSlice("auth.audience"),
    })
    authCfg := routes.AuthConfig{
        Middleware: authMiddleware,
        Enabled:    authEnabled,
        WriteRole:  viper.GetString("auth.write_role"),
    }

    pubSubCfg := routes.PubSubConfig{
        Publisher: publisher,
        TopicARN:  viper.GetString("pubsub.topic_arn"),
    }

    router := initRoutes(obs, todosCreated, authCfg, pubSubCfg)

    router.Handle("/metrics", obs.MetricsHandler())

    router.Get("/api-docs", func(w http.ResponseWriter, r *http.Request) {
        if _, responseFailed := w.Write(openApiSpec); responseFailed != nil {
            slog.Error("failed responding to /api-docs:", slog.Any("error", responseFailed))
        }
    })

    //health check - liveness
    router.Get("/health/liveness", func(w http.ResponseWriter, r *http.Request) {
        render.Status(r, http.StatusNoContent)
        render.NoContent(w, r)
    })

    //health check - readiness
    healthChecker := health.NewHealthChecker(storageAdapter)
    h := middlewares.ErrorHandler{}
    router.Get("/health/readiness", h.Wrap(func(w http.ResponseWriter, r *http.Request) error {
        err := healthChecker.Check(viper.GetBool("health.storage"), viper.GetStringSlice("health.dependencies"))
        if err != nil {
            slog.Error("health check readiness failed", slog.Any("error", err.Error()))
            return &errors.ServiceUnavailable{Message: err.Error()}
        } else {
            render.Status(r, http.StatusNoContent)
            render.NoContent(w, r)
            return nil
        }
    }))

    port := viper.GetString("service.port")
    listenAddress := fmt.Sprintf(":%s", port)

    srv := &http.Server{Addr: listenAddress, Handler: router}
    go func() {
        if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
            logger.Fatal("server failed", slog.Any("error", err))
        }
    }()
    slog.Info("todo-service listening", slog.String("address", listenAddress))

    <-ctx.Done()
    slog.Info("shutdown signal received, stopping server")

    shutdownCtx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
    defer cancel()
    if err := srv.Shutdown(shutdownCtx); err != nil {
        slog.Error("graceful shutdown failed", slog.Any("error", err))
    }
    return nil
}

The `runServer` wiring walk¶

runServer builds the service from the bottom up. In order:

Load the OpenAPI spec. ConfigFS.ReadFile("config/openapi.json") reads the generated spec out of the embedded filesystem — later served at /api-docs. If it's missing, the command fails fast with a reminder to run go generate.
Signal-aware context. signal.NotifyContext(..., os.Interrupt, syscall.SIGTERM) produces a ctx that's cancelled on Ctrl-C or a SIGTERM from the orchestrator. Everything downstream hangs off this context, and the function blocks on it at the end for graceful shutdown.
Observability. observability.Init(ctx, obsCfg) brings up metrics (Prometheus or OTLP, per config) and optional tracing. Its shutdown is deferred immediately, so traces and metrics flush cleanly on exit. See Observability for the configuration surface.
The custom counter. obs.Counter(...) registers todo_service_todos_created_total — the metric the Features section's WithCreatedCounter seam expects.
Storage adapter. storage.StorageAdapterFactory{}.GetInstance(...) builds the adapter from storage.type and storage.config — the same factory call the feature layer makes. See Storage Adapters.
Migrations. storage.NewDatabaseMigration(storageAdapter).Migrate() runs every pending migration against that adapter — the Migrations section's SQL, applied here, before the first request.
Optional pub/sub publisher. When pubsub.enabled is true, an SNS publisher is built; otherwise publisher stays nil. Either way it goes into PubSubConfig for the routes.
Leadership election and scheduler. leadership.NewLeaderElection(...).Start() runs leader election against the storage adapter; a goroutine watches election.Results and starts the background createScheduler() only on the replica that's elected leader.
Auth middleware and AuthConfig. middlewares.EnsureValidToken(...) is built from the auth.* config and wrapped into a routes.AuthConfig along with Enabled and WriteRole. This is the config-gated middleware the Routes section's protected-writes group consumes.
PubSubConfig. The publisher from step 7 and pubsub.topic_arn are packed into a routes.PubSubConfig.
Build the router. initRoutes(obs, todosCreated, authCfg, pubSubCfg) constructs the chi router — base middleware (logging, panic recovery, CORS, observability), then NewTodoRouter mounted at /todos.
Mount the operational endpoints. /metrics serves the Prometheus handler, /api-docs writes the embedded OpenAPI spec, and the two health probes — /health/liveness (always 204) and /health/readiness (runs health.NewHealthChecker against storage and configured dependencies) — are registered directly on the router.
Serve and block. The http.Server runs in a goroutine; runServer blocks on <-ctx.Done(). When the signal arrives it calls srv.Shutdown with a 10-second timeout for an in-flight-safe graceful stop.

Health probes and observability skips¶

The health probes are registered outside the route group that carries the auth middleware — they're unauthenticated by design, so an orchestrator can probe /health/liveness and /health/readiness without a token. The observability middleware is configured to skip them too: ObservabilityOptions sets SkipPathPrefixes: []string{"/health/"} and SkipPaths: []string{"/metrics"}, keeping probe traffic and the metrics scrape itself out of the request metrics and traces.

Where the seams get populated¶

This is where the loose ends from the earlier layers get tied off. The AuthConfig and PubSubConfig structs the Routes section declared but left empty are filled in here from configuration. The WithCreatedCounter and WithPublisher seams the Features section exposed get their real collaborators — the todos_created counter and, when enabled, the SNS publisher — passed down through initRoutes and NewTodoRouter. The layers are independent; the server is what composes them.

With every layer assembled, the only thing left is to start the service and watch it answer.

Run it¶

One build step first: the server fails fast if config/openapi.json is missing, so generate it from the type and route annotations:

go generate ./...

Then start the service. It defaults to the in-memory storage adapter with auth and pub/sub disabled — no database, no tokens, no AWS credentials:

go run . server --config config/development.yaml

In a second shell, exercise it with curl.

Health probes — both return 204 No Content:

curl -i http://localhost:8080/health/liveness
curl -i http://localhost:8080/health/readiness

HTTP/1.1 204 No Content

Create two todos — POST /todos returns 201 with the created record (note the server-minted UUIDv7 id):

curl -s -X POST http://localhost:8080/todos \
  -H 'Content-Type: application/json' \
  -d '{"summary":"buy milk"}'
curl -s -X POST http://localhost:8080/todos \
  -H 'Content-Type: application/json' \
  -d '{"summary":"walk dog","done":true}'

{"id":"01909c42-cc90-75dc-a943-2d87a16e787d","summary":"buy milk","done":false}

POST /todos, no trailing slash

POST /todos/ 301-redirects to /todos (the RedirectSlashes middleware), and curl drops the request body across that redirect. Target /todos directly.

List all todos — GET /todos returns both, plus an empty next cursor (only one page):

curl -s http://localhost:8080/todos

{"todos":[{"id":"01909c42-cc90-75dc-a943-2d87a16e787d","summary":"buy milk","done":false},{"id":"01909c42-d1f0-7a3b-bc77-9e2150f4c8a1","summary":"walk dog","done":true}],"next":""}

Filter with Lucene — ?filter=done:1 returns only the completed todo:

curl -s 'http://localhost:8080/todos?filter=done:1'

{"todos":[{"id":"01909c42-d1f0-7a3b-bc77-9e2150f4c8a1","summary":"walk dog","done":true}],"next":""}

It's done:1, not done:true — the in-memory adapter is SQLite-backed, and its done column stores integers, so the boolean term has to be written as 1. The Features section's note covers why; Search (Lucene) has the full filter syntax.

Metrics — GET /metrics serves the Prometheus exposition. The custom counter wired up in the Server section shows the two creates:

curl -s http://localhost:8080/metrics | grep todo_service_todos_created_total

todo_service_todos_created_total 2

OpenAPI spec — GET /api-docs returns 200 with the generated OpenAPI JSON — the same config/openapi.json that go generate produced from the @openapi annotations:

curl -s http://localhost:8080/api-docs

That's the whole service: the schema migrated itself at startup, the types shaped every payload, the feature layer ran the CRUD and search, the routes mapped them onto HTTP, and the server wired it all together.

Where to next¶

Finishing this tutorial means you've walked todo-service's full stack — migrations, types, features, routes, server — and the running process you just curled is that walk: the real reference service, end to end. You haven't read a toy; you've built and understood the canonical magic service.

The walk kept auth, pub/sub, and the background machinery in the default-off state to stay focused on the request path. todo-service ships all of it, config-gated and ready to turn on:

JWT auth — middlewares.EnsureValidToken with multi-provider issuer support, gating the protected-writes group and the RequireRole check.
SNS pub/sub — the WithPublisher seam, emitting todo.created / todo.updated events when pubsub.enabled is set.
Leadership election + scheduler — leader election over the storage adapter, with the background gocron scheduler running only on the elected replica.
OpenAPI generation — build/generate.go and openapi-godoc turning the @openapi annotations into the served spec.

Flip the relevant config/development.yaml switches and read the corresponding code in tink3rlabs/todo-service to see each one in action.

For the cross-cutting concerns this tutorial pointed at along the way:

Search (Lucene) — the full ?filter= query syntax and the searchable-field rules.
Storage Adapters — the adapter factory and per-adapter configuration.
Observability — metrics, tracing, and the observability config surface.
Contributing — how to work on magic itself.