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Getting started

This tutorial walks you through building your first Katalyst service from an empty Gradle project to a running HTTP endpoint backed by a database. By the end you will have a service, a repository, a table, and a route — all discovered and wired automatically — and you will understand the shape of every Katalyst application.

You will build a small bookmarks API: store a URL, list the stored URLs.

Before you begin

You need:

  • JDK 21 or newer (java -version to check).
  • Gradle (the wrapper below handles the version).
  • Familiarity with Kotlin and basic Ktor concepts (routes, call.respond).

No database server is required — this tutorial uses an embedded H2 database.

Step 1: Create the project

Create a new directory and a Gradle build. This tutorial uses a single-module layout.

mkdir bookmarks && cd bookmarks
gradle init --type kotlin-application --dsl kotlin --package com.example --project-name bookmarks

Replace the generated app/build.gradle.kts with this:

plugins {
    kotlin("jvm") version "2.4.10"
    id("io.ktor.plugin") version "3.5.2"
    id("org.jetbrains.kotlin.plugin.serialization") version "2.4.10"
    application
}

repositories { mavenCentral() }

dependencies {
    val katalyst = "1.0.0-beta02"
    implementation(platform("io.github.darkryh.katalyst:katalyst-bom:$katalyst"))
    implementation("io.github.darkryh.katalyst:katalyst-starter-web")
    implementation("io.github.darkryh.katalyst:katalyst-starter-engine-netty")
    implementation("io.github.darkryh.katalyst:katalyst-starter-persistence")
}

application {
    mainClass.set("com.example.ApplicationKt")
}

You declare three things: the BOM and the starters you need. The BOM (platform(...)) pins every Katalyst artifact to one version, so the starter coordinates below carry no version of their own. Each starter then brings its external dependencies transitively — you never list Ktor, Exposed, HikariCP, or a JDBC driver yourself:

  • katalyst-starter-web pulls in Ktor (server, content negotiation, JSON serialization). It is engine-agnostic, so you also add exactly one engine starter — katalyst-starter-engine-netty here pulls in the Netty engine.
  • katalyst-starter-persistence pulls in Exposed, HikariCP, and the H2 and PostgreSQL drivers.

Run ./gradlew build once to confirm the dependencies resolve. You should see BUILD SUCCESSFUL.

Step 2: Add configuration

Katalyst reads infrastructure settings from YAML. Create app/src/main/resources/application.yaml:

ktor:
  deployment:
    host: 0.0.0.0
    port: 8080

database:
  url: jdbc:h2:mem:bookmarks;DB_CLOSE_DELAY=-1
  username: sa
  password: ""
  driver: org.h2.Driver

The ${VAR:default} syntax is available for environment overrides; you will see it in the configuration how-to. For now, plain values are fine.

Step 3: Write the bootstrap

Create app/src/main/kotlin/com/example/Application.kt:

package com.example

import io.github.darkryh.katalyst.config.yaml.enableYamlConfiguration
import io.github.darkryh.katalyst.di.katalystApplication
import io.github.darkryh.katalyst.koin.KoinBeanEngine
import io.github.darkryh.katalyst.ktor.engine.netty.NettyServer

fun main(args: Array<String>) = katalystApplication(args) {
    engine(NettyServer)
    beanEngine(KoinBeanEngine)
    features {
        enableYamlConfiguration()        // installs the YAML source; must run before database { }
    }
    database { fromConfiguration() }
    scanPackages("com.example")
    schema { createMissing() }          // create tables that don't exist yet
}

schema { createMissing() } tells Katalyst to create any table it discovers but cannot find in the database — convenient for local development. In production you use validateOnStartup() and run migrations.

Step 4: Define the table and entity

Create app/src/main/kotlin/com/example/Bookmark.kt:

package com.example

import io.github.darkryh.katalyst.core.persistence.Table
import io.github.darkryh.katalyst.core.persistence.mapping
import io.github.darkryh.katalyst.repositories.Identifiable
import org.jetbrains.exposed.v1.core.dao.id.LongIdTable

data class Bookmark(
    override val id: Long? = null,
    val url: String,
    val createdAtMillis: Long
) : Identifiable<Long>

object BookmarksTable : LongIdTable("bookmarks"), Table<Long, Bookmark> {
    val url = varchar("url", 2048)
    val createdAtMillis = long("created_at_millis")

    override val mapping = mapping<Long, Bookmark> {
        generatedId(id, Bookmark::id)
        field(url, Bookmark::url)
        field(createdAtMillis, Bookmark::createdAtMillis)

        construct {
            Bookmark(
                id = this[id],
                url = this[url],
                createdAtMillis = this[createdAtMillis]
            )
        }
    }
}

The entity is a plain data class that implements Identifiable<Long> — Table and CrudRepository both require it, so the framework knows where the primary key lives. Keep id nullable with a null default: it is null before the row is inserted, and save uses that to decide between insert and update.

A table is an Exposed LongIdTable that also implements Table<Id, Entity>. The mapping { … } block tells Katalyst how to read a row and how to write inserts and updates — you never touch Exposed's UpdateBuilder directly.

Step 5: Define the repository

Create app/src/main/kotlin/com/example/BookmarkRepository.kt:

package com.example

import io.github.darkryh.katalyst.repositories.CrudRepository
import org.jetbrains.exposed.v1.core.dao.id.LongIdTable

class BookmarkRepository : CrudRepository<Long, Bookmark> {
    override val table: LongIdTable = BookmarksTable
}

Implementing CrudRepository<Id, Entity> gives you save, findById, findAll, deleteById, and more — for free. Because the class is under a scanned package, Katalyst registers it; you never new it up.

Step 6: Define the service

Create app/src/main/kotlin/com/example/BookmarkService.kt:

package com.example

import io.github.darkryh.katalyst.core.component.Service

class BookmarkService(private val repository: BookmarkRepository) : Service {

    suspend fun add(url: String): Bookmark = transactionManager.transaction {
        repository.save(Bookmark(id = null, url = url, createdAtMillis = System.currentTimeMillis()))
    }

    suspend fun list(): List<Bookmark> = transactionManager.transaction {
        repository.findAll()
    }
}

Implementing Service marks the class for discovery and gives you transactionManager, which wraps every database operation in a transaction. The BookmarkRepository constructor parameter is injected by type — no wiring code.

Step 7: Add the route

Create app/src/main/kotlin/com/example/BookmarkRoutes.kt:

package com.example

import io.github.darkryh.katalyst.ktor.builder.katalystRouting
import io.github.darkryh.katalyst.ktor.extension.ktInject
import io.ktor.http.HttpStatusCode
import io.ktor.server.application.install
import io.ktor.server.plugins.contentnegotiation.ContentNegotiation
import io.ktor.serialization.kotlinx.json.json
import io.ktor.server.request.receive
import io.ktor.server.response.respond
import io.ktor.server.routing.Route
import io.ktor.server.routing.get
import io.ktor.server.routing.post
import kotlinx.serialization.Serializable

@Serializable
data class AddBookmarkRequest(val url: String)

fun Route.bookmarkRoutes() = katalystRouting {
    post("/bookmarks") {
        val service = call.ktInject<BookmarkService>()
        val request = call.receive<AddBookmarkRequest>()
        call.respond(HttpStatusCode.Created, service.add(request.url))
    }
    get("/bookmarks") {
        val service = call.ktInject<BookmarkService>()
        call.respond(service.list())
    }
}

You also need JSON content negotiation installed. Add a middleware in app/src/main/kotlin/com/example/JsonMiddleware.kt:

package com.example

import io.github.darkryh.katalyst.ktor.middleware.katalystMiddleware
import io.ktor.serialization.kotlinx.json.json
import io.ktor.server.application.Application
import io.ktor.server.application.install
import io.ktor.server.plugins.contentnegotiation.ContentNegotiation

fun Application.jsonMiddleware() = katalystMiddleware {
    install(ContentNegotiation) { json() }
}

katalystRouting, katalystMiddleware, and katalystWebSockets are the three Ktor entry points Katalyst installs automatically once DI is ready. Inside a route, call.ktInject<T>() resolves a dependency.

Katalyst calls these top-level functions for you, so your own code never references them — your IDE may flag them as unused. Install the Katalyst IDE plugin and the IDE recognizes them as framework entrypoints, so no @Suppress("unused") is needed.

Step 8: Run it

./gradlew run

You should see the bootstrap log, ending with the server starting:

INFO  Katalyst - Bootstrap complete
INFO  Application - Responding at http://0.0.0.0:8080

In another terminal, add and list a bookmark:

curl -X POST http://localhost:8080/bookmarks \
  -H "Content-Type: application/json" \
  -d '{"url":"https://kotlinlang.org"}'
{"id":1,"url":"https://kotlinlang.org","createdAtMillis":1718700000000}
curl http://localhost:8080/bookmarks
[{"id":1,"url":"https://kotlinlang.org","createdAtMillis":1718700000000}]

It works. The table was created at startup, the repository and service were discovered and injected, and the route was registered — and you wrote no DI configuration.

What you built

You now have a complete Katalyst application and have seen its five moving parts:

  • The bootstrap (katalystApplication { … }) selecting an engine, DI adapter, config source, database, scanned package, and schema policy.
  • A table (Table<Id, Entity> + mapping) and a repository (CrudRepository).
  • A service (Service) with transactional methods.
  • A route (katalystRouting) and a middleware (katalystMiddleware).

Where to go next