Ktor App's Main Method Not Called When Deployed to AppEngine - google-app-engine

Question

Issue
The Ktor application's main method is not called when deployed to AppEngine. In the applications's main method is logic to retrieve content from an API request based on a Timer and save that information to a Firestore database which a client consumes.
This logic currently works when deployed in a Jar to AppEngine. However, implementing this with Ktor would save deploy time and help future proof the backend service if endpoints are required.
Expected
The Ktor application's main method is called once the app is deployed to AppEngine similar to how an application's main method is called when ran in IntelliJ.
Actual
The main method is only called once the app's hosted route is called.
ie: https://[yourProjectName].appspot.com
Setup
Main Method
import io.ktor.application.Application
fun Application.main() {
// App logic here.
}
build.gradle
buildscript {
ext.kotlin_version = '1.3.10'
ext.ktor_version = '1.0.0'
ext.appengine_version = '1.9.60'
ext.appengine_plugin_version = '1.3.4'
ext.junitJupiterVersion = '5.0.3'
repositories {
jcenter()
mavenCentral()
}
dependencies {
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
classpath "com.google.cloud.tools:appengine-gradle-plugin:$appengine_plugin_version"
classpath 'org.junit.platform:junit-platform-gradle-plugin:1.0.3'
}
}
plugins {
id 'org.jetbrains.kotlin.jvm' version '1.2.51'
}
apply plugin: 'java'
apply plugin: 'kotlin'
apply plugin: 'war'
apply plugin: 'com.google.cloud.tools.appengine'
sourceSets {
main.kotlin.srcDirs = [ 'src/main/kotlin' ]
}
sourceCompatibility = 1.8
repositories {
jcenter()
mavenCentral()
maven { url "https://kotlin.bintray.com/ktor" }
}
dependencies {
implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
implementation "io.ktor:ktor-server-servlet:$ktor_version"
implementation "io.ktor:ktor-html-builder:$ktor_version"
providedCompile "com.google.appengine:appengine:$appengine_version"
implementation 'com.squareup.retrofit2:retrofit:2.3.0'
implementation 'com.squareup.retrofit2:converter-gson:2.3.0'
implementation 'com.squareup.retrofit2:adapter-rxjava:2.3.0'
implementation 'io.reactivex.rxjava2:rxjava:2.1.1'
implementation 'com.google.firebase:firebase-admin:6.3.0'
implementation 'com.google.apis:google-api-services-youtube:v3-rev204-1.23.0'
testCompile group: 'junit', name: 'junit', version: '4.12'
// JUnit Jupiter API and TestEngine implementation
testCompile("org.junit.jupiter:junit-jupiter-api:${junitJupiterVersion}")
testRuntime("org.junit.jupiter:junit-jupiter-engine:${junitJupiterVersion}")
testCompile("org.assertj:assertj-core:3.10.0")
// To avoid compiler warnings about #API annotations in JUnit code
testCompileOnly('org.apiguardian:apiguardian-api:1.0.0')
}
kotlin.experimental.coroutines = 'enable'
compileKotlin {
kotlinOptions.jvmTarget = "1.8"
}
compileTestKotlin {
kotlinOptions.jvmTarget = "1.8"
}
task run(dependsOn: appengineRun)
appengine {
deploy {
version = 'media-staging-1201181257pm'
}
}
src/main/resources/application.conf
ktor {
application {
modules = [ InitializationKt.main ]
}
}
src/main/webapp/WEB-INF/
appengine-web.xml
<?xml version="1.0" encoding="utf-8"?>
<appengine-web-app xmlns="http://appengine.google.com/ns/1.0">
<threadsafe>true</threadsafe>
<runtime>java8</runtime>
</appengine-web-app>
web.xml
<?xml version="1.0" encoding="ISO-8859-1" ?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" version="3.0">
<servlet>
<display-name>KtorServlet</display-name>
<servlet-name>KtorServlet</servlet-name>
<servlet-class>io.ktor.server.servlet.ServletApplicationEngine</servlet-class>
<!-- path to application.conf file, required -->
<init-param>
<param-name>io.ktor.config</param-name>
<param-value>application.conf</param-value>
</init-param>
</servlet>
<servlet-mapping>
<servlet-name>KtorServlet</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>
</web-app>

Answer 1

Thank you to Gabriel Machado on Kotlin Slack for answering this.
Gabriel recommends using a Cron Job as opposed to a Timer Task because there may be issues with a Timer thread based on scaling type.

Answer 2

GAE probably expects your app to look like a Java app (e.g., a Java style "main", not a Kotlin style "main").
Look at this for more info on how to do this: How to run Kotlin class from the command line?