Exercise: Logger
You are working on a pet store project. You implemented a function that is responsible for adding new pets to the database. Now your task is to add logging to this function. You want to log the following messages:
- Always start with info "Adding pet with name {name}"
- If the pet was added successfully, log info "Added pet with id {id}"
- If the pet was not added because of a conflict, log warning "There already is pet named {name}"
- If the pet was not added because of database error, log error "Failed to add pet with name {name}"
You want to add logger using context receiver of addPet function. The type of this receiver should be Logger, and inside addPet you should use its methods to log messages.
class PetStore(
private val database: Database,
) {
fun addPet(
addPetRequest: AddPetRequest,
): Pet? {
return try {
database.addPet(addPetRequest)
} catch (e: InsertionConflictException) {
null
} catch (e: Exception) {
null
}
}
}
This is what used classes and interfaces look like:
data class AddPetRequest(val name: String)
data class Pet(val id: Int, val name: String)
class InsertionConflictException : Exception()
interface Database {
fun addPet(addPetRequest: AddPetRequest): Pet
}
interface Logger {
fun logInfo(message: String)
fun logWarning(message: String)
fun logError(message: String)
}
This is an example usage, showing what should be logged in different cases:
fun main(): Unit = with(ConsoleLogger()) {
val database = RandomDatabase()
val petStore = PetStore(database)
petStore.addPet(AddPetRequest("Fluffy"))
// [INFO] - Adding pet with name Fluffy
// [INFO] - Added pet with id -81731626
// or
// [WARNING] - There already is pet named Fluffy
// or
// [ERROR] - Failed to add pet with name Fluffy
}
class RandomDatabase : Database {
override fun addPet(addPetRequest: AddPetRequest): Pet =
when {
Random.nextBoolean() ->
Pet(1234, addPetRequest.name)
Random.nextBoolean() ->
throw InsertionConflictException()
else -> throw Exception()
}
}
class ConsoleLogger : Logger {
override fun logInfo(message: String) {
println("[INFO] - $message")
}
override fun logWarning(message: String) {
println("[WARNING] - $message")
}
override fun logError(message: String) {
println("[ERROR] - $message")
}
}
This problem can either be solved in the below playground or you can clone kotlin-exercises project and solve it locally. In the project, you can find code template for this exercise in functional/context/PetStore.kt. You can find there starting code, example usage and unit tests.
Once you are done with the exercise, you can check your solution here.
Playground
import org.junit.After
import org.junit.Test
import kotlin.random.Random
import kotlin.test.assertEquals
class PetStore(
private val database: Database,
) {
fun addPet(
addPetRequest: AddPetRequest,
): Pet? {
return try {
database.addPet(addPetRequest)
} catch (e: InsertionConflictException) {
null
} catch (e: Exception) {
null
}
}
}
data class AddPetRequest(val name: String)
data class Pet(val id: Int, val name: String)
class InsertionConflictException : Exception()
interface Database {
fun addPet(addPetRequest: AddPetRequest): Pet
}
interface Logger {
fun logInfo(message: String)
fun logWarning(message: String)
fun logError(message: String)
}
fun main(): Unit = with(ConsoleLogger()) {
val database = RandomDatabase()
val petStore = PetStore(database)
petStore.addPet(AddPetRequest("Fluffy"))
// [INFO] - Adding pet with name Fluffy
// [INFO] - Added pet with id -81731626
// or
// [WARNING] - There already is pet named Fluffy
// or
// [ERROR] - Failed to add pet with name Fluffy
}
class RandomDatabase : Database {
override fun addPet(addPetRequest: AddPetRequest): Pet =
when {
Random.nextBoolean() ->
Pet(1234, addPetRequest.name)
Random.nextBoolean() ->
throw InsertionConflictException()
else -> throw Exception()
}
}
class ConsoleLogger : Logger {
override fun logInfo(message: String) {
println("[INFO] - $message")
}
override fun logWarning(message: String) {
println("[WARNING] - $message")
}
override fun logError(message: String) {
println("[ERROR] - $message")
}
}
class PetStoreTest {
private val database = FakeDatabase()
private val petStore = PetStore(database)
private val logger = FakeLogger()
@After
fun tearDown() {
database.clear()
logger.clear()
}
@Test
fun `should add pet`() {
val pet = with(logger) {
petStore.addPet(AddPetRequest("Fluffy"))
}
val expected = Pet(0, "Fluffy")
assertEquals(expected, pet)
assertEquals(expected, database.getPets().single())
}
@Test
fun `should return null when database failing`() {
database.startFailing()
val pet = with(logger) {
petStore.addPet(AddPetRequest("Fluffy"))
}
assertEquals(null, pet)
assertEquals(emptyList<Pet>(), database.getPets())
}
@Test
fun `should return null when conflict`() {
database.addPet(AddPetRequest("Fluffy"))
val pet = with(logger) {
petStore.addPet(AddPetRequest("Fluffy"))
}
assertEquals(null, pet)
}
@Test
fun `should log info when added pet`() {
with(logger) {
petStore.addPet(AddPetRequest("Fluffy"))
}
assertEquals(
listOf(
FakeLogger.Level.INFO to "Adding pet with name Fluffy",
FakeLogger.Level.INFO to "Added pet with id 0",
),
logger.getMessages()
)
}
@Test
fun `should log warning when adding conflict`() {
database.addPet(AddPetRequest("Fluffy"))
with(logger) {
petStore.addPet(AddPetRequest("Fluffy"))
}
assertEquals(
listOf(
FakeLogger.Level.INFO to "Adding pet with name Fluffy",
FakeLogger.Level.WARNING to "There already is pet named Fluffy",
),
logger.getMessages()
)
}
@Test
fun `should log error when database error`() {
database.startFailing()
with(logger) {
petStore.addPet(AddPetRequest("Fluffy"))
}
assertEquals(
listOf(
FakeLogger.Level.INFO to "Adding pet with name Fluffy",
FakeLogger.Level.ERROR to "Failed to add pet with name Fluffy",
),
logger.getMessages()
)
}
}
class FakeLogger : Logger {
private val messages = mutableListOf<Pair<Level, String>>()
fun clear() {
messages.clear()
}
fun getMessages(): List<Pair<Level, String>> = messages.toList()
override fun logInfo(message: String) {
messages.add(Level.INFO to message)
}
override fun logWarning(message: String) {
messages.add(Level.WARNING to message)
}
override fun logError(message: String) {
messages.add(Level.ERROR to message)
}
enum class Level {
INFO, WARNING, ERROR
}
}
class FakeDatabase : Database {
private val pets = mutableListOf<Pet>()
private var shouldFail = false
fun startFailing() {
shouldFail = true
}
fun clear() {
pets.clear()
shouldFail = false
}
override fun addPet(addPetRequest: AddPetRequest): Pet {
if (pets.any { it.name == addPetRequest.name }) throw InsertionConflictException()
if (shouldFail) throw Exception()
val pet = Pet(pets.size, addPetRequest.name)
pets.add(pet)
return pet
}
fun getPets(): List<Pet> = pets.toList()
}
