Transaction is a monad

TRANSACTION IS A MONADTRANSACTION IS A MONAD

JAREK RATAJSKIJAREK RATAJSKI
Software Developer, Wizard, AnarchitectSoftware Developer, Wizard, AnarchitectSoftware Developer, Wizard, Anarchitect
at Engenius GmbHat Engenius GmbHat Engenius GmbH

public static void main(String[] args) {
//take mushrooms from the baskets,
// but not more than 20 from each
List<Integer> mushrooms = new ArrayList<>(
Arrays.asList(14, 52, 31, 62, 71, 22, 34));
int sum = 0;
int i = 0;
while (i < 6) {
if (mushrooms.get(i++) > 20) {
sum = sum + 20;
} else {
sum = sum + mushrooms.get(i);
}
}
System.out.println(sum);
}

And there are bugsAnd there are bugsAnd there are bugs
List<Integer> mushrooms = new ArrayList<>(
Arrays.asList(14, 52, 31, 62, 71, 22, 34));
int sum = 0;
int i = 0;
while (i < 6) {
if (mushrooms.get(i++) > 20) {
sum = sum + 20;
} else {
sum = sum + mushrooms.get(i);
}
}
}

bugs in this code have their own bugsbugs in this code have their own bugsbugs in this code have their own bugs

HOW NOT TOHOW NOT TO HAVE BUGS?HAVE BUGS?

You have to be carefulYou have to be carefulYou have to be careful

It does not workIt does not workIt does not work

One cannot be careful 8One cannot be careful 8One cannot be careful 8 hours a dayhours a dayhours a day
ve days a weekve days a weekve days a week

Week after weekWeek after weekWeek after week

Errors will happenErrors will happenErrors will happen

PREVENTING BUGSPREVENTING BUGS

write tests

write tests
use the power of type system

write tests
use safer language features

write tests
use safer languages

write tests
use safer languages
...

TESTSTESTS
they do work
additional code
refactoring paradox

Safer languages / safer featuresSafer languages / safer featuresSafer languages / safer features

LAMBDASLAMBDAS
final List<Integer> mushrooms = new ArrayList<>(
Arrays.asList(14, 52, 31, 62, 71, 22, 34));
final int sum = mushrooms.stream()
.map(m -> Math.min(m, 20))
.reduce(0, (a, b) -> a + b);
}

BETTERBETTER
less imperative
more declarative

BETTERBETTER
less imperative
more declarative
no mutation

BETTERBETTER
less imperative
more declarative
no mutation
separation of concerns

THIS IS ONLY THE BEGINNINGTHIS IS ONLY THE BEGINNING

MUTANTSMUTANTS
final List<Integer> mushrooms = new ArrayList<>(
Arrays.asList(14, 52, 31, 62, 71, 22, 34)
);
int avg = calcAverage(mushrooms);
final int sum = mushrooms.stream()
.map(m -> Math.min(m, 20))
.reduce(0, (a, b) -> a + b);
}

FIGHTING MUTANTS WITHFIGHTING MUTANTS WITH
IMMUTABLE LISTSIMMUTABLE LISTS

List<String> a = [...] ;
a.add("sth"); //no change on a (futile operation)

List<String> a = [...] ;
List<String> b = a.add("sth");

VAVRVAVR
final io.vavr.List<Integer> mushrooms =
List.of ( 14, 52, 31, 62, 71, 22, 34);
int avg = calcAverage(mushrooms); //no problem
final int sum = mushreooms
.map(m -> Math.max(m, 20))
.fold(0, (a,b)-> a+b);

IMMUTABLE (FUNCTIONAL) LISTIMMUTABLE (FUNCTIONAL) LIST

public class Person {
private String firstName;
private String lastName;
private BigDecimal salary;
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public BigDecimal getSalary() {
return salary;
}

Immutability in javaImmutability in javaImmutability in java
public class PersonImmutable {
public final String firstName;
public final String lastName;
public final BigDecimal salary;
public PersonImmutable(
String firstName,
String lastName,
BigDecimal salary) {
this.firstName = firstName;
this.lastName = lastName;
this.salary = salary;
}
PersonImmutable withSalary(BigDecimal newSalary) {
return new PersonImmutable(
this.firstName,
this.lastName,
newSalary);
}
}

Immutability inImmutability inImmutability in kotlinkotlinkotlin
data class Person(
val firstName: String,
val lastName : String,
val salary: BigDecimal)
val x:Person = ...
val y = x.copy( salary = BigDecimal("15000"))

Java recordsJava recordsJava records
record Person(
String firstName,
String lastName,
BigDecimal salary )
var x = ...
//val y = x.copy( salary = BigDecimal("15000"))

Two great things You can do to avoid bugsTwo great things You can do to avoid bugsTwo great things You can do to avoid bugs
stop using java.util. collections
prefer immutable objects
use Kotlin

ASPECTSASPECTS
transactions
security
diagnostics
resource handling

FANTASTICFANTASTIC
FRAMEWORKSFRAMEWORKS

Fantastic frameworksFantastic frameworksFantastic frameworks
What do they promise:What do they promise:What do they promise:
concentrate on business logic
let the magic work
inject whatever wherever

Fantastic frameworksFantastic frameworksFantastic frameworks
What they really do:What they really do:What they really do:
leaking abstractions
mess in code
hard to debug problems
null pointer exceptions
slow tests

@Retryable
void myMethod () {
}

@Transactional
void myMethod () {
}

@Transactional
@Retryable
void myMethod () {
}

Is retry inside transaction or transaction insideIs retry inside transaction or transaction insideIs retry inside transaction or transaction inside
retry?retry?retry?
@Transactional
@Retryable
void myMethod () {
}

Libraries > FrameworksLibraries > FrameworksLibraries > Frameworks

What typical coder thinks when you say you doWhat typical coder thinks when you say you doWhat typical coder thinks when you say you do
not use framework to do transactionnot use framework to do transactionnot use framework to do transaction
code fromcode fromcode from
public void updateCoffeeSales(HashMap<String, Integer> salesForWeek)
throws SQLException {
PreparedStatement updateSales = null;
PreparedStatement updateTotal = null;
String updateString =
"update " + dbName + ".COFFEES " +
"set SALES = ? where COF_NAME = ?";
String updateStatement =
"update " + dbName + ".COFFEES " +
"set TOTAL = TOTAL + ? " +
"where COF_NAME = ?";
try {
con.setAutoCommit(false);
updateSales = con.prepareStatement(updateString);
updateTotal = con.prepareStatement(updateStatement);
for (Map.Entry<String, Integer> e : salesForWeek.entrySet()) {
updateSales.setInt(1, e.getValue().intValue());
updateSales.setString(2, e.getKey());
updateSales.executeUpdate();
updateTotal.setInt(1, e.getValue().intValue());
updateTotal.setString(2, e.getKey());
updateTotal.executeUpdate();
con.commit();
}
} catch (SQLException e ) {
JDBCTutorialUtilities.printSQLException(e);
if (con != null) {
try {
System.err.print("Transaction is being rolled back");
con.rollback();
} catch(SQLException excep) {
JDBCTutorialUtilities.printSQLException(excep);
}
}
} finally {
https://docs.oracle.com/javase/tutorial/jdbc/bashttps://docs.oracle.com/javase/tutorial/jdbc/bashttps://docs.oracle.com/javase/tutorial/jdbc/bas

HIGHER ORDERHIGHER ORDER
FUNCTIONFUNCTION

JOOQ ExampleJOOQ ExampleJOOQ Example
create.transaction(configuration -> {
AuthorRecord author =
DSL.using(configuration)
.insertInto(AUTHOR, AUTHOR.FIRST_NAME, AUTHOR.LAST_NAME)
.values("George", "Orwell")
.returning()
.fetchOne();
DSL.using(configuration)
.insertInto(BOOK, BOOK.AUTHOR_ID, BOOK.TITLE)
.values(author.getId(), "1984")
.values(author.getId(), "Animal Farm")
.execute();
// Implicit commit executed here
});

Hibernate can do that as wellHibernate can do that as wellHibernate can do that as well
TransactionUtil.doInHibernate( this::sessionFactory, session -
...
session.persist( item );
...
} );

SECURITYSECURITY
private <T> void renderSecure(Context ctx,
Function<Session, CompletionStage<T>> async
) {
final Option<String> bearer = Option.of(ctx.getRequest().getHeaders().get("Authorizatio
final Option<String> sessionId = bearer.map(b -> b.replace("Bearer ", ""));
final Option<Session> session = sessionId.flatMap(sessionsRepo::getSession);
ctx.render(JsonMapping.toJsonPromise(session.map(
sess -> (CompletionStage<Object>) async.apply(sess)
)
.getOrElse(
CompletableFuture.completedFuture("unauthorized")
)));
}
private Handler movePaddle() {
return ctx -> {
final String gameId = ctx.getPathTokens().get("id");
ctx.getRequest().getBody().then(body -> {
final float targetY = Float.parseFloat(body.getText());
renderSecure(ctx, session -> gamesRepo.movePaddle(gameId, session.userId, targe
});
};
}
https://github.com/javaFunAgain/ratpong/blob/mhttps://github.com/javaFunAgain/ratpong/blob/mhttps://github.com/javaFunAgain/ratpong/blob/m

What if want:What if want:What if want:
a function that is checked for permissions,
does database access,
and is monitored,
and ...

It can get messyIt can get messyIt can get messy
monitor( ()- >
renderSecure( securityContext, (user) ->
doInTransaction (dbSession -> {
}
....

AlternativeAlternativeAlternative
doInMyMightyStack( callContext -> {
System.out.printn("User is " + callContext.getUser
callContext.dataAccess.insert( )
})

But what if:But what if:But what if:
secure(
trx(
secure(
trx(
cache(

If we only had sth toIf we only had sth toIf we only had sth to chainchainchain such e ectssuch e ectssuch e ects

public void notifyGroupOwner(int userId) {
final User user = userService.getUserFromDB(10001);
final Group group = user.getGroup();
final User owner = group.getGroupOwner();
owner.sendMessage("User "+ userId
+ " is not eating mushrooms");
}

side e ects everywhereside e ects everywhereside e ects everywhere
public void notifyGroupOwner(int userId) {
final User user = userService.getUserFromDB(10001);
//may return null, throw exception
// may return null, is in transaction, throws sometimes excep
//may return null, may come from cache
owner.sendMessage("User "+ userId + " to straszny niejadek!");
//called async
}

NULLNULLNULL - I call it my billion-dollar mistake- I call it my billion-dollar mistake- I call it my billion-dollar mistake

handling missing datahandling missing datahandling missing data
User user = userService.getUserFromDB(10001);
if ( user != null) {
if ( group != null ) {
if ( owner != null ) {
owner.sendMessage("hello there!!!");
}
}
}

WHAT IF WE COULD MAKEWHAT IF WE COULD MAKE
PROBLEMS VISIBLE?PROBLEMS VISIBLE?

root of the problemroot of the problemroot of the problem
User getUserFromDB(int userId) {....

Optional<User> UserService::getUserFromDB(int userId) ...
Optional<Group> User::getGroup(int groupId) ...
Optional<User> Grouo::getGroupOwner() ...

better,better,better,
Optional<User> user = userService.getUserFromDB(10001);
if ( user.isPresent()) {
final Optional<Group> group = user.get().getGroup();
if ( group.isPresent() ) {
final Optional<User> owner = group.get().getGroupOwner
if ( owner.isPresent() ) {
}
}
}

better,better,better,
not smart at allnot smart at allnot smart at all
if ( user.isPresent()) {
final Optional<Group> group = user.get().getGroup();
if ( group.isPresent() ) {
final Optional<User> owner = group.get().getGroupOwner
if ( owner.isPresent() ) {
}
}
}

mapmapmap operationoperationoperation
Optional<String> str = "55";
Optional<Integer> x = str.map( v -> Integer.parseInt(v));

mapmapmap went wrongwent wrongwent wrong
Optional<Optional<Group>> group = user.map(u -> u.getGroup());

flatMapflatMapflatMap to the rescueto the rescueto the rescue
Optional<Group> group = user.flatMap(u -> u.getGroup());

Awesome!Awesome!Awesome!
userService.getUserFromDB(10001)
.flatMap(user -> user.getGroup() )
.flatMap(group -> group.getGroupOwner() )
.ifPresent(user-> user.sendMessage("Hallo there Code4Life!

OptionalOptional
declaration of missing data side e ect
basic handling (code) in
map
flatMap
getOrDefault

THAT IS A POWER OFTHAT IS A POWER OF
MONADMONAD

FAILURE - EXCEPTIONSFAILURE - EXCEPTIONS
class Group {
User getGroupOwner() throws NoOwnerException {...
}
}
class User {
Group getGroup() throws NoGroupMemberException {...
}
void sendMessage(String msg) {...
}
}
class UserService {
User getUserFromDB(long key) throws NoSuchUserException {
}

Either<DataError, User> UserService.getUserFromDB(int userId);
// + other services
enum DataError {
NoOwner, NoGroup, NoUser
}

userService.getUserFromDB(10001)
.flatMap(user -> user.getGroup() )
.flatMap(group -> group.getGroupOwner() )
.forEach(user->
user.sendMessage("Hallo there Code4Life,"
+ "there was no problem !")); //on success

A MONADA MONAD
de nition 1 - formalde nition 1 - formalde nition 1 - formal

aaa monadmonadmonad in X is just a monoid in thein X is just a monoid in thein X is just a monoid in the
category of endofunctors of X, withcategory of endofunctors of X, withcategory of endofunctors of X, with
product × replaced by compositionproduct × replaced by compositionproduct × replaced by composition
of endofunctors and unit set byof endofunctors and unit set byof endofunctors and unit set by
the identity endofunctor.the identity endofunctor.the identity endofunctor.

MONADMONAD
de nition 2 - by codede nition 2 - by codede nition 2 - by code

Something that hasSomething that hasSomething that has
flatMap,
map
of (pure)

Something that hasSomething that hasSomething that has
flatMap,
map
of (pure)
TypeclassTypeclassTypeclass

MONADMONAD
de nition 3 - by comparisonde nition 3 - by comparisonde nition 3 - by comparison

CONTAINERSCONTAINERS
String
a type
List<String>
a container of String
Set<String>
ArrayList<String>

CONTAINERCONTAINER
aaa TypeTypeType + multiplicity+ multiplicity+ multiplicity e ecte ecte ect

MONADMONAD
aaa TypeTypeType +++ somesomesome e ecte ecte ect

EFFECTSEFFECTS
missing data
fails, exceptions
asynchronous computations
multiplicity
transactions
dependency injections
+ combinations+ combinations+ combinations

FunctorFunctorFunctor - is a type(class) that has- is a type(class) that has- is a type(class) that has mapmapmap
Every monad is a functorEvery monad is a functorEvery monad is a functor
Some people call functorSome people call functorSome people call functor mappablemappablemappable

Applicative functorApplicative functorApplicative functor
Something between monad and functorSomething between monad and functorSomething between monad and functor
HasHasHas apapap functionfunctionfunction
ap(F<(A)->B>,F<A>)ap(F<(A)->B>,F<A>)ap(F<(A)->B>,F<A>) -> F-> F-> F

MONADS, FUNCTOR MAKE SIDEMONADS, FUNCTOR MAKE SIDE
EFFECTS EXPLICITLY VISIBLE IN AEFFECTS EXPLICITLY VISIBLE IN A
TYPE SYSTEMTYPE SYSTEM

Applicative functor lets youApplicative functor lets youApplicative functor lets you sumsumsum inside e ectsinside e ectsinside e ects
(potentially parallel sequence)(potentially parallel sequence)(potentially parallel sequence)
Monadic atMap sequences operation one byMonadic atMap sequences operation one byMonadic atMap sequences operation one by
oneoneone

If You have 2 independent db transactions,If You have 2 independent db transactions,If You have 2 independent db transactions,
that can be called concurrently - You can usethat can be called concurrently - You can usethat can be called concurrently - You can use
apapap
If operations on db depend on some otherIf operations on db depend on some otherIf operations on db depend on some other
transaction (success/rollback) use atMaptransaction (success/rollback) use atMaptransaction (success/rollback) use atMap

Monadic transactionMonadic transactionMonadic transaction

It uses JDBI
one of many possible implementations (very
simple)
class Transaction<A> (private val action : (Handle) -> A ) {
fun run(dbi : Jdbi) : A = dbi.inTransaction<A, RuntimeExce
fun map ( f: (A)->B) = Transaction {handle ->
f(action(handle))
}
fun flatMap( f: (A)->Transaction) = Transaction {h
f(action(handle)).action(handle)
}
companion object {
fun <T> pure (obj:T) = Transaction {
obj

Example useExample useExample use
internal fun selectNewJob () = Transaction{
dbHandle ->
dbHandle.createQuery(selectNextJobId)
.mapTo(Long::class.java)
.one()
.map { jobId ->
dbHandle.createUpdate(updateBatch).bind("b
.execute()
jobId
}
}.flatMap { jobId ->
selectNewTasks(jobId)
}

The same can be done with other aspectsThe same can be done with other aspectsThe same can be done with other aspects
/e ects/e ects/e ects
security
cache
diagnostics/monitoring
async

That is a lot ofThat is a lot ofThat is a lot of flatMapflatMapflatMap

Would be better if we hadWould be better if we hadWould be better if we had do notationdo notationdo notation

Four great things You can do to avoid bugsFour great things You can do to avoid bugsFour great things You can do to avoid bugs
stop using java.util. collections
prefer immutable objects
learn lambdas and higher order functions
use kotlin
use monads
use scala

How do we chain multiple e ects?How do we chain multiple e ects?How do we chain multiple e ects?
the worstthe worstthe worst flatMapflatMapflatMap ever...ever...ever...
Future<Secure<Transaction<Optional<Int>>>> myX;
Function<Int, Future<Secure<Transaction<Optional<String>>>>> f
myX.flatMap(f) // ...no go
Function<Secure<Transaction<Optional<Int>>>>, Future<Secure<Tr

Problem of stacking monadsProblem of stacking monadsProblem of stacking monads

Monad TransformersMonad TransformersMonad Transformers FutureTFutureTFutureT,,, OptionTOptionTOptionT

Monad transformers appeared to not be thatMonad transformers appeared to not be thatMonad transformers appeared to not be that
greatgreatgreat
We cannot have use them in kotlin or java (in aWe cannot have use them in kotlin or java (in aWe cannot have use them in kotlin or java (in a
sensible way)sensible way)sensible way)

AllMightyMonadAllMightyMonadAllMightyMonad
write the monad that has all the e ects for
you stack (cache, jdbc, async, etc.)
cry once (writing atMap) enjoy everywhere

Alternatives?Alternatives?Alternatives?

1. zioziozio
2. https://github.com/atnos-org/e /https://github.com/atnos-org/e /https://github.com/atnos-org/e /

No, we will not hack bytecodeNo, we will not hack bytecodeNo, we will not hack bytecode

Project neeProject neeProject nee

class Hasiok {
@Resource
val jdbcConnection: Connection
@Transactional
@Secure
@Cacheable
@Retryable
fun f(p:P) {
//code
}
}

class Hasiok {
val enterprisyF = Nee.pure(
secure
.and(retryable)
.and(cacheable)
.and(transactional)) {conn:ConnectionProvider ->
{p: P ->
//code
}
}
//declaration above means security is checked before retri
//and retrial is made before cache which happens before tr
}

fun readStone() = Nee.pure(cached.andThen(jdbc)) { jdbcProvide
{ id: StoneId ->
val dsl = DSL.using(jdbcProvider.getConnection().g
val record = dsl.selectFrom(Stones.STONES)
.where(Stones.STONES.ID.eq(id))
.fetchOneInto(Stones.STONES)
Stone(record.id, StoneData(record.name, record.pri
}
}.anyError()

fun addNewStone(newStone: StoneData) = seq.next().flatMap
addStone(it, newStone)
}
private fun addStone(stoneId: Long, newStone: StoneData) =
val dsl = DSL.using(jdbcProvider.getConnection().getRe
val insertedRows = dsl.insertInto(Stones.STONES)
.values(stoneId, newStone.name, newStone.price)
.execute()
if (insertedRows == 1) {
Option.some(stoneId)
} else {
Option.none()
}
}

Nee is a hobby, experimental project.Nee is a hobby, experimental project.Nee is a hobby, experimental project.
NeeNeeNee monadmonadmonad has 4 params:has 4 params:has 4 params:
Nee<R,E,P,A>Nee<R,E,P,A>Nee<R,E,P,A>
- R - environment (context)
- E - error type
- P - argument (for cache - I think this was a bas idea)
- A - the real result type

I am slowly adding e ects:I am slowly adding e ects:I am slowly adding e ects:
jdbc,
any transactional resource,
cache,
security,
diagnostic,

DEPENDENCYDEPENDENCY
INJECTIONINJECTION

open class DocsModule(val config: ConfigModule) {
open val dbProvider: HandleProvider by lazy {
DBProvider(config.cfg)
}
open val timeProvider: TimeProvider by lazy { RealTime }
open val jobsRepository by lazy { JobsRepository(dbProvide
open val zipWriter : StatusZipWriter by lazy {
StatusZipWriter(config)
}
open val docsStatusRepository : DocsStatusRepository by la

MATERIALSMATERIALS
Project ZIOProject ZIOProject ZIO
Fun(c) 2018.7: John De Goes - FP to the MaxFun(c) 2018.7: John De Goes - FP to the MaxFun(c) 2018.7: John De Goes - FP to the Max
The Book of MonadsThe Book of MonadsThe Book of Monads Alejandro Serrano MenaAlejandro Serrano MenaAlejandro Serrano Mena
(experiment)(experiment)(experiment)
https://github.com/ziohttps://github.com/ziohttps://github.com/zio
https://www.youtube.com/watch?https://www.youtube.com/watch?https://www.youtube.com/watch?
v=sxudIMiOo68v=sxudIMiOo68v=sxudIMiOo68
https://github.com/nee ect/neehttps://github.com/nee ect/neehttps://github.com/nee ect/nee

FUNCTIONAL PROGRAMMINGFUNCTIONAL PROGRAMMING
Power of abstractionsPower of abstractionsPower of abstractions
we want to have less bugs
we want testability
we want to have more fun
@jarek000000@jarek000000@jarek000000
Thank youThank youThank you

Transaction is a monad

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Transaction is a monad