Scala by the Bay 2017 In a microservice environment, it is painful to handle exceptions thrown by external services. We designed a mechanism for type-safe error handling using EitherT monad transformer to stack Future and Either. Calls to external services are implemented in a stack of EitherT for fail-fast error handling. Errors are separated for each logical external services. Error hierarchy is defined to expose different layers of the system. Location and reason of an error are provided for debugging. In addition, we validated input to each call of external service by Validated to accumulate errors. Validation rules are tested by property-based testing with ScalaCheck. We implemented both fail-fast and accumulated error handling with Cats in payment service for shopbot.

1. Error Handling Without Throwing Exceptions
SBTB 2017
Haeley Yao
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2. About me
Figure 1: Cat in the yard
3+ years coding in Scala
Passionate about FP
Work in New Product Development(NPD) team in eBay
hayao@ebay.com, @HaeleyYao
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3. Projects
Shopbat and mobile app for China market.
Play
Cats
Enumeratum, Scala Scraper
Typesafe config, Typesafe logging
Bigtable
Scalatest, Mockito
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4. Side effect
Partiality
Exceptions
Nondeterminism
Dependency injection
Logging
Mutation
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5. Throwing Exceptions is partial
1def mean(xs: Seq[Double]): Double =
2if (xs.isEmpty)
3throw new ArithmeticException("mean of empty list!")
4else xs.sum / xs.length
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6. Why give up Exceptions
Reduce partiality in program.
Error is defined by type and can be checked at compile time.
Make function composable.
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7. Alternatives to throwing Exceptions
Scala: Option, Try, Either
Scalaz: Disjunction, Validation
Scalactic: One, Many(Accumulation)
Cats: Either, Validated
Collection of experiments link
Test link
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8. Stacking Future and Either
Future[A] comes up in Scala.
We don’t want to block future, so end up with Future everywhere.
Since Future is asynchronous, any errors need to be captured in there.
Future is designed to handle Throwable.
Composing error handling logic.
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9. Simple example on how to use EitherT
1case class EitherT[F[_], A, B](value: F[Either[A, B]])
link
1object SimpleExample {
2sealed trait Error
3case object BadThingHappened extends Error
5def left: EitherT[Future, Error, String] =
6EitherT.fromEither[Future](BadThingHappened.asLeft[String])
8def right: EitherT[Future, Error, String] =
9EitherT.fromEither[Future]("Winning".asRight[Error])
11def failed: EitherT[Future, Error, String] =
12EitherT(
13Future.failed[Either[Error, String]](
14new RuntimeException("FutureFailure ")))
15}
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10. Fast-fail error handling
1// Catch fast-fail error.
2def compose: EitherT[Future, Error, String] = {
3for {
4v1 <- left
5v2 <- right
6} yield {
7v2
8}
9}
11// Hanlde result from composition.
12def handle(result: EitherT[Future, Error, String]):
Future[String] = {
13result.value.map {
14case Left(l) => "BadThingHappened"
15case Right(r) => r.toString
16} recover {
17case NonFatal(e) => e.getMessage
18}
19}
20}
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11. Designing Error type
1trait AppError
3// Error from external services.
5final case class HTTPError(ip: String, host: String) extends
AppError
6final case class BigTableError(tableName: String) extends
AppError
7final case class TokenError() extends AppError
8final case class UserInfoError(userId: String) extends
AppError
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12. Handling error in API call
Validate request header from FE
Validate request body from FE
Call external services
Validate response from external service
Return response to FE
1def validateHeader(headers: Map[String, String]):
Either[AppError, Map[String, String] = ???
2def validateRequest(requestBody: String): Either[AppError,
A] = ???
3def externalService1(header: Map[String, String], body:
String): EitherT[Future, AppError, B] = ???
4def externalServoce2(header: Map[String, String], body:
String): EitherT[Future, AppError, C] = ???
5def validateResponse(response: EitherT[Future, AppError,
C]): EitherT[Future, AppError, C] ???
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13. Composition
1// Fast-fail error handling.
2val result = for {
3validatedHeaders <-
EitherT.fromEither[Future](validateHeader(headers))
4validatedBody <-
EitherT.fromEither[Future](validateRequest(requestBody))
5extResult1 <- externalService1(validatedHeaders,
validatedBody)
6extResult2 <- externalService2(validatedHeaders,
extResult1)
7validatedResponse <- validateResponse(extResult2)
8} yield {
9validatedResponse
10}
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14. Response for good case and bad case
1val response = (for {
2res <- result.value
3} yield {
4res match {
5case Left(e) =>
InternalServerError(e).as(ContentTypes.JSON)
6case Right(r) => Ok(r).as(ContentTypes.JSON)
7}
8}) recover {
9case NonFatal(e) =>
InternalServerError(e).as(ContentTypes.JSON)
10}
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15. Notes for pattern
validateHeader and validateRequest returns Either to keep good
value and bad value.
Transform Either to EitherT for validateHeader and
validateRequest.
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16. References
Stacking Future and Either, Herding Cats (link)
Advanced Scala with Cats by Noel Welsh and Dave Gurnell (link)
Chapter 4. Handling errors without exceptions, Functional
Programming in Scala (link)
Designing Fail-Fast Error Handling by Noel Welsh (link)
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