Use ZIO Failures and Defects Effectively

Mar 29, 2024


Fabio Pinheiro, Shailesh Patil, Pat Losoponkul, Yurii Shynbuiev, David Poltorak, Benjamin Voiturier

#error-handling #zio

Context and Problem Statement

ZIO is a powerful and purely functional library for building scalable and resilient applications in Scala. However, effectively handling errors within the context of ZIO presents challenges that must be addressed.

Within our software development projects utilising ZIO, the management and handling of errors have emerged as areas requiring more clarity and strategy. The existing practices have shown limitations in terms of their efficiency and comprehensiveness.

The key issues are:

  1. Lack of Consistent Error Handling Strategies: There's inconsistency in error handling across different modules and components of our ZIO-based applications, making it challenging to maintain a unified approach.
  2. Understanding and Communicating Errors: There's a need for a clearer method to categorise errors and communicate these effectively within the team and across various layers, facilitating quicker identifications and resolutions.
  3. Optimising Error Recovery Mechanisms: While ZIO provides powerful abstractions for error recovery, there's a necessity to optimise these mechanisms to ensure graceful degradation and resilience in our applications.

This ADR aims to explore and define strategies for utilising ZIO's capabilities more effectively in handling errors. It will outline decision drivers, available options, their pros and cons, and ultimately, the recommended approach to enhance our error management practices with the ZIO framework.

Effective management of errors directly impacts the reliability, maintainability, and customer experience of our applications.

By addressing the challenges of consistent error handling, we aim to enhance the stability of our products, ensuring reduced downtime, clearer communication with customers through structured error messages, and quicker issue resolution.

This not only improves the overall customer experience but also accelerates feature delivery as developers can focus more on implementing new functionalities rather than troubleshooting ad-hoc errors. The resulting streamlined development process contributes to cost reduction and optimised resource allocation.

In essence, these efforts are customer-centric, aiming to deliver a reliable, efficient, and customer-friendly service interface that positively impacts the overall customer experience and product adoption.

Decision Drivers

  1. Consistency and Standardization: A consistent and standardised approach to error handling across different modules and components of our ZIO applications is crucial. This consistency will ease code readability, maintenance, and team collaboration.
  2. Robustness and Resilience: A key driver is to harden our applications against failures by leveraging ZIO's powerful error recovery mechanisms. Enhancing the robustness of our applications will improve their resilience in adverse conditions.
  3. Traceability and Debugging: Reducing debugging time and efforts associated with error resolution is another driving factor. An efficient error-handling strategy should enable traceability and quicker identification, communication, and resolution of errors.
  4. User Experience and Reliability: Improving the quality and clarity of error messages reported to our users is a key driver. We aim to refine error messages to enable users to better understand what’s going on and respond to issues, thereby enhancing the user experience and the overall reliability of our applications.
  5. Alignment with Best Practices: Aligning our error-handling strategies with industry best practices and leveraging the full potential of ZIO's error management features is a driver. Adhering to established standards can lead to more effective and maintainable code.

Considered Options

Option 1: Continue With The Current Error Handling Strategy

Continuing with the existing "so-so" error handling strategy currently in place within our ZIO applications without significant modifications or improvements.


  • Maintains continuity with current practices, potentially requiring minimal adjustments and avoiding immediate disruptions to ongoing projects.


  • Inconsistencies in the code base across the components and services may persist, leading to potential challenges in maintenance and scalability.
  • Engineers may spend time reinventing error-handling solutions rather than leveraging established best practices or frameworks.
  • No significant improvement in terms of traceability and problem debugging, potentially hindering the resolution of issues and defects.

Option 2: Leverage ZIO Failures And Defects Effectively

Adopting best practices for error handling within ZIO applications and effectively utilising ZIO Failures and Defects, defining and implementing stricter guidelines and protocols for error handling.


  • Ensures adherence to established best practices, promoting code consistency, reliability, and scalability across various components and services.
  • Reduces the need for developers to reinvent error-handling solutions, allowing them to leverage proven strategies and frameworks.
  • Improves traceability and problem debugging by employing standardised error-handling practices, facilitating quicker issue identification and resolution.


  • Will require adjustments to current code and practices, necessitating time and effort for implementation.Actual impact and workload still needs to be identified.

Decision Outcome

It has been decided to pursue Option 2: Leverage ZIO Failures and Defects Effectively. This decision aligns with our commitment to enhancing the reliability, scalability, and maintainability of our applications.

While we acknowledge this option requires more refactoring, its long-term benefits in terms of code quality, developer efficiency, and robust error management outweigh the associated refactoring efforts.

Option 2 - General Implementation Rules and Principles

Case 1: When designing a component or service

Carefully Segregate Error Types

When designing a new component or service, the nature of anticipated errors should be carefully considered, and a clear distinction between expected errors (i.e. ZIO Failures or domain-specific errors) and unexpected errors (i.e. ZIO Defects) should be made.

This segregation should be done according to the principles outlined in the ZIO Types of Errors documentation section. That is, carefully distinguishing between:

  • ZIO Failures

    • The expected/recoverable errors (i.e. domain-specific errors).
    • Declared in the Error channel of the effect => ZIO[R, E, A].
    • Supposed to be handled by the caller to prevent call stack propagation.
  • ZIO Defects

    • The unexpected/unrecoverable errors.
    • Not represented in the ZIO effect.
    • We do NOT expect the caller to handle them.
    • Propagated throughout the call stack until converted to a Failure or logged for traceability and debugging purposes by the uppermost layer.

Use ADT to Model ZIO Failures

Use Algebraic Data Types (ADTs) to model domain-specific errors as ZIO failures within the component/service interface.

Implementation tips:

  • Use a sealed trait or abstract class to represent the hierarchy of ZIO Failures, allowing for a well-defined set of error possibilities.
  • Define specific error cases within the companion object of the sealed trait. This practice prevents potential conflicts when importing errors with common names (e.g. RecordNotFoundError), allowing users to prefix them with the name of the parent sealed trait for better code clarity.


sealed trait DomainError

object DomainError {
  final case class BusinessLogicError(message: String) extends DomainError

  final case class DataValidationError(message: String) extends DomainError

Use Scala 3 Union Types to Be More Specific About ZIO Failure Types

Using the Scala 3 Union Types feature to declare the expected failures of a ZIO effect should be preferred over using the broader and more generic top-level sealed trait. This allows for a more explicit and detailed definition of potential failure scenarios and enhances error handling accuracy on the caller side.

This principle is outlined in the following section of the ZIO Error Management Best Practices documentation.


trait MyService {
  def myMethod(): ZIO[Any, BusinessLogicError | DataValidationError, Unit]

Don’t Type Unexpected Errors (i.e. ZIO Defects)

When we first discover typed errors, it may be tempting to put every error into the ZIO failure type parameter/channel. That is a mistake because we can't recover from all types of errors. When we encounter unexpected errors we can't do anything with, we should let the application die (i.e. the ZIO fiber). This is known as the “Let it Crash” principle, and it is a good approach for all unexpected errors.

This principle is outlined in the following section of the ZIO Error Management Best Practices documentation.

Case 2: When calling an existing component or service

Only Catch Failures You Effectively Handle

As a user of an existing component or service, you should exclusively catch failures that you are prepared to effectively handle. Any unhandled failures should be transformed into defects and propagated through the call stack. You should not expect callers of your component to handle lower-level failures that you do not handle.

Use Failure Wrappers To Prevent Failures Leakage From Lower Layers

Do not directly expose their failure types in your component interface when invoking lower-level components. Use wrappers to encapsulate and abstract failure types originating from lower-level components, thus enhancing loose coupling and safeguarding against leakage of underlying implementation details to the caller.

Using failure wrappers and propagating them should not be the default strategy. Lower-level failures should primarily be managed at your component implementation level, ensuring that it appropriately handles and recovers them.

Failures not handled within the component's boundaries should preferably be transformed into defects whenever possible.

Do not reflexively log errors

Avoid catching a ZIO failure or defect solely for the purpose of logging it. Instead, consider allowing the error to propagate through the call stack. It's preferable to assume that the uppermost layer, commonly known as 'the end of the world' will handle the logging of those errors. This practice promotes a centralised and consistent logging approach for better traceability and debugging.

This principle is outlined in the following section of the ZIO Error Management Best Practices documentation.

Adopt The “Let it Crash” Principle For ZIO Defects

Adopt the “Let it Crash” principle for ZIO defects. Let them bubble up the call stack and crash the current ZIO fiber. They will be handled/recovered at a higher level or logged appropriately “at the end of the world” by the uppermost layer.

Option 2 - Practical Implementation

Repository Layer

Try using defects only (UIO or URIO)

The repository layer leverages Doobie, which natively relies on unchecked exceptions. Doobie will report any database error as a subclass of Throwable, and its specific type will be directly linked to the underlying database implementation (i.e. PostgreSQL). Handling it this way means there is no deterministic way to recover from an SQL execution error in a database-agnostic way.

A good approach is to use ZIO Defects to report repository errors, declaring all repository methods as URIO or UIO(example). Conversely, declaring them as Task assumes that the caller (i.e. service) can properly handle and recover from the low-level and database-specific exceptions exposed in the error channel, which is a fallacy.

trait ConnectionRepository {
  def findAll: URIO[WalletAccessContext, Seq[ConnectionRecord]]

Converting a ZIO Task to ZIO UIO can easily be done using ZIO#orDie(example).

class JdbcConnectionRepository(xa: Transactor[ContextAwareTask], xb: Transactor[Task]) extends ConnectionRepository {
  override def findAll: URIO[WalletAccessContext, Seq[ConnectionRecord]] = {
    val cxnIO =
           | SELECT
           |   id,
           |   created_at,
           |   ...
           | FROM public.connection_records
           | ORDER BY created_at


For those cases where one has to generate a defect, a common way to do this is by using the following ZIO construct (example):

class JdbcConnectionRepository(xa: Transactor[ContextAwareTask], xb: Transactor[Task]) extends ConnectionRepository {
  override def getById(recordId: UUID): URIO[WalletAccessContext, ConnectionRecord] =
    for {
      maybeRecord <- findById(recordId)
      record <- ZIO.fromOption(maybeRecord).orDieWith(_ => RuntimeException(s"Record not found: $recordId"))
    } yield record

Apply the get vs find pattern

Follow the get and find best practices in the repository interface for read operations:

  • getXxx() returns the requested record or throws an unexpected exception/defect when not found (example).
  • findXxx() returns an Option with or without the request record, which allows the caller service to handle the found and not-found cases and report appropriately to the end user (example).
trait ConnectionRepository {
  def findById(recordId: UUID): URIO[WalletAccessContext, Option[ConnectionRecord]]

  def getById(recordId: UUID): URIO[WalletAccessContext, ConnectionRecord]

Do not return the affected row count

The create, update or delete repository methods should not return an Int indicating the number of rows affected by the operation but either return Unit when successful or throw an exception/defect when the row count is not what is expected, like i.e. an update operation resulting in a 0 affected row count (example).

class JdbcConnectionRepository(xa: Transactor[ContextAwareTask], xb: Transactor[Task]) extends ConnectionRepository {
  override def create(record: ConnectionRecord): URIO[WalletAccessContext, Unit] = {
    val cxnIO =
           | INSERT INTO public.connection_records(
           |   id,
           |   created_at,
           |   ...
           | ) values (
           |   ${},
           |   ${record.createdAt},
           |   ...
           | )

extension[Int](ma: RIO[WalletAccessContext, Int]) {
  def ensureOneAffectedRowOrDie: URIO[WalletAccessContext, Unit] = ma.flatMap {
    case 1 => ZIO.unit
    case count =>"Unexpected affected row count: $count"))

Do not reflexively log errors

The upper layer will automatically do so appropriately and consistently using Tapir interceptor customization.

Service Layer

Reporting 404 Not Found to user

How can a service appropriately report a 404 Not Found to a user that i.e. tries to update a record that does not exist in the database? Following the above rules, the update method will throw a defect that will be caught at the upper level and returns a generic 500 Internal Server Error to the user.

For those cases where a specific error like 404 should be returned, it is up to the service to first call find() before update() and construct a NotFound failure, propagated through the error channel, if it gives a None (example).

Relying on the service layer to implement it will guarantee consistent behavior regardless of the underlying database type (could be different RDMS flavor, No-SQL, etc.).

class ConnectionServiceImpl() extends ConnectionService {
  override def markConnectionRequestSent(recordId: UUID):
  ZIO[WalletAccessContext, RecordIdNotFound | InvalidStateForOperation, ConnectionRecord] =
    for {
      maybeRecord <- connectionRepository.findById(recordId)
      record <- ZIO.fromOption(maybeRecord).mapError(_ => RecordIdNotFound(recordId))
      updatedRecord <- updateConnectionProtocolState(
    } yield updatedRecord

Do not type unexpected errors

Do not wrap defects from lower layers (typically repository) in a failure and error case class declarations like this should be prohibited.

Considering that failures are viewed as expected errors from which users can potentially recover, error case classes like UnexpectedError should be prohibited (example).

Extend the common Failure trait

Make sure all service errors extend the shared trait org.hyperledger.identus.shared.models.Failure. This allows handling "at the end of the world“ to be done in a consistent and in generic way.

Create an exhaustive and meaningful list of service errors and make sure the value of the userFacingMessage attribute is chosen wisely! It will present "as is" to the user and should not contain any sensitive data (example).

trait Failure {
  val statusCode: StatusCode
  val userFacingMessage: String

sealed trait ConnectionServiceError(
                                     val statusCode: StatusCode,
                                     val userFacingMessage: String
                                   ) extends Failure

object ConnectionServiceError {
  final case class InvitationAlreadyReceived(invitationId: String)
    extends ConnectionServiceError(
      s"The provided invitation has already been used: invitationId=$invitationId"

User Input Validation

Enforcing user input validation (business invariants) should primarily sit at the service layer and be implemented using the ZIO Prelude framework.

While partially implementing user input validation at the REST entry point level via OpenAPI specification, it is crucial to enforce validation at the service level as well. This implementation ensures consistency and reliability across all interfaces that may call our services, recognizing that REST may not be the sole interface interacting with our services.

class ConnectionServiceImpl() extends ConnectionService {
  def validateInputs(
                      label: Option[String],
                      goalCode: Option[String],
                      goal: Option[String]
                    ): IO[UserInputValidationError, Unit] = {
    val validation = Validation
        ValidationUtils.validateLengthOptional("label", label, 0, 255),
        ValidationUtils.validateLengthOptional("goalCode", goalCode, 0, 255),
        ValidationUtils.validateLengthOptional("goal", goal, 0, 255)

Modeling validation errors should use a dedicated error case class and, when possible, provide validation failure details. One could use a construct like the following:

sealed trait ConnectionServiceError(
                                     val statusCode: StatusCode,
                                     val userFacingMessage: String
                                   ) extends Failure

object ConnectionServiceError {
  final case class UserInputValidationError(errors: NonEmptyChunk[String])
    extends ConnectionServiceError(
      s"The provided input failed validation: errors=${errors.mkString("[", "], [", "]")}"

Use Scala 3 Union Types

Use Scala 3 union-types declaration in the effect’s error channel to notify the caller of potential failures (example)

class ConnectionServiceImpl() extends ConnectionService {

  override def receiveConnectionRequest(request: ConnectionRequest, expirationTime: Option[Duration] = None):
  ZIO[WalletAccessContext, ThreadIdNotFound | InvalidStateForOperation | InvitationExpired, ConnectionRecord] = ???


Do not reflexively log errors

The upper layer will automatically do so appropriately and consistently using Tapir interceptor customization.

Controller Layer

Reporting RFC-9457 Error Response

All declared Tapir endpoints must use org.hyperledger.identus.api.http.ErrorResponse as their output error type (example) This type ensures that the response returned to the user complies with the RFC-9457 Problem Details for HTTP APIs.

object ConnectionEndpoints {

  val createConnection: Endpoint[
    (ApiKeyCredentials, JwtCredentials),
    (RequestContext, CreateConnectionRequest),
  ] = ???


Use "Failure => ErrorResponse" Implicit Conversion

If all the underlying services used by a controller comply with the above rules, then the only error type that could propagate through the effect’s error channel is the parent org.hyperledger.identus.shared.models.Failure type and its conversion to the ErrorResponse type is done automatically via Scala implicit conversion.

Do not reflexively log errors

The upper layer will automatically do so appropriately and consistently using Tapir interceptor customization.